Prepared by:

Douglas B. Bausch and David S. Brumbaugh

Northern Arizona University

P.O. Box 4099

Flagstaff, Arizona 86011

Two products were prepared at the state level at a scale of 1:1,000,000. The first product was a State of Arizona Maximum Intensity Ground Shaking Map (1887-1987) (Morrison and others, 1991) that contours the maximum levels of historical ground shaking experienced within the state. The second product prepared for the state was the Arizona 100-Year Accelerations contour map (Bausch and others, 1993). The acceleration mapping required a detailed analysis of seismic sources affecting Arizona, and their probability for recurrence. These interpretations were input into the U.S. Geological Survey computer program SEISRISK III (Bender and Perkins, 1987). The 100-year contour map illustrates the force of gravity expressed as a percentage of 1.0 g that has a 90-percent chance of not being exceeded during the next 100 years. The output consisted of a grid of about 4,000 data points that were contoured for Arizona and outlying regions. The preparation of these two map products, conclusions and recommendations are explained in the State Report (Bausch and Brumbaugh, 1994). More detailed seismic hazard mapping was provided for seven key communities in Arizona. The communities were selected based on their proximity to faulting, historical seismicity, and population. The communities of Grand Canyon Village, Flagstaff, Winslow, Prescott, Phoenix, Tucson and Yuma were selected for these studies. The community-based seismic hazard evaluation provides mapping of key geologic units that are expected to exhibit different intensities of ground shaking, as well as neotectonic faults, selected critical facilities, and the 50, 100 and 250 year acceleration data points and values from the state-wide project. The mapping is prepared utilizing 7.5-minute U.S. Geological Survey base maps that cover the urbanized area of each community. Data from these previous projects are utilized in this report for La Paz County that analyzes the vulnerabilities of the County and provides conclusions and recommendations.

• Maximum Intensity Ground Shaking Map (1887-1987)

• Arizona 100-Year Probabilistic Acceleration Contour Map

• Seismic Hazard Mapping of Local Geology and Accelerations for Seven Key Arizona Communities

The earthquake risk to La Paz County is dominated by its relative proximity to the major fault systems of southern California. Portions of La Paz County are as close as 100 miles from the San Andreas fault system that represents the boundary of the North American and Pacific tectonic plates. The magnitude 7.1 May 1940 Imperial Valley earthquake resulted in strong ground shaking in La Paz County, however, no damage was reported.

La Paz County has experienced several strong earthquakes from seismogenic sources within southern California, and several national earthquake catalogs list numerous earthquake epicenters within La Paz County. However, as a result of relocation studies by the Arizona Earthquake Information Center (AEIC) in 1992, nearly all these earthquakes were relocated in northern Mexico or southern California. Our catalog now lists only two earthquake epicenters within La Paz County, occurring in 1875 near Camp La Paz and 1964 underlying the Trigo Mountains of southwesternmost La Paz County. The first earthquake reported to have occurred in the La Paz County area was felt at Camp La Paz on the Colorado River in 1875.

The M 7.1 Imperial Valley earthquake occurred on May 18, 1940. Shaking effects in La Paz County included MMI VI and V effects at Parker and Quartzsite, respectively. The 1979 Imperial Valley earthquake also occurred on the Imperial fault near the international border. Shaking in the epicentral region was MMI IX and MMI V in Parker and IV in Quartzsite.

Seismic station coverage in La Paz County is poor, therefore, earthquakes smaller than M ±3.0 likely go undetected. In addition, because of La Paz County's sparse population many of the events are not felt and reported.

Historically, earthquakes originating southern California have been strongly felt in La Paz County. Generally two source areas in southern California have impacted the County. The first includes the Salton Trough region that generated the large earthquakes of 1940 and 1979, while the second source area includes the Mohave Shear Zone of the western Mohave Desert. Both of these earthquake sources range from 90 to 130 miles from Parker and western La Paz County.

Regions of La Paz County meet the criteria for liquefaction to occur. Areas along the Colorado River corridor, especially regions that are irrigated for farming, are underlain by relatively unconsolidated soil and shallow ground water. These areas could include upstream critical facilities, such as Davis, Parker or Alamo Lake dam. An assessment of the foundation materials of these dams are not part of this study. Typically, the dam owner's, such as the U.S. Bureau of Reclamation, etc., are required to study the safety of their dams on a regular basis. Several dams each year undergo seismic retrofit based on the results of these studies.

Failure of Hoover, Davis or Parker dams along the Colorado River as a result of an earthquake would greatly extend the scope of an earthquake disaster for all Colorado River corridor Counties. While Hoover and Parker are concrete arch dams, that are historically the most resistant to earthquake damage, Davis Dam is an earthen dam, a design that historically performs much more poorly in earthquakes. Earthen dams are often founded in areas underlain by alluvial materials that are subject to secondary earthquake hazards, such as liquefaction and/or subsidence and frequently require seismic retrofit based on their poor performance in recent earthquakes.

Because of these risks, Arizona is designated by the Federal Emergency Management Agency National Earthquake Hazards Reduction Program as a "High Risk" state for earthquakes.

1.1 What to do Before, During and After an Earthquake

2.0 FEDERAL PROGRAMS RELATED TO LOCAL SEISMIC HAZARDS

2.1 Federal Legislation

2.2 Earthquake Insurance

2.2.1 Federal Earthquake Insurance Proposals

3.0 EARTHQUAKE HAZARD EVALUATION: PROCEDURE

3.1 Ground Shaking

3.1.1 Predicting Ground Motion

3.2 Ground Failure

3.2.1 Slope Stability

3.2.2 Liquefaction

3.2.3 Ground Rupture

4.0 PREPARATION OF GROUND SHAKING MAPS FOR LA PAZ COUNTY, ARIZONA

4.1 Peak Ground Acceleration Mapping for La Paz County

4.2 Effects of Local Geology

5.0 HISTORICAL SEISMICITY

5.1 La Paz County Historical Earthquakes

5.2 Imperial Valley Earthquake of May 19, 1940, M_L 7.1

5.3 Imperial Valley Earthquake of October 15, 1979, M_L 6.5

5.4 Discussion of Historical Seismicity

6.0 EARTHQUAKE SOURCES

6.1 Southern San Andreas Fault

6.2 San Jacinto Fault

6.3 Imperial Fault

6.4 Cerro Prieto Fault

7.0 DESIGN EARTHQUAKES

7.1 Impact of the Design Earthquakes on La Paz County

8.0 VULNERABILITY OF LA PAZ COUNTY TO SEISMIC HAZARDS

8.1 Ground Shaking Parameters

8.2 Hazardous Buildings and Structures

8.3 Critical Facilities

8.4 Lifelines

9.0 SUMMARY AND CONCLUSIONS

10.0 MITIGATION OPPORTUNITIES

10.1 Special Development Regulations

10.2 Hazard Reduction

10.3 Recovery and Reconstruction

10.4 Recommended Goals and Policies

10.4.1 Retrofit and Strengthening of Existing Facilities

10.4.2 Strengthening and Enforcing Seismic Codes

10.4.3 Public Education

11.0 REFERENCES

12.0 GLOSSARY

TABLES  

1) GEOLOGIC TIME SCALE

2) ABRIDGED MODIFIED MERCALLI INTENSITY SCALE

3) NEOTECTONIC FAULTS OF THE LA PAZ COUNTY AREA

4) COMPARISON OF PROBABILISTIC ACCELERATION VALUES FROM SEVERAL STUDIES

5) PROBABILISTIC ACCELERATION VALUES FOR THE DAVIS AND PARKER DAM AREAS

6) SUMMARY OF EARTHQUAKES FELT IN LA PAZ COUNTY

7) DESIGN EARTHQUAKES

FIGURES

1) SEISMOGRAPH STATION LOCATIONS IN ARIZONA

2) U.B.C. SEISMIC ZONATION OF THE UNITED STATES

3) RELATIVE MOTION OF PACIFIC AND NORTH AMERICAN PLATES

4) GENERAL GROUND SHAKING RISK MAP FOR ARIZONA

5) GENERAL GROUND SHAKING RISK MAP FOR LA PAZ COUNTY

6) PROBABILISTIC ACCELERATION CONTOUR MAPPING FOR ARIZONA

7) NEOTECTONIC FAULTS OF THE LA PAZ COUNTY AREA

8) ACCELERATION PROBABILITIES-ARIZONA KEY COMMUNITY COMPARISON

9) ARIZONA EARTHQUAKES (1830-1993)

10) LOCAL FAULTING AND SEISMICITY

11) ISOSEISMAL MAP OF THE 1940 M_L 7.1 IMPERIAL VALLEY EARTHQUAKE

12) ISOSEISMAL MAP OF THE 1979 M_L 6.5 IMPERIAL VALLEY EARTHQUAKE

13) UNREINFORCED MASONRY CONSTRUCTION

14) SOFT STORY-TIMBER POLE CONSTRUCTION

15) PRECAST CONCRETE FRAME CONSTRUCTION

1.0 CAUSES, NATURE AND MEASUREMENT OF EARTHQUAKES

Earthquakes occur when stresses within the earth's crust are relieved by slippage along rupture surfaces known as faults. The rupture process generates waves that radiate from the fault source, affecting people and structures on the surface of the earth. Although the process is conceptually simple, the factors controlling the precise nature of an earthquake are not completely understood. Further geological and seismological research is needed to assess where and when earthquakes will occur in the future, as well as how large they are likely to be, and to anticipate the probable effects on various types of man made structures.

The focus, or hypocenter, of the earthquake is the point within the earth's crust where the initial rupture of the rocks occurs and where the elastic waves from the earthquake are first released. The majority of earthquakes recorded in the United States have had shallow focal depths; 15 km or less; and have occurred in regions containing faults outcropping at the surface. In other regions, however, earthquakes occur at deeper locations within the earth's crust, so that a surface rupture is not often observable in the field. The later process is perhaps the most common within the study area. Relatively deep earthquakes often exceeding 15 km are recorded for Arizona, and especially for the Colorado Plateau (AEIC Catalog of Earthquakes; Wong and Chapman, 1986). In addition, no surface rupturing events associated with earthquakes within Arizona have been observed during historic time.

The epicenter of an earthquake is the projection of the focus up onto the earth's surface. In the absence of instrumental data, epicenters have often been established on the basis of felt reports and the damage that is observed. However, epicenters are now typically located by the relative arrival times of seismic wave components received at various instruments operating within a seismograph network. In Arizona, there are twelve seismic stations located throughout the state at Tucson, Yuma, Phoenix, Flagstaff, Williams, Jerome, Sunset Crater, Blue Ridge Reservoir, Pipe Springs National Monument and the south and north rims of the Grand Canyon (Figure 1). However, in view of Arizona's earthquake risk and size, this is considered sparse seismic station coverage. Historically, seismic station coverage for Arizona is considered very sparse. The earthquake risk to the state can best be determined by adequate seismic station coverage that collects and processes accurate earthquake data. The accuracy of epicenter and hypocenter locations depends upon: (1) The number of reliable recording stations; (2) geologic interpretations of crustal structures; and (3) knowledge of local earthquake wave propagation velocities in various areas.

Figure 1 - Locations of seismograph stations within Arizona. Stations maintained by the AEIC are shown by open triangles, and include remote analog stations PSNM and SCN operated by the National Park Service, and JRAR operated by the State Parks service. Other operators in Arizona include: Arizona State University (ASU), Caltech (YMA), and University of Arizona (TUC).

Earthquakes are normally classified as to severity according to their magnitude (usually using the Richter scale), or their seismic intensity. Richter magnitude is a logarithmic measure of the maximum motions of the seismic waves as recorded by a seismograph. Because this size classification is based on a logarithmic scale, a magnitude 8 earthquake is not twice as big as a magnitude 4 earthquake, but rather, 10,000 (i.e., 10⁴ or 10x10x10x10) times larger. More recently, seismologists have shown that magnitude is also proportional to the energy released during an earthquake, but at a level 32 times greater between earthquake magnitudes (e.g., a magnitude 6 earthquake releases 32 times the energy as a magnitude 5 earthquake).

The magnitude of an earthquake is intended to be a measurement of its size, independent of the place of observation. It is calculated from measurements on seismographs. Physically, the magnitude can be correlated with the energy released by an earthquake, as well as with the fault rupture length and the maximum fault displacement. At present, at least four different magnitudes are in common use for classifying earthquakes: (1) local magnitude (M_L), the classic Richter magnitude based on peak response of a calibrated instrument; (2) body-wave magnitude (m_b), based on the response amplitude of the primary (P-wave) body-wave; (3) surface wave magnitude (M_S), based on the response amplitudes of long-period surface waves; and (4) the moment magnitude (M_W), which is the most complete measure of earthquake size. Moment magnitude (M_W) is directly based on the amount of energy released during an earthquake and can be measured by a geologist in the field examining the fault geometry, as well as by a seismologist studying the digital waveforms. Each of these magnitudes are used in this report, and are derived from a well-calibrated instrument, knowledge of the characteristics of the rock through which the seismic waves must travel and the local conditions at the seismograph station.

In the absence of instrumental recordings of ground motion, seismologists have described the ground movement by assigning intensity numbers according to subjective intensity scales. Following an earthquake, the assignment of an intensity to a given location is based on interviews with inhabitants of the area and on observations of damage in the area. Assigned intensity values from different locations are then combined to formulate a map containing a series of isoseismals, contours that separate regions of successive intensity rating. The shape and extent of the isoseismals are influenced by the tectonic features of the area, indicating predominant directions along which seismic waves are transmitted and the manner in which the earthquake originates (NUREG, 1975). In addition, several other factors influence the felt intensity of an earthquake, including: population density, local geology, shallow ground water, and building type.

The destructiveness of an earthquake at a particular location is commonly reported using the Modified Mercalli Scale of seismic intensity. Seismic intensities are subjective classifications based on reports of ground shaking and damage caused by past earthquakes. There are several seismic intensity scales; the one used most often is the Modified Mercalli Intensity (MMI) scale. The MMI scale was modified in the 1930's to address construction practices and affects on new inventions such as automobiles, and the scale is undergoing modification during the writing of this report to address modern construction practices, such as steel frame buildings. This scale has 12 levels of intensity; the higher the number, the greater the ground shaking intensity and/or damage.

Earthquakes have only one magnitude, but they have variable intensities that generally decrease with increasing distance away from the source. However, other factors such as local geology, shallow ground water and building type affect the intensities of earthquakes at a site. For example, greater intensities are associated with poorly consolidated alluvial soils, high ground water levels, poor construction practices and unreinforced masonry structures. Certain soils greatly amplify the shaking in an earthquake. Seismic waves travel at different speeds in different types of rock, and when seismic waves pass from rock to soil they generally slow down and get bigger. The looser and thicker the soil is, the greater the amplification will be. For example, ground motion that damaged regions underlain by poorly consolidated sediment in the Loma Prieta earthquake were 10 times greater than neighboring regions. In addition, earthquakes such as Northridge 1994 and Kobe 1995 have demonstrated the influence of fault rupture directivity on intensity distribution. When the earthquake rupture moves along the fault, it focuses energy in the direction it is moving so that a site in that direction will receive more shaking than a site the same distance away but in the opposite direction.

1.1 What to do Before During and After an Earthquake

The following is a list of tasks that individuals at the home or office should undertake to lessen the overall impact of a major earthquake.

Before an Earthquake:

• Remove or correct interior nonstructural hazards, such as top-heavy bookcases and storage cabinets, water heaters and other appliances. Anchor furniture and water heaters against the wall and provide gas-fired appliances with flexible connections.

• Set aside a supply of emergency food and water, and obtain first aid materials, a gas shut-off wrench, fire extinguisher, and battery-powered radio. Identify neighbors with first aid training and check for an emergency supply of medication for all members of the family, especially children, disabled, and elderly.

• Practice taking cover. This exercise will make people aware of the safest places during an earthquake, such as under a desk, table, bed or strong doorway. The maximum duration of shaking from an earthquake impacting La Paz County is expected to be roughly 20 to 30 seconds.

• Practice exiting. Walk the possible escape routes from your house or office and plan to avoid light fixtures, masonry chimneys, unsupported walls and other overhead hazards. Power for elevators and escalators may fail in high-occupancy facilities, so be aware of alternate exits. Do not panic or run; crowded exits should be evacuated in an orderly manner to avoid additional injuries in a rush for the door, emergency loud speaker systems may give instructions.

• Practice turning off electricity and water and know how to turn off gas at the main. Replace rigid inlet gas connection lines to water heaters with flexible line. For safety reasons, do not practice gas shut-off; only the utility company should turn the gas back on. Be sure anyone in the household can locate main switches and valves.

• Review the responsibilities of each family member after an earthquake. Plans for picking up children from schools, day-care centers, or other facilities with dependents should be regularly checked and reviewed. Have the phone number available of the person outside of the area for management of family messages.

• Contact the County and neighbors about forming a neighborhood co-op self-help group.

During an Earthquake:

• Stay calm.

• If you are indoors remain indoors.

• If in your place of residence, crouch under a desk or table, or brace yourself in a doorway (Be aware that it is possible for doors to swing shut during a quake). Try to protect your head with a coat, cushions, etc. Stay away from windows or brick masonry (fireplaces), china cabinets, hanging cabinets, or anything else that might possibly fall on you.

• If you are in a high-rise building stay away from outside walls and windows. Do not use the elevator.

• If outdoors remain outdoors. Try to move away from buildings, powerlines, trees, or anything that might fall on you.

• If you are in a car try to move away from overpasses. Stop slowly and remain in your car. If possible try not to park where building material may fall on your car.

After an Earthquake:

• Check for injuries in your family and neighborhood.

• Extinguish small fires and check for additional fire hazards, such as cracked walls, roof lines and attics, and other physical signs of structural damage that can cause a malfunction in the electrical wiring.

• Check for the smell of leaking gas, and if detected, shut off gas at the gas meter. Unanchored gas heaters or gas-fired hot water heaters may experience damage to valves and service connections, especially those without flexible line connections.

• Shut off electrical power if there is damage to the wiring or there is a gas leak. The main switch is usually located in or next to the main fuse or circuit breaker box.

• Clean up flammable liquids, medicines, and other harmful substances.

• Check for structural and nonstructural damage, such as cracked chimneys, fallen power lines, and other objects that may become unstable and fall during an aftershock.

• Try not to use water, it may result in a drop in water pressure for firefighting purposes (fire flow). Toilets should not be flushed until both incoming water lines and outgoing sewer lines have been checked to see if they are open.

• Try not to use the phone unless it is a genuine emergency. Emergency and damage report alerts, and other information may be obtained by turning on your radio.

• Report serious injuries and significant damage to a nearby city or county emergency reception center.

2.0 FEDERAL PROGRAMS RELATED TO LOCAL SEISMIC HAZARD ANALYSIS

2.1 Federal Legislation

At the federal level, there are two important pieces of legislation relating to local seismic hazard assessment. These are Public Law 93-288, amended in 1988 as the Stafford Act that establishes basic rules for federal disaster assistance and relief, and the Earthquake Hazards Reduction Act of 1977, amended in 1990, which establishes the National Earthquake Hazards Reduction Program (NEHRP).

The Stafford Act briefly mentions "construction and land use" as possible mitigation measures to be used after a disaster to forestall repetition of damage and destruction in subsequent events. However, the final rules promulgated by the Federal Emergency Management Agency (FEMA) to implement the Stafford Act (44 CFR Part 206, Subparts M and N) require post-disaster state-local hazard mitigation plans to be prepared as a prerequisite for local governments to receive disaster assistance funds to repair and restore damaged or destroyed public facilities. Under the regulations implementing Section 409 of the Stafford Act, a city or county must adopt a hazard mitigation plan acceptable to FEMA if it is to receive facilities restoration assistance authorized under Section 406.

The overall purpose of the National Earthquake Hazards Reduction Act is to reduce risks to life and property from earthquakes. This is to be carried out through activities such as: hazard identification and vulnerability studies; development and dissemination of seismic design and construction standards; development of an earthquake prediction capability; preparation of national, state and local plans for mitigation, preparedness and response; conduct basic and applied research into causes and implications of earthquake hazards; and, education of the public about earthquakes. While this bears less directly on earthquake hazards for a particular local government, much of the growing body of earthquake-related scientific and engineering knowledge has been developed through NEHRP funded research, including this study.

2.2 Earthquake Insurance

After every major earthquake, the problem of financing recovery and reconstruction reemerges. As urban settlement has expanded worldwide, disasters have been experienced with increasing frequency and publicity. So it would seem reasonable for private and public sector organizations to plan in advance to provide more adequately for such contingencies.

Yet disaster relief and recovery resources are not consistently adequate or timely. Federal and state disaster assistance covers only a portion of the loss encountered in major earthquakes. Sometimes it is not received until long after the critical needs for such assistance are experienced. Consequently, many households, businesses and industries are significantly disrupted and many smaller enterprises go out of business after a major disaster.

The optimum solution would be to build cities strong enough and located so as to withstand the worst damage likely to be caused by natural disasters. Gradually, as older cities are renewed with each cycle of rebuilding and reinvestment and as new cities are built with better codes and land use practices, this goal will come closer to being achieved. But the experience of hazard mitigation to date, together with the evolving state of scientific knowledge and incomplete coverage of federal and state disaster assistance, suggest that additional sources of financial support for post-disaster recovery and reconstruction are needed. One possible source is earthquake insurance.

Historically, insurance coverage for earthquake damage has been either unavailable or prohibitively expensive. This was because there were previously no established actuarial methods for accurately estimating earthquake losses in advance of such disasters. Because scientific methods of earthquake prediction and loss estimation were in their infancy, insurance companies have found it difficult to reasonably estimate what probable maximum loss they might incur by insuring against for a catastrophic disaster. Therefore, they could not be sure whether or not they could remain in business after such an event if payment of claims plus operating costs were to exceed premiums collected.

Improvements in loss estimation have proceeded sufficiently in the past decade, however, so that more insurance companies have begun to provide earthquake insurance, although still costly, with high deductibles. Some insurance companies are today better able to distinguish more clearly the level of risk by geographic area. Computerized methods, including Geographic Information Systems (GIS), have made it possible for some of the more sophisticated companies to begin to model and forecast potential losses, based upon information gathered and maintained about localized areas and the structures now being insured.

Although this growing trend in sophistication has helped insurance companies improve their available coverage to some degree, less than a third of all property owners in earthquake prone areas are estimated to be participating in earthquake insurance. Moreover, sophisticated new industry technology cannot overcome the problem of a nationwide impact likely to be created by a catastrophic event. Destruction anticipated in various catastrophic earthquake scenarios is so large and difficult to estimate that a national program to cover seismically induced losses has been slow to evolve and faces serious difficulties in enactment. Direct losses of such an event have been estimated varyingly in the tens of billions of dollars and indirect losses at several magnitudes more.

2.2.1 Federal Earthquake Insurance Proposals

In recent years, the insurance industry has approached the federal government to enact legislation which would require 100% mandatory coverage in all homeowner and commercial risk insurance, backed up by a federally sponsored reinsurance pool to which loans could be made by the federal government to offset losses incurred in a catastrophic event (Earthquake Project, 1989). Such loans would be paid back through future premium receipts.

Passage of such legislation has been stalled by disagreement over the issue of whether or not federally required earthquake insurance should be accompanied by hazard mitigation in high risk areas to reduce the potential magnitude of losses over time. The argument for hazard mitigation as part of a national earthquake insurance program is predicated on principles similar to those underlying the federal flood insurance program which has been in place for nearly two decades. That program has identified high risk areas by issuance of Flood Insurance Rate Maps which cities and counties are obligated to observe through requiring hazard mitigation measures in their local planning and zoning practices in high flood hazard areas.

The argument for mandatory mitigation goes, in short, why penalize those who are not in high risk areas by requiring them to absorb costs of losses which might otherwise be avoided through proper hazard mitigation? The counter argument is that mandatory mitigation would increase local development costs in many communities where earthquake losses may not be experienced. Although involved interests are far from agreement on the role and level of required mitigation, there is a reasonable expectation that a compromise will be worked out as the probabilities of a catastrophic earthquake disaster increase each year.

3.0 EARTHQUAKE HAZARD EVALUATION: PROCEDURE

The seismic hazard evaluation for La Paz County is determined by the analysis of the following factors:

1.) Preparation of probabilistic acceleration maps. Maps were prepared for the State of Arizona (Bausch and Brumbaugh, 1994) based on 50, 100 and 250 year probabilistic horizontal accelerations at bedrock. These data are provided by this report for La Paz County.

2.) Geographical factors such as the pattern, type, and movement of a nearby potentially active fault or fault system, and the distance of the fault to the area under investigation. An evaluation of the previous work performed in the area provides information for this segment of the earthquake hazard evaluation.

3.) The spatial and temporal distribution of historic earthquake epicenters. Historic records are utilized for this portion of the analysis.

4.) Evaluation of isoseismal maps (based on the Modified Mercalli Intensity scale that uses felt reports to map the extent and magnitude of earthshaking), to provide information on the expected intensity, type of ground motion, and the distribution of future earthquakes.

5.) Geologic criteria such as slope stability, ground rupture, liquefaction and other seismically induced geologic hazards. The geometry of the underlying fracture system(s), the profile of the overlying surficial deposits and the basement/soil interaction are important in the evaluation of seismic risk. Seismic amplification and dampening are controlled by the soil/basement profile and topographic effects, while mass movement may result from the applied seismic force (Seed and others, 1969).

6.) Vulnerability of critical facilities and lifelines based on structural type and seismic building code conformance. For this information the Uniform Building Code (U.B.C.) should be consulted. The U.B.C. for a given area varies with seismic zonation and by the importance of the structure to the community. La Paz County lies within U.B.C. Zone 3 and 2b, as shown on the current U.B.C. zonation mapping (Figure 2).

Given the generalized nature of this study, the seismic history of the area is significant. The frequency, ground acceleration, magnitude and intensity of past earthquakes are essential data (Haley and Hunt, 1974). The maps prepared for this study are recommended for planning purposes only; site specific investigation, especially for critical facilities are warranted.

Several faults have the potential of generating earthquakes that will cause strong ground motions in Arizona, including La Paz County. The southwestern one-half of La Paz County, due to its relative proximity (Figure 3), is especially vulnerable to San Andreas system earthquakes. Earthquakes on the San Andreas fault proper, as well as the Imperial and Cerro Prieto faults can cause strong ground shaking in southwestern Arizona as illustrated in Figures 4 and 5. Each of these potential earthquakes will affect La Paz County differently, depending on the distance between the earthquake-generating fault and La Paz County, the size and rupture mechanism of the earthquake, and the local geologic conditions. Some faults are also more likely to cause an earthquake than others. The faults of the San Andreas system have the fastest displacement rates of any in the western U.S., and therefore, produce earthquakes more frequently than most faults.

Seismic waves propagating through the earth's crust are responsible for the ground vibrations normally felt during an earthquake. Seismic waves vibrate up and down and side to side at different frequencies, depending on the frequency content of the earthquake rupture mechanism, the distance from the earthquake source to a particular site, and the path and material through which the waves are propagating. As seismic waves travel through the earth's crust, their energy is lost due to the inelastic behavior of the ground motion, and due to scattering, diffraction and deflection of the waves as they cross materials of different physical properties. The overall effect, known as attenuation, alters the form and frequency content of the seismic waves with distance away from the earthquake's source.

Near-field earthquakes, which occur within approximately 10 miles of the site of reference, generate rough, jerky, high-frequency seismic waves that are generally more efficient in causing short buildings, such as single-family residential structures, to vibrate. Longer period wave forms, characteristic of far-field earthquakes, are felt at greater distances from the earthquake source. These longer-period waves, manifested as a slow rolling motion, are more likely to cause high-rise buildings and buildings with large floor areas to vibrate vigorously. An earthquake on the southern San Andreas fault system would be an example of a far-field earthquake affecting La Paz County.

Faults in Arizona have formed over millions of years as a response to various tectonic stress regimes. Some of these faults are generally considered inactive under the present geologic conditions, that is, they are unlikely to generate future earthquakes. Other faults are known to be accumulating strain as a result of current shifting of the earth's plates. Such faults have either generated earthquakes in historical times, or show geologic and geomorphic characteristics that suggest they might move in the relatively recent future, within a time span of concern to the residents of the area, or for long-term consideration in building design.

In general, the probability of an earthquake occurring on a given fault decreases with age of the latest proven faulting. That is, geologically young faults (Quaternary) are more likely to move than pre-Quaternary faults. However, it is at times difficult to determine with a certain degree of confidence, which faults are capable of moving in the future, and which ones are not likely to move under the present stress regime. Geologic evidence suggests that some faults may remain dormant for hundreds to thousands of years between major displacements. The geologic time scale (Table 1) is often used as a yardstick of latest proven faulting to evaluate the risk a fault may pose to development. For Arizona, existing studies (Scarborough and others, 1983; Menges and Pearthree, 1983; Pearthree and others, 1983; and Scarborough and others, 1986) define neotectonic faults as those that exhibit signs of surface displacement within the last about 4 million years (Late Pliocene-Quaternary).

Faults with infrequent recurrence, however, should be considered in the design phase and seismic analyses of many types of projects, such as nuclear facilities, dams and emergency operation centers. When their risk cannot be established, these faults may also be treated in the same manner as active faults, including designating building setbacks if necessary. The activity classification of faults may also change as geologic field studies along the trace of the fault are conducted, or if an earthquake occurs on a fault previously considered inactive. Some historical earthquakes have occurred along previously unknown faults.

Maximum Probable Earthquake: A maximum probable earthquake is the largest earthquake a fault is predicted capable of generating within a specified time period of concern, say 30 or 100 years. Maximum probable earthquakes are most likely to occur within the time span of most developments, and therefore, are commonly used in assessing seismic risk.

TABLE 2 - ABRIDGED MODIFIED MERCALLI INTENSITY SCALE

Primary Source: Bolt (1993)

Maximum Credible Earthquake: The maximum credible earthquake, i.e. the largest earthquake a fault is believed capable of generating, is nevertheless, often considered in a number of planning and engineering decisions. For example, maximum credible earthquakes are considered in the design of critical facilities such as dams, nuclear power plants, and emergency operation centers. They are also used in urban and emergency planning to identify and mitigate the risk of worst-case scenarios.

Earthquakes are normally classified as to severity according to their magnitude (usually using the Richter scale), or their seismic intensity. Richter magnitude is a logarithmic measure of the maximum motions of the seismic waves as recorded by a seismograph. Because this size classification is based on a logarithmic scale, a magnitude 8 earthquake is not twice as big as a magnitude 4 earthquake, but rather, 10,000 (i.e. 10⁴ or 10x10x10x10) times larger. The destructiveness of an earthquake at a particular location is commonly reported using a seismic intensity scale. Seismic intensities are subjective classifications based on reports of ground shaking and damage caused by past earthquakes. There are several seismic intensity scales; the one used most often is the Modified Mercalli Intensity (MMI) scale (Table 2). This scale has 12 levels of intensity; the higher the number, the greater the ground shaking intensity and/or damage. Earthquakes have only one magnitude, but they have variable intensities that generally decrease with increasing distance away from the source. However, other factors such as local geology, shallow ground water and building type affect the intensities of earthquakes at a site.

3.1.1 Predicting Ground Motion

Ground motion caused by earthquakes is generally characterized using the parameters of ground displacement, velocity, and acceleration (Figure 6). Engineers traditionally work with ground acceleration, rather than with velocity or displacement, since acceleration is directly related to the dynamic forces that earthquakes induce on structures. The most often used measure of the strength of ground motion is peak ground acceleration. Peak ground accelerations are generally calculated using empirical attenuation equations that describe the behavior of the ground motions as a function of the magnitude of the earthquake, and the distance between the site and the seismic source (the causative fault). The increasingly larger pool of seismic data recorded in the world, and particularly in the western United States, has allowed researchers to develop reliable empirical attenuation equations that are used to model the ground motions generated during an earthquake.

Although computer models are now routinely used to predict the ground motions expected at a given site as a result of an earthquake, it is still difficult to anticipate the damage sustained by different types of structures during an earthquake. This is so because the response of structures to ground shaking depends on many parameters, including the amplitude and frequency content of the seismic waves, and the duration of shaking. The frequency content of the ground motion, in turn, depends on the rupture mechanism of the earthquake, the properties of the materials that attenuate the seismic energy, and the regional and local site conditions that may amplify, focus, or defocus the seismic waves arriving at the site of interest. In addition, different structures, because of differences in their natural frequencies and modes of vibration, respond differently to a given ground motion. For planning of critical facilities, therefore, it is often best to study the effects of the worst-case scenario, using as standard the maximum credible earthquake of the fault nearest to the site, such as an M 7 event on a fault within La Paz County. Structures are then designed accordingly, assuming that earthquakes of lesser magnitude and intensity will effect the study area to a lessor degree.

The ground shaking data presented in this report, if used together with inventories of potentially hazardous buildings, can help identify areas most likely to be damaged during an earthquake. The maps can also be used to identify areas where response capability operations, such as heavy rescue operations, will be vital in the case of an earthquake. The ground shaking data should be used only for general planning purposes, and should not be used for specific building design requirements. Site-specific studies are required to adequately characterize the seismic parameters used in the design of a structure.

Modified Mercalli Intensity levels for La Paz County may be calculated from the acceleration values presented from a combination of Richter's (1958) and Evernden and Thompson's (1988) empirical relationships for predicting intensity in terms of peak and root-mean-square (RMS) accelerations. Evernden and Thompson (1988) made a comprehensive study on the correlation between intensity and different seismic parameters. It can be assumed that RMS acceleration amplitudes for alluvial sites are about 70% of the corresponding peak ground acceleration values. The ratios of the RMS to peak amplitude for alluvial sites usually are higher than the ratios of the RMS to peak amplitude for stiff soil or rock sites. This is also consistent with Evernden and Thompson's (1988) recommendation of negative corrections for intensity scales for sites that are not underlain by alluvial materials. Richter (1958) reported an acceleration-intensity relationship based on the Modified Mercalli Intensity scale. Evernden and Thompson (1988) stated that Modified Mercalli Intensities are not a linear scale in terms of the level of ground shaking. They argued that the Rossi-Forel scale provides a better linear relationship. The relation between the two scales are qualitative.

Predicted intensity may have advantages over probabilistic bedrock accelerations for sites underlain by thick sequences of alluvium, such as the La Paz County community. Alluvium is known to amplify the effects of ground shaking in both intensity and duration of shaking. Other parameters that increase reported earthquake intensities such as shallow ground water, building type and soil properties may be included in the conversion of the predicted bedrock acceleration values to predicted intensity.

3.2 Ground Failure

In areas where ground failure might occur, seismic intensity values may increase by one or two levels. Areas of suspected ground failure in the event of a large magnitude near-field earthquake, including liquefaction and slope instability, are discussed further in this report. Geologic effects caused by earthquakes are divided into two principal categories: primary effects and secondary effects. Primary effects are those caused by deep-seated forces in the earth and include fault rupture, tectonic uplift and subsidence. Secondary geologic effects are those caused by ground shaking and include liquefaction, compaction of sediment and various forms of mass movement (Youd, 1986).

3.2.1 Slope Stability

Earthquake induced landslides and rockfalls often result in a considerable portion of the damage associated with historical earthquakes. Falls of precarious rocks may be triggered by small earthquakes in steep terrain. Slopes in their natural condition are generally far less susceptible to instability than those that are altered by activities of man. Mining activities within La Paz County can greatly enhance slope instability. Therefore, design of man-made slopes should include parameters for seismically induced ground shaking based on acceleration mapping. The rapid population growth in La Paz County will result in more development within vulnerable hillside regions. The current risk to the La Paz County community as a result of earthquake-induced slope instability is expected to be low.

3.2.2 Liquefaction

Liquefaction occurs primarily in saturated, loose, fine to medium-grained soils in areas where the ground water table is 50 feet or less below the ground surface. When these sediments are shaken, such as during an earthquake, a sudden increase in pore water pressure causes the soils to lose strength and behave as a liquid. The resulting features are called sand boils, sand blows or "sand volcanoes". Liquefaction-related effects include loss of bearing strength, ground oscillations, lateral spreading, and flow failures or slumping (Yerkes, 1985). Ground failure caused by liquefaction is a major cause of earthquake damage. For example, most of the extensive damage caused by the 1964 Alaska and the 1989 Loma Prieta earthquake was a consequence of liquefaction.

Regions of La Paz County meet the criteria for liquefaction to occur. Areas along the Colorado River corridor, especially regions that are irrigated for farming, are underlain by relatively unconsolidated soil and shallow ground water. These areas could include upstream critical facilities, such as Davis, Parker or Alamo Lake dam. An assessment of the foundation materials of these dams are not part of this study. Typically, the dam owner's, such as the U.S. Bureau of Reclamation, etc., are required to study the safety of their dams on a regular basis. Several dams each year undergo seismic retrofit based on the results of these studies.

3. Ground Rupture

Within the greater La Paz County area there are very few mapped neotectonic faults (Scarborough and others, 1983; Menges and Pearthree, 1983; Pearthree and others, 1983; and, Scarborough and others, 1986). An analysis of fault rupture hazard for a particular fault requires that the subject fault be located exactly, and that its potential for fault rupture be known, if only approximately. The historical record, is too short a time period to characterize the earthquake recurrence of most faults. Geologists use repeatedly offset stratigraphic or physiographic features along a fault to reconstruct the earthquake history of the fault. Approximate recurrence intervals for major earthquakes on that particular fault can often be obtained from the analysis of field data. These recurrence intervals are then used in engineering design and planning decisions. Unfortunately, these kinds of data are available for only a very few faults affecting Arizona.

The analysis of fault rupture potential also assumes that a fault will slip along the same or nearly the same surface on which the fault last slipped. This assumption is generally true, based on observations from past surface-rupture events that show most ground ruptures do follow closely pre-existing fault traces. However, during an earthquake some sections of a fault surface may rupture, while others may not. In conducting a fault-rupture hazard analysis the worst-case scenario is assumed, that is, that during a moderate to major earthquake the subject fault surface will rupture in the area of study. An earthquake producing surface rupture along a fault in La Paz County could be associated with an earthquake of magnitude 7. Surface rupture occurs when part of the stress released during an earthquake ruptures the fault plane at the earth's surface. In general terms, if the displacement is more than a few inches, structures that straddle the fault trace will be damaged. It is very costly to design structures to withstand large vertical or horizontal displacements.

Reconnaissance mapping of neotectonic faults in La Paz County has been completed by previous researchers (Figure 7). The major neotectonic faults affecting La Paz County include the Lost Trigo fault in southwesternmost La Paz County, and the Blythe Graben about five miles west of west-central La Paz County within California. The later exhibits geologically more youthful movement, as summarized in Table 3, below.

4.0 PREPARATION OF GROUND SHAKING MAPS FOR LA PAZ COUNTY

State-wide studies by Bausch and Brumbaugh (1994) provide probabilistic peak ground acceleration data for La Paz County.

4.1 Peak Ground Acceleration Mapping

The Peak Ground Acceleration (PGA) mapping represents peak horizontal acceleration of the ground at bedrock. The approach of representing peak horizontal ground acceleration on bedrock is a common and widely used method of showing ground accelerations. Indeed it has been utilized in national reports (Algermissen and others, 1982; 1990) and on one other report in Arizona (Euge and others, 1992). In fact, such an approach is often the only feasible one because of a lack of adequate data on spectral accelerations of different rock types. Over the last decade more rock type acceleration numbers have become available through the installation of accelerometers in the western U.S. At present, however, Arizona is sadly lacking in such data. The National Hazard Maps and subsequently the Uniform Building Code, which is based upon the national maps prepared periodically by the U.S. Geological Survey, are a result of probabilistic acceleration mapping. The construction of probabilistic acceleration maps are a result of three types of basic input parameters:

1) Attenuation of ground shaking with distance from the earthquake source;

2) Frequency of earthquakes within an area or region, termed recurrence; and

3) The character and extent of regions and faults that generate earthquakes.

Several probabilistic assessments have been performed for the study area. The mapping indicates a significant difference in the ground shaking potential for northern and southern La Paz County. Table 4 below provides the general range of values predicted for northern and southern La Paz County. The northern County generally includes the communities of Parker and Empire Landing, as well as the Parker Dam. Southern La Paz County includes the community of Quartzsite and the sparsely populated regions to the south.

For specific values indexed by latitude and longitude, one should refer to the report from Bausch and Brumbaugh (1994) and/or the AEIC World Wide Web site at http://vishnu.glg.nau.edu/aeic/aeic.html

The studies listed above utilized several different methods in determining the probabilistic accelerations for Arizona. Bausch and Brumbaugh (1994), and Euge and others (1992) represent regional reports, while Algermissen and others (1990) and Frankel and others (1996) represent the national mapping of the U.S. Geological Survey. The regional reports all utilized the computer program SEISRISK III (Bender and Perkins, 1987), and include line sources (faults), as well as historic seismicity.

A new method of determining probabilistic accelerations was utilized during preparation of the most recent national maps by Frankel and others (1996). Rather than defining seismogenic source zones, this method moves a one-square kilometers grid across the historic seismicity database, thereby providing a running average of the occurrence values for the region. The primary advantage of this method is in eliminating the uncertainties in defining source zone boundaries.

Values of horizontal accelerations exceeding 0.10 g, or 10-percent of the force of gravity, are generally accepted as being destructive to weakly constructed structures (Richter, 1958). Figure 8 was prepared to compare the acceleration values with other Arizona communities that are considered high (Yuma) to low (Phoenix). The anticipated accelerations for the San Francisco region, which has the highest earthquake risk in the U.S., are also illustrated for comparison.

Table 5, below, has been prepared to illustrate the values of Bausch and Brumbaugh (1994) determined for Davis and Parker dams. This referenced study included seismic sources throughout southern California, southern Nevada and Arizona. Because of the distance from these sources, the predicted accelerations in the areas of Davis and Parker dam are relatively low (Table 5).

4.2 Effects of Local Geology

An analysis of ground shaking intensity reported during historic earthquakes affecting Arizona indicated differences in ground shaking based on the underlying geology (Morrison and others, 1991). For our studies regarding Arizona key communities, geologic earth units that occur throughout the state were categorized into three groups: 1) alluvium; 2) sedimentary and volcanic bedrock; and, 3) granitic bedrock. The analysis of historic intensities indicated higher reported intensities for alluvial sediments compared to bedrock areas, as well as slightly higher intensities noted for sedimentary and volcanic rock as compared to areas underlain by granitic rock.

Shallow ground water can increase the expected seismic intensity values at a site. For most earthquake scenarios, seismic intensity values increase by one level on the Modified Mercalli Intensity scale (see Table 2) in those areas of La Paz County where shallow ground water is present. The accuracy of these interpretations are dependent on the accuracy of the ground water data available for La Paz County.

An increase of one level in the expected seismic intensities for a given scenario earthquake may be applied to any area of the county where shallow ground water (less than 30 feet) may be reported in the future, such as areas where development or agriculture may increase ground water levels.

Figure 8: Acceleration probability expressed as a percentage of the force of gravity against time. The graph illustrates the very high values anticipated for the Yuma community in comparison with high values for southern and relatively low values for northern La Paz County. San Francisco, the U.S. city with the highest earthquake risk, is shown for comparison. Ten-percent of the force of gravity is generally accepted as the onset of damage to weakly constructed structures (Richter, 1958).

5.0 HISTORIC SEISMICITY

La Paz County has experienced several strong earthquakes from seismogenic sources within southern California. Several national earthquake catalogs list numerous earthquake epicenters within La Paz County. However, as a result of relocation studies by the Arizona Earthquake Information Center (AEIC) in 1992, nearly all these earthquakes were relocated in northern Mexico or southern California. The AEIC studies involved searching archives of regional seismic networks and combining data to obtain better constrained earthquake locations. Our catalog now lists only two earthquake epicenters within La Paz County, occurring in 1875 near Camp La Paz and 1964 underlying the Trigo Mountains of southwesternmost La Paz County (Figures 9 and 10). As mentioned the most significant ground shaking in La Paz County resulted from southern California earthquakes in 1940 and 1979.

• The first earthquake reported to have occurred in the La Paz County area was felt at Camp La Paz on the Colorado River in 1875 and generated felt effects of MMI V effects at Camp La Paz and IV at Ehrenberg.

• The M_L 3.2-3.46 western Arizona earthquake of September 6, 1964 locates in the vicinity of the Trigo Mountains.

• The M 7.1 Imperial Valley earthquake occurred on May 18, 1940 and resulted in shaking effects in La Paz County of MMI VI at Parker and V at Quartzsite.

• The 1979 Imperial Valley earthquake also occurred on the Imperial fault near the international border. Shaking in the epicentral region was MMI IX and MMI V in Parker and IV in Quartzsite.

• Seismic station coverage in La Paz County is poor, therefore, earthquakes smaller than M ±3.0 likely go undetected, and because of La Paz County's sparse population many of the events are not felt and reported.

• Two source areas in southern California have impacted the County. The first includes the Salton Trough region that generated the large Imperial Valley earthquakes of 1940 and 1979. The second source area includes the Mohave Shear Zone of the western Mohave Desert that generated the 1992 M 7.3 Landers earthquake. Both of these earthquake sources range from 90 to 130 miles from Parker and western La Paz County.

Figure 9: Seismicity of Arizona 1830 to 1993 showing the Northern Arizona Seismic Belt (NASB) (Arizona Earthquake Information Center Archives).

Felt reports at Camp La Paz included, "Most of the soldiers left the barracks for the night, and took chances of catching cold in preference to having the old adobe buildings fall down on them. Heavier at La Paz than at Ehrenberg" (DuBois and others, 1982).

Reports from Ehrenberg indicated, "Then it was quite till 1 pm, when 5 more shocks, but light ones, followed. From Friday [1/22] night 9 pm, up to Saturday [1/23] 4 o'clock am, there were five distinct shocks" (DuBois and others, 1982).

Western Arizona M_L 3.2-3.46, September 6, 1964: No felt reports for this earthquake have been found. The event was recorded by the Tonto Forest Observatory and a focal depth of 15 km was reported (Figure 10).

The M 7.1 Imperial Valley earthquake occurred on May 18, 1940. Shaking in the epicentral region was MMI X, with $6 million in damages, 8 fatalities, and 20 injuries (Bolt, 1993). Approximately 65 kilometers (40 miles) of the Imperial fault ruptured with maximum right-lateral horizontal displacement of 4.5 meters (14 feet 10 inches) near the international boundary. Shaking effects in La Paz County included MMI VI and V effects at Parker and Quartzsite, respectively (Figure 11). Observations at Parker approximately 115 miles from the epicenter included, "Shock felt by many. Felt outdoors by some in automobile. Bureau of Reclamation dam structures were said to have been unscathed. A few window panes cracked. Some structures were cracked" (DuBois and others, 1982). At Quartzsite notable effects were, "Motion rapid; beginning gradual. Felt outdoors. Felt by all. Awakened few. Frightened few" (DuBois and others, 1982). To the south, in Yuma, MMI X effects were noted approximately 70 kilometers (50 miles) from the epicenter (Figure 11). In the Yuma Valley, liquefaction as the result of the earthquake buckled bridges and flumes, and ruptured the extensive canal network (DuBois, 1982). Dollar damage within the Yuma community exceeded $50,000 in 1940 dollars, with the majority located primarily near Gadsen.

The 1979 earthquake also occurred on the Imperial fault near the international border. Shaking in the epicentral region was MMI IX and MMI V in Parker and IV in Quartzsite (Figure 12). The October 15, 1979 M 6.5 earthquake injured 91 persons, damaged 1565 homes and 450 businesses. The estimated dollar damage was $30 million, including heavy damage to the All-American Canal. Approximately 25 kilometers of the Imperial fault ruptured with maximum right-lateral horizontal displacements of 55 centimeters and vertical displacements of 19 centimeters.

5.4 Discussion of Historical Seismicity Affecting La Paz County

Seismic station coverage in La Paz County is poor, therefore, earthquakes smaller than M ±3.0 likely go undetected. In addition, because of La Paz County's sparse population many of the events are not felt and reported. These factors should always be considered when evaluating an historic data base for a region.

Historically, earthquakes originating southern California have been strongly felt in La Paz County. Generally two source areas in southern California have impacted the County. The first includes the Salton Trough region that generated the large earthquakes of 1940 and 1979 discussed above, while the second source area includes the Mohave Shear Zone of the western Mohave Desert. Both of these earthquake sources range from 90 to 130 miles from Parker and western La Paz County. A listing of the events felt in La Paz County, and their intensities are provided in Table 6, below:

Earthquakes that occur outside of Arizona are frequently felt in La Paz County (see Table 6). The San Andreas, San Jacinto and Imperial fault zones are located within 110 miles of western La Paz County, and the Cerro Prieto fault is located within about 130 miles. The segment of the San Andreas fault nearest La Paz County has not ruptured in a major earthquake in more than 300 years, and is considered a likely segment to rupture in a magnitude 8 or greater earthquake.

Some faults are also more likely to cause an earthquake than others, such as the plate boundary faults listed above. La Paz County is located within the Basin and Range Physiographic Province. Faults of the Basin and Range have the potential for producing large damaging earthquakes, such as the 1887 M 7.2 Sonora, Mexico and the 1983 M 7+ Borah Peak, Idaho events. However, Basin and Range faults have relatively long and uncertain recurrence times, that is, the time between large earthquakes occurs on a scale of tens to hundreds of thousands of years. Whereas, large earthquakes occur on plate boundary faults on a scale of hundreds of years, or less.

Therefore, a more probable earthquake hazard to La Paz County, than a nearby event, is from large distant earthquakes in California or Mexico. The Maximum Probable Earthquake (MPE) for La Paz County is considered a large magnitude plate boundary earthquake, while the Maximum Credible Earthquake (MCE) would be a local Basin and Range earthquake within the County. Although the later has very long recurrence intervals, it should be considered in the design of critical facilities.

6.1 Southern San Andreas Fault

The southern San Andreas fault passes within 65 miles west of Arizona, and 110 miles west of La Paz County (see Figure 3). The San Andreas fault is a right-lateral transform fault extending for more than 600 miles from the Salton Sea to off the coast of northern California at Cape Mendocino. The San Andreas fault is perhaps the most studied in California (Wesnousky, 1986), and has been the primary laboratory for modern probabilistic seismic hazard analysis (Grant and Sieh, 1993). The southern San Andreas consists of three segments as defined by the Working Group on California Earthquake Probabilities (WGCEP) (1988). From north to south they include the Mojave, San Bernardino Mountains and the Coachella Valley segments. The WGCEP (1988) data on each of these segments is presented as follows:

New developments since the WGCEP 1988 report include: 1) increase of regional earthquake activity since 1985 (Jones, 1992); 2) the Landers earthquakes have occurred; and 3) the stress towards failure has been increased on parts of the San Andreas fault.

This southernmost segment is located approximately 110 miles from western La Paz County. The data presented above indicates the southernmost segment, the Coachella Valley, is the most likely segment of the San Andreas fault to fail in the next 30 years (WGCEP, 1988). The stress towards failure on the Coachella Valley segment was increased by the Landers earthquake sequence of 1992 (Stein and others, 1992). The Coachella Valley segment extends from the Salton Sea on the southeast to San Gorgonio Pass on the northwest. Very low levels of aseismic creep have been noted on this segment (Louie and others, 1985). Paleoseismic data near Indio indicate an average time interval between earthquakes of about 230 years (Sieh, 1986), however, the most recent rupture occurred about 300 years ago (1680±40).

On June 28, 1992, a M_W 7.4 earthquake occurred near Landers within the Mojave Desert of southern California. This earthquake was widely felt in Arizona, and residents of Yuma and Tucson reported that water spilled over the edges of their pools during the shaking (Wallace, 1992). This phenomena is known as a seiche, which can occur within lakes and reservoirs as well. The Landers earthquake triggered seismicity throughout much of the western United States, and is believed to a brought the southern San Andreas fault (Coachella Valley segment) closer to a failure point (Jaumé and Sykes, 1992; Jones, 1992; Stein and others, 1992; and Working Group, 1992).

6.2 San Jacinto Fault

The San Jacinto fault has historically produced more large earthquakes than any other fault in southern California, although no great earthquakes such as the 1857 and 1906 San Andreas earthquakes have been produced. The San Jacinto fault is a right-lateral strike-slip fault with reported slip rates ranging from 1.6 to 18 mm/yr. The average slip rate is reported at 8-12 mm/yr (Wesnousky, 1986). The fault is divided into five segments based on geologic characteristics and seismicity patterns (Sanders, 1986; WGCEP, 1988).

According to the U.S. Bureau of Reclamation (USBR, 1976) southwestern Arizona was strongly shaken by the April 8, 1968 Borrego Mountain earthquake on a southerly segment of the San Jacinto fault. In addition, the Superstition Hills earthquake sequence of November 1987 was strongly felt throughout southwestern Arizona. The largest events of this sequence were M_W 6.2 and 6.6 (Sipkin, 1989). About 9 km of ground rupture with a maximum slip of 12.5 cm was observed along the Superstition Hills and Superstition Mountains faults of the southern San Jacinto fault zone (Sharp and others, 1989). Two 12-15 mile segments of the San Jacinto fault have not slipped since at least the late 1890s (Sanders and others, 1986). For this study we assign a Maximum Credible Earthquake (MCE) of M 7.5 for the San Jacinto fault.

6.3 Imperial Fault

The Imperial fault is the right-lateral transform system that connects the oceanic-type spreading centers located at the Brawley seismic zone (Johnson, 1979) of the Imperial Valley, and the Cerro Prieto geothermal area. The Imperial fault is about 60 miles in length and passes within 110 miles of western La Paz County. It has produced at least two, and possibly five, large historic earthquakes (Johnson and Hill, 1982). The largest events were the M_L 7.1 May 18, 1940 and M_W 6.5 October 15, 1979 Imperial Valley earthquakes. Both of these events caused damage from high intensity ground shaking and soil liquefaction in the Yuma Valley area of Arizona (USBR, 1976). The most extensive liquefaction damage in Yuma was a result of the larger 1940 event, the areal extent of this damage is illustrated on the maps prepared for the Seismic Hazard Evaluation for Yuma, Arizona (Bausch and Brumbaugh, 1994). Aseismic surface creep on the northern 30 km of the fault measured by alignment arrays and creep meters (Louie and others, 1985) is about 8 mm/yr. Right-lateral displacement at depth on the southernmost 30 km of the Imperial fault during the 1940 earthquake ranged from 4 to 8 meters, and 1.8 to 2.2 meters for the northern 30 km. The 1940 event rupture extended 35 km further south than the 1979 event rupture (Snay and others, 1982). The fault slip for the 1979 event was about 0.8 meters. Slip rates for the Imperial fault range from 8.6 to 20 mm/yr (Wesnousky, 1986), and a Maximum Credible Earthquake of 7.25 has been assigned for this analysis.

6.4 Cerro Prieto Fault

7.0 DESIGN EARTHQUAKES

The design earthquakes for La Paz County are the maximum credible and probable earthquakes presented in Table 7. Because most earthquakes are believed to originate as a result of fault breaks, the rock motion at any particular site will depend on: (1) the amount of energy released along the fault during the earthquake; and (2) the distance of the site from the zone of energy release. In general the amplitude of these motions decrease with increasing distance from the zone of energy release, although other factors, such as geologic structure and orientation, will also have some effects (Seed and others, 1969). The magnitude of an earthquake is a convenient indication of the amount of energy released when a fault breaks. Because the magnitude, M, of an earthquake and the amount of energy released, E, are related by:

an increase of 1 unit on the magnitude scale corresponds approximately to a 30-fold increase in the amount of energy released. Another factor that needs to be considered is the depth of focus or hypocenter. Depths of earthquakes in California and northern Mexico are usually 6 to 15 miles (10 to 25 km) and are considered to be shallow. For events this shallow the hypocentral and epicentral distances are not appreciably different when the distance between the site and causative fault exceeds about 40 miles (60 km) (Seed and others, 1969). In the case of the maximum probable design earthquake the causative fault would be at least 40 miles (60 km) from the study area. The length of the fault break must also be considered, as the site to fault distance and duration of shaking are dependent on the length and direction of rupturing. In the case of the maximum probable earthquake for La Paz County the causative faults are short in length relative to the distance of the site from the fault. Therefore, the distance of the site from the fault can be expressed by the epicentral distance for the La Paz County area.

In relating the design earthquake to engineering seismology the following parameters must be considered: (1) magnitude of the event; (2) maximum amplitude of the horizontal acceleration; (3) duration of strong motion; and (4) the predominant frequency or period of motion of the site (Haley and Hunt, 1974). These parameters are given in Table 7 for MMI of VIII and VIII-IX events, the MPE and MCE, respectively. The design parameters are calculated from the acceleration curves and from the predominant period calculations of Seed and others (1969). These equations result in an increase in period of the earthquake with increasing distance from the epicenter.

7.1 Impact of the Design Earthquake to La Paz County

The following compilation of potential earthquake hazards are based on the response of the materials underlying La Paz County to the design earthquake parameters listed in Table 7.

A) Within regions of La Paz County that are underlain by alluvial deposits and areas of shallow ground water, the potential exists for greater damage than normal to structures. The earthquake damage index to structures increases with increasing thickness of the alluvial layer and softness (compaction) of the subsoil (Kanai, 1983).

B) The natural period of one story wooden structures ranges (i.e. most single-family residences) from 0.2 to 0.3 seconds (Kanai, 1983). The predominant period of the MPE and MCE has a range of values from 0.20 to 1.40 seconds that may result in developing resonance (period of structure = period of earthquake) and the accompanying increase in vibration and destruction. Multi-story buildings will display higher periods of vibration making resonance possible. However, the predominate period of individual buildings may vary depending on design and construction materials.

C) Documented earthquake damage studies indicate brick buildings experience an overall increase in damage on soft ground with an increase in the number of stories. The thickness of the sediment package is a controlling factor in the expected damage to concrete structures.

D) However, thickness can be offset by variations in the natural density of the material underlying a specific reinforced concrete structure. An increase in the softness of the ground causes a concomitant increase in the extent of damage to reinforced concrete buildings (Kanai, 1983). The demonstration of low values for penetration tests during geotechnical investigations are a suitable measure of ground softness.

E) By using the empirical equation of the natural period for reinforced concrete buildings a range of periods for different heights of structures can be developed:

1) 1 story = .06 - .12 seconds

2) 2 story = .12 - .21 seconds

3) 3 story = .21 - .30 seconds

4) 4 story = .30 - .40 seconds

5) 5 story = .40 - .60 seconds

6) 8 story - 10 story = .60 - .90 seconds

The duration of motion and the period of .55 seconds resulting from earthquakes in Tokyo provided suitable conditions for the development of resonance in 5 story structures (Kanai, 1983). The MCE has a predominate period of 0.2-0.8 sec., thereby, resulting in resonance to 3 to 5-story structures. This table should be used as a guide, a more precise value of predominant period should be determined by a structural engineer taking into account the specific building design and construction.

• M_L = 8.0

• DISTANCE FROM EPICENTER =100-130 miles

Predicted characteristics of ground motion for La Paz County, Arizona. Caused by a magnitude 8.0 earthquake on any fault of the San Andreas system. No primary ground rupture in La Paz County would be expected with the MPE.

• MAXIMUM CREDIBLE EARTHQUAKE: INTENSITY VIII-IX

• M_L = 6.5

• DISTANCE FROM EPICENTER = 0-10 miles

Predicted characteristics of ground motion for La Paz County, Arizona produced by a surface rupturing event along a local Basin and Range fault.

8.0 VULNERABILITY OF LA PAZ COUNTY TO SEISMIC HAZARDS

This section assesses the e