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Evaluation of Active Faulting and Associated Hazards

D.BURTON SLEMMONS and CRAIG M.DEPOLO

University of Nevada, Reno

ABSTRACT

Active faulting is a geologic hazard with a causative relation to earthquakes and associated strong ground motion, surface faulting, tectonic deformation, landslides and rockfalls, liquefaction, tsunamis, and seiches. Plate-tectonic models for the Earth’s crust show that most active faults occur near plate boundaries, and research has been concentrated in interplate regions. Intraplate regions have less fault activity and represent a potential hazard that only recently has been recognized. Faults are delineated by geologic, remote-sensing, seismic reflection, gravity, magnetic, and trenching methods. Fault activity is assessed using geologic, geomorphic, geodetic, and seismologic data. Correlations of fault length, displacement, and area with earthquake magnitude are utilized to assess earthquake hazards of faults and form the principal data for risk analysis. Estimation of earthquake recurrence rates and characterization of fault behavior provide additional input data for risk analysis.

Recent attention has been focused on the character of subduction in the northwestern United States. An absence of large seismic events along this convergent zone has led to the speculation that the zone is aseismic. Recent studies indicate, however, that if this is true, this zone may be unique when compared with other subduction zones of the world. Ongoing studies are trying to determine if this region has had large earthquakes in the geologic past. The recent discovery of the active geomorphic features along the Meers Fault in Oklahoma has prompted studies of this and adjoining regions.

INTRODUCTION

The assessment of earthquake hazards involves consideration of earthquake magnitude, intensity, frequency, recency of the last event, probability of occurrence, and human experience and values.

The appraisal of earthquake potential is feasible because historical data show a good correlation between earthquake size and the fault rupture parameters of length, maximum displacement, and fault rupture area. Most earthquakes have proportional tectonic effects, with earthquakes of below magnitude 6 having fault ruptures of up to 10-km length and a few centimeters maximum displacement, but magnitude 8+ earthquakes have up to 400-km length and 10 m of displacement. This relation also can be used for paleoseismicity evaluations to infer from prehistorical evidence the potential size of future activity on the same fault segments.

The historical record of worldwide earthquakes



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Active Tectonics: Studies in Geophysics 3 Evaluation of Active Faulting and Associated Hazards D.BURTON SLEMMONS and CRAIG M.DEPOLO University of Nevada, Reno ABSTRACT Active faulting is a geologic hazard with a causative relation to earthquakes and associated strong ground motion, surface faulting, tectonic deformation, landslides and rockfalls, liquefaction, tsunamis, and seiches. Plate-tectonic models for the Earth’s crust show that most active faults occur near plate boundaries, and research has been concentrated in interplate regions. Intraplate regions have less fault activity and represent a potential hazard that only recently has been recognized. Faults are delineated by geologic, remote-sensing, seismic reflection, gravity, magnetic, and trenching methods. Fault activity is assessed using geologic, geomorphic, geodetic, and seismologic data. Correlations of fault length, displacement, and area with earthquake magnitude are utilized to assess earthquake hazards of faults and form the principal data for risk analysis. Estimation of earthquake recurrence rates and characterization of fault behavior provide additional input data for risk analysis. Recent attention has been focused on the character of subduction in the northwestern United States. An absence of large seismic events along this convergent zone has led to the speculation that the zone is aseismic. Recent studies indicate, however, that if this is true, this zone may be unique when compared with other subduction zones of the world. Ongoing studies are trying to determine if this region has had large earthquakes in the geologic past. The recent discovery of the active geomorphic features along the Meers Fault in Oklahoma has prompted studies of this and adjoining regions. INTRODUCTION The assessment of earthquake hazards involves consideration of earthquake magnitude, intensity, frequency, recency of the last event, probability of occurrence, and human experience and values. The appraisal of earthquake potential is feasible because historical data show a good correlation between earthquake size and the fault rupture parameters of length, maximum displacement, and fault rupture area. Most earthquakes have proportional tectonic effects, with earthquakes of below magnitude 6 having fault ruptures of up to 10-km length and a few centimeters maximum displacement, but magnitude 8+ earthquakes have up to 400-km length and 10 m of displacement. This relation also can be used for paleoseismicity evaluations to infer from prehistorical evidence the potential size of future activity on the same fault segments. The historical record of worldwide earthquakes

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Active Tectonics: Studies in Geophysics shows an excellent spatial correlation with plate-tectonic movements. Most of the historical or geologically young fault ruptures are located on or near boundaries between plates and microplates. Importance to Society Most evaluations of active faults are conducted at or near plate boundaries, where consideration of design, siting, zoning, communication, and response to earthquake hazards is necessary for all types of major engineering structures in order to reduce potential loss of life, injury, or property damage. The seismic motion or deformation effects on facilities such as nuclear generators, dams, communication centers, and other lifelines is critical because of great potential harm to society. The importance of earthquake evaluation in intraplate regions has been recognized recently. In the intraplate region east of the Rocky Mountains, earthquakes affect larger areas than in the western United States. Although intraplate earthquakes are more infrequent and unexpected, the impact of earthquakes on society can be greater than is generally perceived because of the large affected areas and greater population density in the eastern United States. Difficulties in Making Evaluations The evaluation of faults, particularly assessment of their seismic potential, is often difficult because of the following factors: poor conditions of surface exposure (concealment by bodies of water or young sediments); plastic deformation of near-surface materials; transitional or branching rupture character; detachment, décollement, or listric faulting of shallow materials; conflicting or incomplete geologic, seismologic, or geophysical observations; incomplete bases for analysis; and basic assumptions about activity or nonactivity of faults. These factors have led to smaller, shorter, more discontinuous expression of surface faulting parameters in almost one-fourth of the historical examples of surface faulting in North America. Approach for Earthquake Evaluation Two approaches for earthquake-hazard assessment have been used in the United States. The western United States is dominated by active-tectonic processes and many active faults. Those faults with proper orientations or connections to plate boundaries may be active and can be evaluated by methods that are discussed in this paper. Faults within this region are commonly assumed to be active unless there are contrary data. In the central and eastern United States—in intraplate regions east of the Rocky Mountains—most faults are inactive and rarely have the potential for being sources of damaging and hazardous earthquakes. Earthquakes of these zones may not show characteristics typical of active faults, with lower magnitudes (commonly less than 5.75 or 6), and may be assumed to have random or “floating” epicenters within a province. Man’s structures may need to be designed conservatively for the largest historical earthquake of the region or province or may be designed conservatively for a higher magnitude or intensity. Most faults for such regions are commonly assumed, or may appear to be inactive, although the Meers Fault (Oklahoma) and the scarp at Reelfoot Lake (near New Madrid, Missouri) suggest that some faults of central United States may be active, and deterministic assessments should be used. Many different disciplines are used for studying active faults. Some of these are shown in Figure 3.1 in a time perspective. FIGURE 3.1 Time relations of different disciplines associated with active faulting. (From Kasahara, 1981.)

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Active Tectonics: Studies in Geophysics SEISMOGENIC MODEL Earthquakes are generally assumed to be elastic waves radiating out from a rupture in the Earth that slips suddenly and generally in a brittle manner. Most evaluations of the potential for surface faulting and earthquakes assume that earthquakes of above magnitude MS=6 are by brittle failure and are represented by fault-rupture parameters. This simple model is effective for many examples of surface faulting, particularly for faults with moderate to steep dips and with good surface exposure. The release of energy is a function of fault rupture parameters and is also affected by the elastic rebound of the strained volume of rock, associated folding (King and Stein, 1983; Hill, 1984; Molinari, 1984), detachment faulting (Hardyman, 1978; Berberian, 1982), fault type and attitude (low-angle thrust faults), and surface exposure. HAZARDS RELATED TO FAULTS Earthquake and Ground Motion Perhaps the best-known hazards of active faulting are the destructive effects of earthquake shaking, often called “strong ground motion.” Sudden movement along a fault or fault zone radiates elastic waves that are generally strongest near the causative fault and taper off or attenuate away from the fault. Strong ground motion is the single largest natural factor in causing earthquake damage, including loss of life and property, failure of structures, disruption of utilities, and numerous secondary effects such as landsliding and liquefaction. The characteristics and intensity of strong ground motion at different sites usually varies with a number of factors, including earthquake size, attentuation, and local ground response. Variation of strong ground motion with distance from a causative fault has been one of the most discussed factors, based on many analyses of strong-motion records from historical earthquakes and various theoretical considerations. Recent studies by McGarr (1984) show that the intensity of strong ground motion may vary with the type of motion along a fault (e.g., reverse motion versus normal motion). Details of the geometry of fault zones can have a large influence on strong ground motion. Local areas where the fault is not so free to slide concentrate stress (Bakun et al., 1980, 1984; Aki, 1984); when these places are broken and the stress is released a large amount of high-frequency energy is generated resulting in high-frequency ground motion. Source directivity, a phenomenon involving the propagating fault rupture and its relationship with the elastic waves being radiated, can have a pronounced effect on the frequency and amplitudes of the radiating waves and should be addressed for sites near the causative fault (Singh, 1981). Other factors such as the radiated wave’s travel path characteristics (Singh, 1981), topographic effects at a site (Davis and West, 1973), and site materials and geology (Rogers et al., 1983) can also affect strong ground motion. Many siting and design studies use a sophisticated approach to strong ground motion estimation involving the combined expertise of the seismologist, the geologist, and the engineer for deterministic and probabilistic studies. The effect of strong ground motion is incorporated into building codes and may influence zoning (Berg, 1983). Geotechnical studies of soils as well as geologic and seismologic fault studies are vital to this ground motion potential assessment. Surface Rupture One of the most direct hazards (effects) of active faulting is displacement or offset at the foundation of a structure (Swiger, 1978). Ruptures can occur suddenly during earthquakes or slowly or gradually by creep. Three main types of fault movements associated with a faulting event are primary, secondary, and sympathetic movements. A primary rupture occurs along the main fault responsible for the earthquake and can be estimated from observations and regression analyses (Slemmons, 1982b; Bonilla et al., 1984) of historical earthquakes and fault displacement. These are commonly based on rupture length or maximum displacement; actual observed effects can be reduced by drag and distributed rupture, plastic failure, detachment, and other causes. The construction of some brittle structures may not tolerate even small fault rupturing. The proposed Auburn Dam in California was in advanced stages of site preparation when the possibility of fault displacement of about 15 cm in the foundation was estimated by Woodward-Clyde Consultants (1977). The consultants and a special review panel essentially concurred that, although the recurrence interval was very long, the maximum expected ground displacement of about 15 cm could occur with an associated earthquake of magnitude 6.5. The concrete double arch dam could not accommodate this large a foundation displacement, and the proposed dam was abandoned as inappropriate for the site even though $200 million was spent for site preparation. Secondary ruptures are those that occur along branch faults and other faults subordinate to the principal fault trace. These faults locally accommodate deformation

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Active Tectonics: Studies in Geophysics from the main fault and generally have lesser displacements. Bonilla (1970) showed that secondary rupture displacements decrease with increasing distance from the fault. Sympathetic offsets occur when strain release along the main fault or vibratory ground motion disturbs the state of stress of another fault, causing it to undergo displacement. During the 1968 Borrego Mountain earthquake, sympathetic displacement occurred on three faults, the Imperial, Superstition Hills, and San Andreas. Investigations by Allen et al. (1972) reported sympathetic offsets of 1- to 2.5-cm right-lateral displacement and that the longest of these ruptures was 30 km long and 50 km from the epicenter. Surface displacement can occur gradually in areas of tectonic creep and nontectonic fluid withdrawal. In California, several faults are undergoing tectonic creep, with a maximum reported creep of 3.2 cm/yr along the San Andreas Fault in San Benito County (Burford and Harsh, 1980). Tectonic Deformation Tectonic deformation refers to areal or regional deformation that may or may not be associated with moderate or large earthquakes. Dramatic effects of tectonic deformation have been noted by observing the vertical displacement and tilting of shorelines during the 1964 Alaskan earthquake, which shows evidence of land movement relative to sea level. An area of south central Alaska, of probably over 110,000 mi2 of land and sea bottom, was affected by warping, horizontal distortion, and faulting (Plafker, 1972). The upper growth limit of barnacles showed a maximum of 37.8 feet of vertical displacement when the pre-earthquake and post-earthquake shorelines at Montague Island were compared (Plafker, 1972). In the Kodiak Islands, a maximum subsidence of 6.3 feet of the shoreline was recorded (Plafker, 1972). Plafker commented, “Regional uplift and subsidence occurred mainly in two nearly parallel elongate zones, together about 600 miles long and as much as 250 miles wide, that lie along the continental margin.” Earth movements, detected by geodetic measurements, were recorded after the 1971 San Fernando, California, earthquake (ML=6.5). These measurements showed the mountains to the northeast of the causative fault shifted upward as much as 2 m and horizontally as much as 2 m (Savage et al., 1975). Folds and large crustal uplifts and tilts have generally been assumed to be aseismic and to represent gradual plastic failure. Recent activity at Coalinga, California, has shown that this, however, is not always the case. The damaging 1983 Coalinga earthquake (MS=6.6) occurred in an area where active faults and large-magnitude earthquakes were previously unrecognized. Post-earthquake investigations concluded that no surficial faulting accompanied the earthquake (Clark et al., 1984). Studies by King and Stein (1983) showed that uplifted Holocene terraces on the main fold associated with the earthquake could be identified and are consistent with the regional deformation accompanying the earthquake (Stein, 1983). Other earthquakes that were associated with folding and compressional regimes are the 1978 Tabas-e-Golshan, Iran, earthquake (MS= 7.5) and the 1980 El Asnam, Algeria, earthquake (MS= 7.25). The relationships recognized at Coalinga have led to a re-examination of major folds of the California Coast and Transverse Ranges and adjustment of seismic design of nearby major engineering structures. Yeats (1982) suggested that surface faults along anticlines may be weakly seismic or low-shake faults. These faults, also called flexural-slip faults, are thought to be related to the folding structure, with displacement occurring along bedding planes of the units being folded (see Yeats, Chapter 4, this volume). Hill (1984) suggested that some of the Coalinga earthquakes may be related to flexural-slip events. Late Quaternary fault scarps are associated with the Toppenish anticline of the Yakima fold belt in Washington (Campbell and Bentley, 1981), but whether these are seismogenic has not been resolved. Criteria for discriminating between seismogenic and aseismic geologic structures have yet to be developed for evaluating folds or areas actively undergoing tectonic deformation. Resolution of these issues is important for seismic evaluation of many engineering structures in areas with Late Cenozoic folding. Tectonic tilting or warping must also be considered for level or gradient-sensitive structures, such as aqueducts in Owens Valley and also on the western edge of the San Joaquin Valley. Secondary Effects Secondary effects associated with earthquakes include landslides and rockfalls, liquefaction, seiches, and tsunamis. These effects can cause severe and widespread damage; although the effects may be severest in the epicentral region, they may extend out to distances of as much as 1000 km. Landslides are often induced by earthquake shaking. The scale of these features can vary from slides a few meters long to slides kilometers in length. During the 1976 Guatemalan earthquake (MS= 7.5) over 10,000 landslides were generated in an area of 16,000 km2 (Harp et al., 1981). One of the most notable slides occurred in Peru in 1970, where a large, seismi-

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Active Tectonics: Studies in Geophysics cally induced landslide originating 130 km from the earthquake claimed at least 18,000 and possibly as many as 25,000 lives (Gere and Shah, 1984). Splashes or waves caused by landslides may also have extensive effects; the 1958 earthquake induced a landslide into Lituya Bay, which caused a giant wave reaching 1720 ft above sea level (Miller, 1960). Liquefaction is a phenomenon in which near-surface water-saturated sediments are shaken, lowering their rigid strength and behaving as a semiliquid material. Structures such as buildings and pipelines built on ground that liquifies can tilt or sink or may be moved as the ground flows. This phenomenon has been noted in many earthquakes including classic effects of large apartment houses tilting in Niigata, Japan, during the June 1964 earthquake and spectacular effects during the March 1964 earthquake at Turnagain Heights and Valdez Harbor, Alaska. Liquefaction effects can be predicted in a gross way using simple linear diagrams (Youd, 1978) or more precisely with sophisticated computer models for specific sites (Youd et al., 1978). A seiche is a wave set up in a body of water in response to earthquake waves and also can occur great distances from the earthquake source. The danger of seiche is temporary flooding of areas near lakes and reservoirs and overtopping of dams. During the 1954 Dixie Valley-Fairview Park, Nevada, earthquakes, a seiche was set up in a covered water reservoir in Sacramento, California, 300 km away, damaging support pillars, concrete walls, and gunite panels (Steinbugge and Moran, 1957). Large ocean waves created by uplift or downdropping of the seafloor during an earthquake are called tsunamis. Tsunamis can move hundreds of kilometers per hour and destroy facilities and structures along the coast, thousands of kilometers from the earthquake. Warnings are issued when there is a large earthquake in oceanic areas, which allows coastal residents to go to safety on higher ground until the tsunami danger has passed. Detailed observations and investigations of secondary effects have led to the understanding, prediction, and mitigation of these effects in many areas. Today, secondary effects can often be identified and risks assessed thanks to geologic, geophysical, and engineering laboratory and field studies. TECTONIC SETTING Plate-Tectonic Relationships Plate-tectonic concepts are accepted by most earth scientists as a working model of the crustal behavior of the Earth. This model suggests that the Earth’s surface is composed of several large plates and numerous smaller plates that are slowly moving and rotating with respect to each other. Since this behavior is dynamic, faulting, earthquake activity, and rates of fault slip and folding are closely related to the rate of movement between plates. Most seismic activity occurs along plate boundaries, areas known as interplate areas. Intraplate areas are areas within plates and have less seismic activity and lower rates of tectonic activity than interplate areas. Closer inspection of interplate regions reveals smaller “microplate” tectonic domains characterized by a particular faulting style, such as the Basin and Range province in the western United States. Interplate Regions These regions have many active faults with a potential for future displacements and associated earthquake activity. The methods described in this chapter are especially appropriate for many faults in these regions. There is a great range in rates of fault activity with recurrence intervals that range from decades to hundreds of thousand years. Additionally, the plate and microplate boundaries vary from sharp and narrow to broad (“soft”) zones that may extend hundreds to thousands of kilometers from the main boundary, including most of the western United States, although there are “islands” of inactive subplates within this region and variations in rate of activity within active provinces. Wallace and Whitney (1984) described an example of variable rates of tectonic activity within the Great Basin province. Complete evaluations of a fault are needed to assess the following characteristics: seismogenic character, segmentation, recurrence interval and slip rate, recency of fault activity, and the relation to the site or area being evaluated. The maximum magnitude of earthquakes varies with seismotectonic province or fault and includes a range in values for each type of plate boundary. The maximum historical values include Mw (moment magnitude) values of over 9 for some subduction zones, MS (surface wave) magnitudes of up to about 8.3 for strike-slip faults, and MS magnitudes of up to about 7.5 or 8 for extensional zones. Intraplate Regions Seismic hazard evaluation of intraplate regions has evolved rapidly in the last few years, although processes will become more refined and better understood. Rates of activity are generally orders of magnitude lower within intraplate areas as compared with interplate areas. The low rates of faulting and warping and sub-

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Active Tectonics: Studies in Geophysics dued geomorphic expression of deformation may be due to broad basinal or domal uplift. These lower tectonic rates are generally accompanied by less earthquake activity than for interplate regions. In general this has caused more focus and awareness of earthquake hazard in interplate regions and less concern in the intraplate regions. An excellent summary of intraplate seismicity (and other intraplate tectonism) is given by Sykes (1978), who noted that intraplate seismicity appears to be localized along pre-existing zones of crustal weakness. Seismicity in the eastern United States within the North American plate exemplifies this concept. Seismic activity in the New Madrid area, where the large earthquakes of 1811 and 1812 occurred, appears to be localized along a pre-existing rift structure in the continent (Sykes, 1978). Although at the time the rift was formed the continent was under extension in that region, recent activity appears to be related to compression. This change in stress regime shows how a pre-existing structure (e.g., rift) can be reactivated in later, different strain episodes. The crustal weakness appears to be the locus of earthquake-related strain release. The 1886 Charleston earthquake was located in a Paleozoic orogenic belt, an inactive or relic interplate region on the eastern edge of the continent. Low seismicity and cover of young sediments has not allowed the tectonics and source zones of the Charleston earthquake to be understood as well as in the New Madrid area. The Meers Fault of Oklahoma is another example of reactivation of an ancient geologic structure. The Meers Fault, a segment of the Frontal Fault zone, was formed about 500 m.y. ago as part of the southern Oklahoma rift. During a later orogeny it underwent extensive compressional (reverse) as well as left-lateral displacement. Studies of the Meers Fault show that the fault is currently active and has a dominant lateral component (Ramelli and Slemmons, 1985; Slemmons et al., 1985). The potential size of intraplate earthquakes, although they are infrequent, is great. The New Madrid, Missouri, earthquakes of 1811 and 1812 are estimated to have MS magnitudes of over 8; the Charleston, South Carolina, earthquake of 1886 was about MS=7. The recently discovered Meers Fault has characteristic features that suggest MS=6.5 to 7.5 earthquakes in the late Quaternary (Slemmons et al., 1985). IDENTIFICATION AND DELINEATION OF ACTIVE FAULTS Geologic Methods Many active faults are poorly delineated on most pre-1950 geologic, structural, or tectonic maps. These commonly emphasize ancient and inactive tectonic features rather than neotectonic structures. Recognition and detailed mapping of historical and Quaternary faults in many zones of neotectonic activity, particularly at or near plate boundaries, have led to recent improvements in the delineation of active faults. In addition, the possible reactivation of intraplate faults such as the Meers Fault has emphasized the need to re-examine other faults and folds in central and eastern United States. Remote-Sensing Methods Remote-sensing methods can be effective in detecting, delineating, and describing the character of active faults and neotectonic features. The most effective methods accentuate fault scarps by employing imaging techniques using special illumination angles, wavelengths, or stereographic effects. Some of the methods for earthquake-hazard analysis are summarized by Glass and Slemmons (1978), but newer equipment and methods and rapid developments in analytical techniques require continued adaptation of many of these concepts. Examples of instrumental and structural analysis are in Williams (1983) in the section on geological applications, and for applications in nuclear power-plant site investigations in McEldowney and Pascucci (1979). Low Sun angle and radar imagery methods are especially effective in detecting and delineating active faults and folds. Special low-Sun angle photography of faults can have the advantages of relatively low cost and appropriate scale and optimum shadowing or highlighting of scarps by selection of solar azimuth and altitude. Since this is the most effective single method of assessment of active faults, it is one of the most widely used methods for aerial photography and reconnaissance (Glass and Slemmons, 1978). Radar imagery of some areas is available at much higher cost and on smaller scales, may have the advantages of some ground penetration in arid regions, and can be taken at any azimuth, time of day or night, and in cloud or fog cover. Recent studies using ground penetrating radar for fault trace identification have been very successful. Black et al. (1983) studied an area along the San Andreas Fault zone that has been extensively trenched and found good correlation of the ground penetrating radar records and the trench logs. Bilham and Seeber (1985) used subsurface radar profiling to detect colluvial wedges associated with former movements along the Lost River Fault and wide zones of faulting along the San Andreas Fault system. As this method is refined it will become an even more powerful method of fault detection and delineation.

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Active Tectonics: Studies in Geophysics Geophysical Methods Observations of seismicity can sometimes help to delineate active faults. Persistent alignments of seismicity, especially at the ends of identified faults, can occasionally be considered seismic sources or seismogenic extensions of a fault. Many active faults have associated seismicity, including the Calaveras, Hayward, and central San Andreas Fault zones of central California, which, however, may only indicate the creeping segments of these faults. Other sections of the San Andreas Fault system that are not currently creeping are not clearly delineated by small earthquakes. Areas of relatively high seismicity may warrant examination for active faults. In the eastern United States, alignments of high seismicity such as near New Madrid are associated active subsurface faults. Other major basement faults are associated with seismicity and may be active (Gordon, 1985); these also could be examined. Seismic reflection techniques can help to delineate subsurface faults in sedimentary basins, both on land and beneath lakes and oceans. These techniques are used for recent fault detection and delineation studies, particularly in offshore California (Greene et al., 1973) and along the central California coastal margin near the Hosgri Fault (Crouch et al., 1984) near Point Conception (Pipkin and Ploessel, 1985), and in the offshore zone of deformation between the Inglewood Fault and Rose Canyon Fault (San Diego). Seismic reflection profiling by the Consortium for Continental Reflection Profiling (COCORP) has revealed the down-dip nature of many faults and a major detachment surface under the Sevier Desert of Utah (Allmendinger et al., 1983). Gravity methods are most effective for studying fault zones where a strong density contrast exists between materials on either side of the fault. This situation occurs along faults where basement rocks are displaced against sediments or fault offsets in basins where the thickness of sediment differs across the fault. These methods are especially effective for regions of extensional faulting. Zoback (1983) used gravity techniques to delineate the geometry of range bounding normal faults in the Basin and Range province along the Wasatch Fault zone in Utah. Application of surface magnetic and aeromagnetic survey methods for evaluation of active faults is discussed by Cluff et al. (1972), Krinitzsky (1974), and Sherard et al. (1974). These methods can be used to detect and delineate faults concealed by recent sediments and provide a relatively inexpensive method of contouring the thickness of basin fill. Smith (1967) located intra-basin, largely concealed, major fault grabens within the Dixie Valley graben using aeromagnetic methods. Some of these graben boundary faults were also accurately delineated by the faulting of the 1954 Dixie Valley earthquake. Smith provided a detailed outline for applying magnetic methods to the Basin and Range province with normal- and oblique-slip faults. Bailey (1974) used a magnetometer to determine the surface fault location of the Chabot Fault, California, where anomalous drops in magnetism suggested locations of fracturing and subsequent leaching related to faulting. The eastern limit of the Chabot Fault zone was identified so that buildings could be sited to avoid potential surface rupture. Similar applications may be useful in defining active faults that are concealed by young alluvium or bodies of water. Exploratory Methods Exploratory methods for fault assessment advocated by Louderback (1950) were little used until the late 1960s when they assumed an important role in fault evaluations to assess such features as for activity, age dating, paleorupture and liquefaction events, slip direction, recurrence intervals, and slip rates. An adequate exploratory trenching and borehole program is critical in evaluation of active faults and is a major part of both domestic and foreign assessments. Specific applications to fault assessment are included in Taylor and Cluff (1973). The use of trenching as an exploratory method for nuclear power plant siting is discussed in Hatheway and Leighton (1979). DETERMINATION OF FAULT ACTIVITY Definition of Activity Before 1950, most geologists did not distinguish between inactive faults and those with a potential for renewed displacements and associated earthquake activity, yet this is a critical part of man’s planning and design. Slemmons (1982a) listed over 30 definitions for “active” or “capable” faults of which only three were made before 1950. No definition for active faults is universally accepted, although two elements are present in most definitions: (1) the potential or probability of future displacements in the present tectonic setting and (2) the time of most recent displacement (e.g., historical, Holocene, Quaternary, or “in the present seismotectonic regime”). The first element, potential, is critical to all assessments for larger earthquakes; the second, recency, relates indirectly to rate of activity, which provides a more quantitative measure of degree of fault activity. Fault activity can also be classified by fault slip rate. Figure 3.2 shows the general relationships between

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Active Tectonics: Studies in Geophysics three factors: (1) the time since the last event or recurrence interval, (2) the surface wave magnitude, and (3) fault slip rate. This figure also shows the general geomorphic expression for each interval of fault slip rates, although such factors as climate, time since the last event, variations in fault slip rate, or noncharacteristic activity can affect the landforms. Cluff and Cluff (1984) noted that the common current use of “active” and “inactive” can be a scientific oversimplification that may lead to improper siting or design of engineered structures. Use of quantitative measures for degree of activity, such as fault slip rate and recurrence interval with probabilities and variance, can lead to deterministic values providing meaningful numbers for analysis by probabilistic methods. Geologic Indicators One of the most convincing arguments or evidence of fault activity is the cross-cutting or non-cross-cutting relationship with a datable unit. If Holocene activity is the criterion for activity, then a Holocene age unit crossing the fault could be an ideal location for a trench site. If the unit is offset, then the age of the unit and the amount of offset can be used to estimate a slip rate and a recurrence interval if the nature of characteristic earthquake is known. A wide variety of types of Holocene deposits have been used for evaluation of fault activity, most commonly alluvial and volcanic deposits. Deposits are dated by carbon-14 radiometric methods, tephrochronology, soil development, fossil stratigraphy, and many other techniques. Pierce (Chapter 13, this volume) presents a good summary and review of Quaternary dating methods. Exposures of faulted units may be found in stream cuts and landslide scars or in road cuts or other man-made excavations. To prove whether a fault or strand of a fault system is active, a trench may be dug at the proposed site and the geologic units and soils inspected for faults. If no demonstrated fault activity has taken place in these geologic units within the defined “active” fault period, the proposed structure can be considered reasonably safe from damage from surface faulting. The structural aspects of young geologic units adjacent to faults may also provide information about activity of a fault. Adjacent units may be brecciated and shattered, have open fissures, be tilted or warped, or have secondary effects of faulting and liquefaction effects (e.g., sand boils and sand dikes). In a detailed study of a fault, the youthful geologic units should be described, delineated, and inspected for evidence of young faulting. Geomorphic Indicators The freshness of appearance and type of geomorphic expression of faults is related to the age of faulting (Matsuda, 1975; Slemmons, 1977, 1982a; Wallace, 1977, 1978). Geomorphic investigations into faulting are relatively easy and can yield considerable information. Many landforms such as depressions and sag ponds, open rifts, and prominent high-angle scarps suggest youthfulness and further help to identify the active traces or strands of faults zones (Figure 3.3). A geomorphic investigation begins with examination of aerial photographs or an aerial reconnaissance. Overview of the geomorphology allows delineation of key lo- FIGURE 3.2 Relation between time or recurrence interval between earthquakes, earthquake magnitude, and slip rate across the fault zone. This chart assumes that most of the energy is released by seismogenic rather than aseismic activity and that the average displacement is one half the maximum. [Modified from Matsuda (1975) and Slemmons (1977).]

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Active Tectonics: Studies in Geophysics FIGURE 3.3 Geomorphic features related to active faulting. cations for ground investigations. Low-Sun-angle techniques for reconnaissance or photography have been extremely useful in detecting subtle geomorphic features that would have otherwise been missed (Glass and Slemmons, 1978), such as in cities where geomorphic expression of scarps may have been smoothed out or altered, but general elevation differences still exist. Key areas found through geomorphic investigation are often used as sites for further geologic investigations as exploratory trenching. Geomorphic investigations form a large part of the data base used in paleoseismic investigations. Freshness and continuity of geomorphic expression in space strongly suggest a surface rupture created during one event or over multiple events closely spaced in time. Recently there have been numerous efforts to quantify the degree of degradation of fault scarps relative to age (see Chapters 7, 8, and 12, this volume). Geodetic Indicators Recognition of activity along some faults is possible by repeated geodetic surveys. Geodetic methods are capable of detecting and measuring tectonic strain of regions or across active faults. Reduction of the geodetic data permits determination of rate and direction of ground movement. The data provide another measure of fault displacement, both seismic or aseismic, and can assist in locating active branch faults or focus on areas of current movement within complex zones of faulting. Sylvester (Chapter 11, this volume) presents several methods of near-field geodetics including level lines, alignment surveys, trilateration, triangulation, and creepmeters. Regional geodetic leveling and trilateration surveys are made to monitor regional strain accumulation and release (Prescott et al., 1979; Vanicek et al., 1980). Advances using satellite geodesy, e.g., the Navstar Global Positioning System (GPS), offer surveying techniques with a precision superior to classic surveying at one-twentieth the cost (Kerr, 1985). Seismologic Indicators Detailed studies of earthquake epicentral and hypocentral distributions of many fault zones can indicate the activity, continuity, location, dip and strike, seismogenic depth, and possible stress regime of the fault zone. However, this is often a difficult task. The quality of seismic data must be scrutinized and understood. The best-quality data come from dense seismometer networks that are limited to a few areas and are often temporary. Quarry blasts and possible geothermal, volcanic, and reservoir-induced seismicity must be separated from fault-related seismicity and can be analyzed as an additional seismic hazard. The remainder may be a well-defined zone of activity or a diffuse pattern of distributed activity. Well-defined zones of activity are common in aftershock areas and along creeping sections of faults. Diffuse patterns are harder to interpret but at least indicate that some strain is taking place in the area. A diffuse pattern of historical

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Active Tectonics: Studies in Geophysics seismicity suggests a maximum historic earthquake (for the area including the fault) and warrants further investigation of faulting in the area. Earthquake activity along a fault zone clearly indicates that the fault zone is active at least at seismogenic depths. However, the type of activity needs to be evaluated to assess the hazard and risk. A fault that slips aseismically represents a different type of risk than a fault that slips with large rupture events. Adjoining sections of the same fault may behave differently and, for example, the San Andreas Fault between Parkfield and San Juan Bautista appears to be characterized by creep and frequent lower magnitude (less than 6) earthquakes, whereas the adjoining section to the south has abrupt brittle failures, about 150-yr recurrence intervals, and up to magnitude 8.3 earthquakes. Rates of Activity Rates of activity may be learned from the study of geomorphic features (Matsuda, 1975); faults with high slip rate have abundant and well-developed landforms and steep scarps (Wallace, 1977; Hanks et al., 1984; Chapters 7, 12, and 14, this volume). The interrelationships of time between characteristic earthquakes (the recurrence interval), fault slip rate, and earthquake magnitude are shown in Figure 3.2. Slip rates presented in this figure are based on average displacement, which is more likely to be measured during field studies, rather than maximum displacement. We have assumed that the average displacement is one-half the maximum displacement. EARTHQUAKE SIZE AND ACTIVE FAULT PARAMETERS Earthquake Magnitudes and Moment Magnitude Earthquake magnitude scales are one of the most important earthquake size source parameters used today in seismology and active-tectonic studies. Magnitude scales have different forms such as local magnitude (ML), surface-wave magnitude (MS), body-wave magnitude (Mb), and coda-duration magnitude (Mc) (Kanamori, 1977; Bath, 1981; Chung and Bernreuter, 1981). These different forms were originally created to accommodate different kinds of seismic networks (e.g., near field versus far field) in the magnitude determination of earthquakes. One of the principal differences between the magnitude scales is the period of the wave measured. This difference of measured frequency arises from the variety of instrument responses of seismometers used and the changing frequency spectrum of the waves reaching seismometers at various distances from the earthquake sources. The local magnitude scale was introduced by C.F.Richter for earthquakes in southern California with epicentral distances of 600 km or less and focal depths of 15 km or less. Both ML and Mb are determined from the amplitude of waves with a period of about 1 sec, and the values of these magnitudes are thought to saturate at about magnitude 7.25 (Chung and Bernreuter, 1981). Thus, as the rupture gets larger in area from that of a magnitude 7.25 earthquake, the values of ML and Mb increase very little and do not represent the entire energy released from the earthquake. The surface wave magnitude is suitable for global distances and is measured at periods near 20 sec. The MS scale saturates at about magnitude 8.6 (Chung and Bernreuter, 1981). Another useful seismic estimate of earthquake size, the seismic moment (M0), is defined as the product of the rupture area (A), the average displacement of the rupture (D), and the shear modulus (µ) along the rupture (Brune, 1968), Hanks and Kanamori (1979) developed a moment magnitude scale (Mw) based on seismic moment, where The moment magnitude is an important scale because it relates directly with the physical properties of the rupture and does not, by definition, saturate. Earthquake size is one of the most important factors in seismic-hazard analysis and can be estimated using specific fault parameters. When using magnitude data, it is important to understand the characteristics of the scale being used, such as saturation, and to understand the quality or range in errors and uncertainties of the data used for the magnitude estimate. Fault Rupture Parameters and Earthquake Size Tocher (1958) recognized that there was a good relationship in large historical earthquakes between size or magnitude and the logarithmic parameters of total surface rupture length, maximum displacement, or length times maximum displacement. Subsequent reports by Iida (1959, 1965), Bonilla (1967, 1970), and Bonilla and Buchanan (1970) refined the original linear regressions. Bonilla with his colleagues (Bonilla and Buchanan, 1970; Mark, 1977; Mark and Bonilla, 1977) and Slemmons (1977) added new data points and rejected poor data, improved magnitude values and statistical practices, and scrutinized the quality of field measurements of faulting parameters. More compatible linear regres-

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Active Tectonics: Studies in Geophysics sions with lower standard deviations and better fit were obtained by Slemmons (1982b) and using more sophisticated magnitude values and statistical methods by Bonilla et al. (1984). Where there is good field expression of the length and/or maximum displacement from geomorphic expression of fault scarps, or where these parameters can be measured from soil-stratigraphic relations, it is possible to infer the approximate magnitude of paleoseismic events. These correlations between fault parameters and earthquake magnitude have been made for siting and engineering design of vital structures in many parts of the world for diverse tectonic settings, including extensional areas such as the Basin and Range province (Wallace, 1977, 1978; Cluff et al., 1980; Schwartz and Coppersmith, 1984) and regions of strike-slip faulting (Sieh, 1984; Sieh and Jahns, 1984) and for thrust faulting in regions with compressional tectonics (Woodward-Clyde Consultants, 1984). An application to intraplate locations is shown by the left-oblique faulting of the Meers Fault zone in Oklahoma. There a 26- to 38-km-long fault scarp can be mapped. Using correlations of length to MS magnitude, the scarp can be inferred to have formed during prehistoric earthquakes of between 6.5 to 7.5 magnitude (Slemmons et al., 1985). This paleoseismic evidence is especially important since no historical earthquakes have been recorded in the area. Elsewhere in this zone a general alignment of epicenters of small earthquakes has been noted (Gordon, 1985). In summary, active fault parameters such as length and displacement can be used to estimate earthquake magnitudes through the regression formula presented by Slemmons (1982b) or Bonilla et al. (1984), and the parameters can be used directly in a moment magnitude calculation. An example of these correlations is shown in Figure 3.4. Segmentation The segmentation of fault systems involves the identification of individual fault segments that appear to have continuity, character, and orientation; these suggest that a segment will rupture as a unit (Slemmons, 1982b). Individual fault segments have different characteristics relative to adjacent segments or are separated from adjacent segments by identifiable discontinuities. Figure 3.5 illustrates the concept that fault zones rupture in segments with an example from Turkey. During the period 1939 to 1967, the North Anatolian Fault system ruptured as segments and not as a single through-going event. The delineation of segments involves the identification of discontinuities in the fault system. Discontinui FIGURE 3.4 Relationship between earthquake magnitude (MS) and maximum displacement for strike-slip faults. (From Slemmons, 1982b.) ties can be divided into two categories, geometric and inhomogeneous; these categories are borrowed from seismologists who have used these terms for asperities and barriers (Aki, 1984). Examples of geometric discontinuities include fault intersections, such as branch faults or cross-fault terminations; fault-zone features, such as en echelon segments, separations, and changes in attitude; and fault terminations. Inhomogeneous discontinuities include variations in fault width, local stress regimes, and rates of displacement. Segall and Pollard (1980), acknowledging that fault traces consist of numerous discrete segments, have developed a two-dimensional mathematical solution for nonintersecting cracks. Their solution describes the mechanical behavior of left-stepping versus right-stepping en echelon cracks in a right lateral stress regime. Segall and Pollard state, “for right lateral shear and left-stepping cracks, normal tractions on the overlapping crack ends increase and inhibit frictional sliding, whereas for right-stepping cracks, normal tractions decrease and facilitate sliding.” Bakun et al. (1980) studied the seismicity and behavior of the San Andreas Fault zone in central California where strain release is characterized by creep and moderate earthquakes. In their analysis, they modeled fault segments in an en echelon fashion (Figure 3.6). Behavior of these segments was as would be pre-

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Active Tectonics: Studies in Geophysics FIGURE 3.5 Sequence of faulting along the North Anatolian Fault zone, Turkey, for the period 1939–1983. (Modified from Ambraseys, 1978.) dicted by Segall and Pollard’s model, strengthening the idea that fault systems behave in a segmented manner. If the discontinuities are large enough, segmentation of a fault system may be intuitively easy (e.g., a 10-km en echelon step appears to be a substantial discontinuity with a good chance of stopping a rupture). In less clear cases of discontinuities, several lines of geologic, seismologic, and geometric evidence must be gathered to suggest or substantiate the existence of the discontinuity. An understanding and delineation of the entire earthquake history of a fault can be used as relatively strong evidence of segmentation. Once convinced that a particular segmentation is reasonable, various approaches including correlation with faulting parameters can be used to estimate earthquake magnitudes. Schwartz and Coppersmith (1984; Chapter 14, this volume) suggested examples of fault zones for which the segmentation model may be applicable. EARTHQUAKE RECURRENCE ESTIMATION Recurrence Models Earthquake recurrence intervals can vary markedly from fault to fault. Historical seismicity of the Parkfield segment of the San Andreas Fault system suggests a recurrence rate of 21±4 yr (Bakun and McEvilly, 1984), whereas soils and trenching data suggest to Machette (1978) that the County Dump Fault in New Mexico has a recurrence interval of 90,000 to 190,000 yr. As pointed out by Wallace (1970) and Schwartz and Coppersmith (1984; Chapter 14, this volume), the slip rate of a fault directly affects recurrence rates. Various models have been used to explain earthquake recurrence, including the time-predictable (Shimazaki and Nakata, 1980; Bufe et al., 1977), slip-predictable (Shimazaki and Nakata, 1980), and the periodical model (Bakun and McEvilly, 1984). Recurrence Data Wallace (1970) discussed recurrence for the San Andreas Fault. Subsequent refinements in dating, exploratory trenching, geomorphic expression, and low-Sun-angle aerial reconnaissance or photography have greatly expanded knowledge of faulting recurrence, paleoseismicity, and scarp morphologic change. These new methods are critical to active fault evaluations and timing and probability analyses. Studies determining paleoseismic history have been conducted recently at Pallett Creek (Sieh, 1984) and Wallace Creek (Sieh and Jahns, 1984) and at Cajon Pass (Weldon and Sieh, 1985) along the San Andreas Fault zone. Cross-cutting relationships and radiocarbon dates

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Active Tectonics: Studies in Geophysics limit the ages of the prehistoric ruptures, and can be used to determine a local recurrence interval and slip rate for the fault. Although the 1886 Charleston earthquakes apparently did not rupture the ground surface, preventing a direct analysis of recurrence rate, studies of liquefaction-related sand blows in the Charleston area by Obermeier et al. (1985) suggest at least two prehistoric events occurred that may be used to establish the recurrence interval of large earthquakes for the area. Thatcher (1984) noted examples in which geodetic measurements may also lead to recurrence estimates. Seismic Gaps Mogi (1979) pointed out that the term “seismic gap” has been used to describe two different phenomena. Mogi termed a seismic gap of the first kind as a gap in the spatial distribution of rupture zones of the largest earthquakes in a seismic belt. A second kind of seismic gap is a gap in the seismicity of smaller-magnitude earthquakes before larger earthquakes. FIGURE 3.6 (Top) Schematic fault model structure. Interior of the smooth, continuous, nearly planar segments are creeping patches (C). Stuck patches are classified as pinned (P), unpinned (U), or bent (B) if they occur at a left-stepping offset, a right-stepping offset, or a change in strike, respectively. (Bottom) Specific model for the Bear Valley-Limekiln Road section of the San Andreas Fault in central California. (From Bakun et al., 1980.) Wallace (1981) described seismic gaps as active-tectonic zones between recently active fault segments with high potential for reactivation in the near future. Wallace and Whitney (1984) examined the paleoseismic history of three segments of the Central Nevada Seismic Belt—the Dixie Valley Fault segment, the Stillwater Fault segment, and the Pleasant Valley Fault zone segment. They found that scarps 104 to 105 yr of age and approximately Holocene age (less than 104 yr) are present in places along all three segments. However, within historical time, only the Pleasant Valley faults in 1915 and the Dixie Valley faults in 1954 ruptured. Within the intervening Stillwater seismic gap there are no free faces preserved on Holocene scarps, indicating that they are probably older than 300 yr. Wallace and Whitney (1984) commented that “the Stillwater gap is a likely site for future major faulting, but the low level of seismicity in the gap area suggests that the next major earthquake is not imminent.” Other fault systems where seismic gaps of the first kind have been identified are plate boundary systems. McCann et al. (1979) conducted a comprehensive study of large earthquakes and seismic gaps along major plate boundaries. Figure 3.7 shows large earthquakes and seismic gaps along the major plate boundaries near Alaska. The second type of seismic gap applies mainly to the earthquake prediction and management. CASE STUDIES Character of the Subduction in Northwestern United States—Seismic or Aseismic? Several lines of evidence suggest the Juan de Fuca and Gorda plates are being subducted underneath the North American plate in the northwestern United States. These include (1) the Cascade Range, an active andesitic chain of volcanoes; (2) seismicity related to the Benioff-Wadati zone (Smith and Knapp, 1980; Cockerham, 1984; Tabor and Smith, 1985); (3) geodetic deformation consistent with subduction (Ando and Balazs, 1979; Savage et al., 1981); and (4) deformation of marine terraces consistent with subduction (Adams, 1984). Although active subduction seems clear, this system has been relatively aseismic with respect to great subduction-zone earthquakes. Subduction is either proceeding with relatively little coupling with the overriding plate and thus large elastic strains are not being stored, or the subducting Juan de Fuca/Gorda plate is strongly coupled with the overriding plate, producing conditions in which a large earthquake could occur in the future and the historical period is a period between

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Active Tectonics: Studies in Geophysics FIGURE 3.7 Recent large earthquakes along the Alaska-Aleutian seismic zone. Areas along this zone that have not ruptured are considered seismic gaps and may be the locations of future large earthquakes. (From McCann et al., 1979.) earthquakes (Ando and Balazs, 1979; Weaver and Smith, 1983; Adams, 1984). Recent studies have been undertaken to assess the seismic potential associated with subduction in this region (Heaton and Kanamori, 1984; Adams, 1984). Heaton and Kanamori examined the seismic coupling process and compared the Juan de Fuca subduction zone with subduction of other young oceanic plates. In particular, they compared convergence rates, ages of lithosphere, presence of active back-arc basins, depth of oceanic trench, dip of the Benioff-Wadati zone, topography of the subducted slab, and seismic quiescence. Present-day convergence across the Juan de Fuca subduction zone has been estimated as 3 to 4 cm/yr, a moderate rate for subduction zones. The age of the subducted slab has been estimated at 10 to 15 m.y., a relatively young lithospheric plate (Heaton and Kanamori, 1984). They associated the young buoyant crust with strong coupling with the overriding plate; and when rates of plate movement and the age of ocean floor are used (Ruff and Kanamori, 1980), they estimated a maximum moment magnitude of Mw=8.3±0.5. They also proposed that other parameters such as the dip of the Benioff-Wadati zone (10 to 15° beneath Puget Sound), topography of the subducted slab, absence of a back-arc basin, and depth of trench suggest strong coupling of the plates. In their worst-case model of strong coupling, the Juan de Fuca subduction zone could rupture in one event (approximately 600 km by 200 km; convergence rate=4 cm/yr), with an estimated maximum moment magnitude of Mw=9.0. Heaton and Kanamori (1984) commented, “this 500-km gap in seismic activity is one of the most remarkable to be found anywhere in the Circum-Pacific belt,” and “if slip is occurring aseismically on the shallow part of the subduction zone, then this particular example would have to be considered unique.” They concluded, “that there is sufficient evidence to warrant further study of the possibility of a great subduction zone earthquake in the Pacific Northwest.” Adams (1984) suggested types of geologic investigations of paleoseismic activity that could help resolve the seismic hazard. A possible example is Sims’s (1975) study of disturbances of glacio-lacustrine deposits in the western Puget Sound area, which considered 14 disturbed zones in the 40,000-yr-old sediments to be caused by earthquakes. Such evidence needs to be verified as truly seismogenic, synchronous with earthquakes, and distributed over a larger part of the Pacific Northwest. Definitive studies have yet to resolve the issue of whether this Benioff-Wadati zone is seismogenic or aseismic. Resolution is important to future building, siting, design, and zoning within Washington and Oregon. Many existing engineered structures may have inadequate design for earthquakes of magnitude 7.5, 8.0, or 8.5. Implications of the Meers Fault, Oklahoma The Meers Fault (Figure 3.8) is a relatively short fault of about 75-km length in the Frontal Fault zone between the Amarillo-Wichita Uplift and the Anadarko Basin. It was previously recognized to be a major ancient fault with about 3 km of total vertical component of offset in a zone of faults that has a total, mainly Paleozoic, offset of over 10 km. Gilbert (1983) recognized that

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Active Tectonics: Studies in Geophysics FIGURE 3.8 Aerial view of the late Holocene Meers Fault in Oklahoma. The fault scarp is about 5 m high and shows left-lateral offset of drainage lines and ridges. this fault showed well-defined steep scarps suggesting late Quaternary to prehistorical activity with earthquakes of magnitude 6 to 7. This statement was unexpected, as this area has no historical seismicity and is almost 600 km east of the nearest recognized Quaternary faults of the Rio Grande-Rocky Mountain Belt. Previous seismic assessments of earthquake risk assumed that maximum future earthquakes would be of magnitude 5.5 or less, the maximum historical earthquake within the region. As noted by Kerr (1985), this area would have been an ideal one for vital engineering structures, because the fault appeared to be inactive and not a seismic threat. Studies by Ramelli and Slemmons (1985) and Tilford and Westen (1985) confirmed that there is a young fault scarp of about 26- to 28-km length along part of the Meers Fault zone. Ramelli and Slemmons subsequently identified another scarp that extends the main scarp for a total length of about 38 km. The compound scarp of 5-m height developed from repeated surface fault offsets. Donovan (1985), Kerr (1985), Slemmons et al. (1985), and other workers also showed that the fault also had a left-lateral component. Past offsets were at least 1 or 2 m per event. These data suggest that past offsets were accompanied by earthquakes of surface wave magnitude of 6.5 to 7.5. The Holocene activity is confirmed by radiometric dating (Madole and Meyer, 1985) and youthful scarp morphology (Ramelli and Slemmons, 1985). The lack of significant historical seismicity is indicated by Gordon (1985), Gordon and Dewey (1985), and Lawson (1985), although Gordon recognized a general epicentral alignment with the Wichita-Ouachita zone, with left-lateral focal mechanisms in this trend. This unexpected young fault leads to the speculation that there are possible connections between the Frontal Fault zone and the Mississippi trend of epicenters in the New Madrid, Missouri, area (with short zones of surface faulting in 1811–1812). Further connections were suggested by Hinze et al. (1980, 1985) between the New Madrid area and the east-west trending zone of faults in the complex group of faults in the Cottage Grove-Moorman Syncline-Rough Creek-Kentucky River Fault zone. Pleistocene fault activity along the Kentucky River Fault zone has been suggested by VanArsdale and Sergeant (1985). They examined trenches in the Plio-Pleistocene terrace deposits along the Kentucky River Fault zone and found the deposits folded, faulted, and injected with clay dikes. These connections suggest a possible extensive Quaternary breakup of the midcontinental region along reactivated faults; a potential for high-magnitude earthquakes may be present. In addition it indicates the need for more studies of tectonic activity in Central and Eastern United States and the reassessment of the current rationale for evaluating earthquake activity in this intraplate region. ACKNOWLEDGMENTS Much of the research that contributed to this paper is related to special studies for the U.S. Nuclear Regulatory Commission. We gratefully acknowledge their support and encouragement. We are especially grateful to Jim Lienkaemper of the U.S. Geological Survey in Menlo Park; his careful and perceptive review led to many discussions and beneficial changes in the paper. Barbara Matz of the Mackay School of Mines provided

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Active Tectonics: Studies in Geophysics both word processing and editorial assistance. We also benefitted from technical and editorial comments from Steve Brocoum, Tom Usselman, David R.Slemmons, Sunny Meriweather, and Hank Ohlin. REFERENCES Adams, J. (1984). Active deformation of the Pacific Northwest continental margin, Tectonics 3(4), 449–472. Aki, K. (1984). Asperities, barriers, characteristic earthquakes and strong motion prediction, J. Geophys. Res. 89, 5867–5872. Allen, C.R., M.Wyss, J.N.Brune, A.Grantz, and R.E.Wallace (1972). Displacements on the Imperial, Superstition Hills and the San Andreas Faults triggered by the Borrego Mountain earthquake, U.S. Geol. Surv. Prof. Paper 787, 87–104. Allmendinger, R.W., J.W.Sharp, D.Von Tish, L.Serpa, L.Brown, S.Kaufman, J.Oliver, and R.B.Smith (1983). Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic-reflection data, Geology 11, 532. Ambraseys. N.N. (1978). 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