Bulletin of the Seismological Society of America, Vol. 94, No. 6, pp , December 2004

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1 Bulletin of the Seismological Society of America, Vol. 94, No. 6, pp , December 2004 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake: An Analog for Disastrous Shaking from a Major Thrust Fault in the Los Angeles Basin by James N. Brune, Abdolrasool Anooshehpoor, Baoping Shi, and Yuehua Zeng Abstract Precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault provide constraints on ground motion during the 1952 M s 7.7 Kern County earthquake, a possible analog for an anticipated large earthquake in the Los Angeles basin (Shaw et al., 2002; Dolan et al., 2003). On the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves. On the other hand, on the hanging-wall, there is evidence of intense ground shattering and lack of precarious rocks, consistent with the intense hanging-wall accelerations suggested by foam-rubber modeling, numerical modeling, and observations from previous thrust fault earthquakes. There is clear evidence of the effects of rupture directivity in ground motions on the hanging-wall side of the fault (from both precarious rocks and numerical simulations). On the southwest part of the fault, which is covered by sediments, the thrust fault did not reach the surface ( blind thrust). Overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall, an effect that may not be as effective when the rupture reaches the surface (is not blind ). Transformers near the up-dip projection of the fault tip have been damaged or overturned on both the hanging-wall and footwall sides of the fault. The transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in Los Angeles. We suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the Los Angeles basin, specially given the fact that there is so little instrumental data from large thrust fault earthquakes. Introduction Recent studies have emphasized the seriousness of the threat of large thrust faults, both outcropping and blind, to the city of Los Angeles (Shaw et al., 2002; Dolan et al., 2003). Until the Chi Chi, Taiwan, earthquake of 20 September 1999 there were few data for estimating ground motion and damage from large earthquakes (M 7 8) on such faults, and thus it is not certain whether the ground motions recorded for that earthquake are typical of large thrust faults (Boore, 2001; Anderson et al., 2002; Aagaard et al., 2004). We suggest that the ground-motion constraints we have obtained for the 1952 M s 7.7 (the range of magnitudes reported in the references cited here are M w and M s ) Kern County earthquake can provide important additional evidence for ground motions to be expected from large thrust fault earthquakes. Recent evidence from foam rubber modeling (Brune, 1996), lattice numerical modeling (Shi et al., 1998), and finite-element modeling (Oglesby et al., 1998, 2000), along with field evidence from several large thrust fault earthquakes (Allen et al., 1998), has indicated extremely severe ground motions (accelerations near 1 g, velocities exceeding 100 cm/sec) on the near-fault-trace hanging wall of some large thrust fault earthquakes. Allen et al. (1998) documented clear evidence of extreme motions on the hanging wall in the 1971 M w 6.7 San Fernando earthquake. The ubiquitous presence of a particular pattern of shattered rock on the hanging wall of thrust faults in southern California also supports the occurrence of these extreme motions (Brune, 2001). Such motions could have disastrous consequences for certain parts of Los Angeles and other cities threatened by such earthquakes. On the other hand, precarious rock data suggest much lower ground motions on the footwall side of outcropping thrust faults. Modeling studies also indicate much lower ground mo- 1993

2 1994 J. N. Brune, A. Anooshehpoor, B. Shi, and Y. Zeng tions in the footwall of thrust faults. In the foam rubber model studied by Brune (1996), which exhibits clear fault separation near the surface, the ground motions are about a factor of 5 lower in the footwall than in the hanging wall. Similar asymmetry was found for the numerical models of thrust faults of Shi et al. (1998) and Oglesby et al. (1998). This same type of asymmetry was spectacularly displayed in the ground motions recorded from the M s 7.6 Chi Chi, Taiwan, earthquake (Zeng and Chen, 2001). In this article we use precariously balanced rock and overturned transformer evidence on both the footwall and hanging wall of the White Wolf fault to provide constraints on the ground motion during the M s 7.7 Kern County earthquake of In as much as geodetic evidence gives a good estimate of the fault slip and direction of rupture propagation, we can compare various ground-motion synthesis programs and regression curves with the ground-motion constraints and, in particular, look for effects of directivity. Precarious Rock Methodology Estimates of peak ground acceleration necessary to topple a balanced rock (or a rigid transformer), through rigidbody rocking motion, can provide constraints on ground motion from earthquakes that have affected that site. The time scale for evolution and stability of precarious rocks is of the order of thousands of years (Bell et al., 1998). We use a pseudo-3d finite-difference method developed by Shi et al. (1996), as well as shake-table tests, to estimate the dynamic toppling acceleration of balanced rocks, subjected to inputs with various waveforms, from the quasi-static toppling accelerations measured in the field. Rocks with quasi-static toppling accelerations less than 0.3 g are classified as precarious and those between 0.3 and 0.5 g as semiprecarious (Brune, 1996). A detailed explanation of the methodology is given by Anooshehpoor et al. (2004). Survey Methodology The data presented in this study are based on extensive reconnaissance surveys for precarious and semiprecarious rocks wherever crystalline rocks were exposed on either the hanging wall or the footwall of the White Wolf fault. All available roads were traversed using 4-wheel-drive vehicles. Optical surveys using binoculars were carried out from the roads, commonly covering both sides of the roads to a distance of a few kilometers. Then hikes were made to cover likely sites (sites with the kind of weathering of granitic rocks shown by experience to be favorable to producing precarious rocks). In most areas where precarious or semiprecarious rocks existed it was soon obvious that no rocks existed more precarious than corresponded to a certain rough level of acceleration. The critical rocks were then photographed from a number of different directions to show the shape of the rock and the rocking points, both necessary to estimate the toppling accelerations using the methodology described by Anooshehpoor et al., (2004). The toppling accelerations were then estimated from photographs by three different persons. The estimates typically agreed with one another to within 0.1 g. Although the estimates presented here are based on reconnaissance surveys and approximate estimations of toppling acceleration, rather than the full quantitative methodology described by Anooshehpoor et al. (2004), our extensive field experience covering many thousands of rocks in many different types of terrain has indicated that our estimates are approximately this accurate. We believe that the results presented here should eventually be checked by more extensive studies as described by Anooshehpoor et al., (2004), but that in the meantime our preliminary results are important enough to warrant publication, if not to give final values to accelerations produced in the 1952 Kern County earthquake (which of course will probably never be available), certainly to outline preliminary conclusions and to point out important new areas for research. Of course it would be preferable to have hundreds of instrument recordings from numerous thrust fault earthquakes, but since this may not be achieved for centuries, the hundreds of approximate estimates from precarious rocks are clearly useful, given the importance of even approximate estimates of ground motions for large earthquakes in metropolitan areas such as Los Angeles. Even in areas where no precarious or semiprecarious rocks are found, we may make approximate estimates of upper bounds by observing the sensitivity to toppling of the numerous remaining, untoppled, rocks. These shaken down areas have a typical appearance that can be easily recognized with enough field experience (rounded cliff profiles with no precarious or semiprecarious rocks, numerous boulders at the bottoms of canyons, and almost all rocks in a setting of minimum potential energy). This methodology is not as quantitative as the case where numerous measurable precarious and semiprecarious rocks remain available for testing but nevertheless can provide important constraints on ground motion for recent earthquakes. This procedure is based primarily on the extensive field experience of the first author (J.N.B.), and thus has not been checked by independent investigators. The method was applied to the region between the Elsinore and Newport-Inglewood faults in southern California by Brune (2002). In this study it is used in areas that have apparently been shaken down by ground motions greater than about 0.5 g (no precarious or semiprecarious rocks remaining), specifically, the northeast hanging wall. The approximate acceleration contours shown in Figure 1 are based on this method and, although not as accurate as for the case where precarious and semiprecarious rocks remain, are generally consistent with the more accurate measurements and provide important data for areas of more intense shaking.

3 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake 1995 Figure 1. The location of the White Wolf fault that ruptured during the 1952 Kern County earthquake (Oakeshott, 1955). The approximate location of the overturned and damaged transformers are shown by hatching on the map. The diamond indicates the location of precarious rocks on the footwall (Fig. 2a c); square, the location of precarious rocks on the hanging wall (Fig. 2d); triangle, the location of semiprecarious rocks near the fault trace on the footwall (Fig. 3); circle, the location of shattered rock on the hanging wall (Fig. 4); and hexagon, the location of the Weedpatch substation. Numbers 1 and 2 indicate the location of calculated synthetic ground acceleration shown in Figure 7. The fault has been divided into three sections depending on the structure of the near-surface part of the fault: (1) the southwest section, which is a blind thrust fault with sediments on both the footwall and the hanging wall; (2) the central section, with sediments on the footwall and crystalline basement on the hanging wall; and (3) the northeast section, with crystalline basement on both the hanging wall and footwall. Precarious Rock Data Constraining Ground Motion for the M s 7.7 Kern County Earthquake The M s 7.7 Kern County earthquake caused about 2 m of slip on the White Wolf fault (Dunbar et al., 1980; Stein and Thatcher, 1981; Bawden et al., 1997; Bawden, 2001). The location of the inferred rupture is indicated in Figure 1. A collection of articles describing all aspects of the earthquake may be found in Bulletin 171 of the California Division of Mines (Oakeshott, 1955). Although there was no clear evidence of simple thrust faulting that reached the surface (because of the presence of complex landslide deposits from oversteepening of Bear Mountain, possibly triggered by earlier earthquakes), geodetic and geologic evidence indicate that the oblique thrust slip of more than a meter occurred along the White Wolf fault (Fig. 1). Moreover, it is clear that although slip reached very close to the surface for the northeast part of the fault, it did not reach the surface (i.e., was blind ) for the southwest part of the fault.

4 1996 J. N. Brune, A. Anooshehpoor, B. Shi, and Y. Zeng We have divided the fault into three sections depending on the structure of the near-surface part of the fault: (1) the southwest section, which is a blind thrust fault with sediments on both the footwall and the hanging wall, (2) the central section with sediments on the footwall and crystalline basement on the hanging wall, and (3) the northeast section with crystalline basement on both the hanging wall and footwall. These three sections are marked off in Figure 1. The hypocenter was at a depth of about 15 km, and the rupture propagated toward the northeast, which would be expected to focus energy (by rupture directivity) in this direction. Standard regressions for reverse-oblique-slip earthquakes with this magnitude and geometry give peak ground acceleration values greater than 0.7 g to distances of 7 km on both the footwall and hanging wall of the fault. Precarious Rocks on the Footwall of the Northeast Section of the White Wolf Fault Perhaps surprisingly, from the point of view of current attenuation and simulation estimates, the granite outcrops on the footwall along the northeastern part of the fault show many precarious or semiprecarious rocks (estimated toppling accelerations of about 0.3 g) that have been in place for thousands of years. Examples are shown in Figure 2. Rocks 2a, 2b, and 2c are on the footwall; rock 2d is on the hanging wall, but back about 10 km from the fault trace, and not in the direction of rupture propagation. Two examples very close ( 1 km) to the fault trace on the footwall side of the White Wolf fault are shown in Figure 3. Although not as precarious as the rocks shown in Figure 2, they are much closer to the fault trace and thus provide important constraints on the near-fault ground motion (including footwall directivity) from thrust faults. Preliminary estimates of the constraint on the ground motion are about 0.5 g, whereas typical regression curves give values of about 1 g. Shattered Rock, Shaken-Down Rocks and Semiprecarious Rocks on the Hanging Wall of the White Wolf Fault: Evidence for Thrust Rupture Directivity Near the fault trace, the hanging wall region around the White Wolf fault is characterized by large landslides, possibly triggered by earlier earthquakes. Back from the trace of the fault on the hanging-wall side the bedrock is intensely shattered (Fig. 4), presumably from the Kern County and earlier earthquakes (Brune, 2001). The slopes are generally smoothed with no evidence of precarious rocks, even though the geologic formations are granite of the type that commonly forms precarious rocks. Numerous rock falls were generated on the hanging-wall side of the fault in the 1952 earthquake (Buwalda and St. Amand, 1955). The evidence of intense ground shattering and lack of precarious rocks on the hanging-wall side of the fault is consistent with the intense hanging-wall accelerations suggested by foam rubber modeling, numerical modeling, and previous thrust fault earthquakes (Brune, 1996; Allen et al., 1998; Oglesby et al., 1998; Shi et al., 1998; Oglesby et al., 2000; Brune, 2001). Several reconnaissance surveys of the granite boulder terrain on the hanging-wall side of the fault reveal clear evidence of rupture directivity in ground motions. These consisted of vehicle surveys on all accessible roads, including Jeep trails, and hikes over the terrain in between by the first author (J.N.B.) and members of his family, and in addition a two-week survey by Nathan Robison and his wife as part of a Southern California Earthquake Center (SCEC) Intern program (Robison, 1999). For the northeast part of the fault, in the direction of rupture propagation, no precarious or semiprecarious rocks are found at distances less than 15 km from the outcrop of the fault (hanging wall). At a distance of about 20 km, near the town of Tehachapi, rocks that may be close to semiprecarious begin to appear (estimated toppling accelerations of about 0.5 g). On the other hand, for the central part of the fault, closer to the epicenter and expected to be less affected by directivity, semiprecarious rocks begin to appear at a distance of about 10 km from the fault trace (Fig. 1). One example is shown in Figure 2d. The effects of rupture directivity on the hanging wall are also clear in the numerical simulations described in a later section. Transformers Overturned by the 1952 Kern County Earthquake To the southwest, the footwall of the White Wolf fault is covered by thick sediments of the Great Valley sequence. This presents the possibility of determining the effect of overlying sediments on ground motions for blind thrust faults, since bedrock estimates of ground motion on the nonblind, northeast section of the fault are constrained by the precarious rock data. Evidence from the overturning of numerous transformers in the 1952 earthquake tends to support the significant transfer of energy from the hanging wall to the footwall. The approximate location of overturned or damaged transformers is shown in Figure 1. Transformers near the projected fault outcrop have been damaged or overturned on both the hanging-wall and footwall sides of the fault, but predominantly at distances of less than about 6 km. Figure 5 shows a toppled transformer at the Weedpatch substation. Figure 6 shows estimates for the minimum amplitude of a full-sine acceleration pulse versus ground frequency, which would topple the transformer. The minimum peak amplitude at 1.5 hz is about 0.5 g, consistent with Intensity VIII (Wald et al., 1999) estimated for this region (Fig. 7) (Neumann and Cloud, 1955; Wald et al., 1999). The methodology for using overturning and nonoverturning of equipment and precarious rocks is documented by Shi et al. (1996), Anooshehpoor et al. (1999, 2000, 2004), and Zhang and Makris (2000, 2001). These results, along with the upper-bound constraints on ground motion for the nearby rock outcrops on the footwall, give us a fair idea of the effect of the thick sedimentary cover (along with modeling studies

5 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake 1997 c) a) b) d) Figure 2. Examples of precarious rocks on the footwall (a, b, c), and the hanging wall of the White Wolf fault (d).

6 1998 J. N. Brune, A. Anooshehpoor, B. Shi, and Y. Zeng a) b) Figure 3. Examples of semiprecarious rocks very close ( 1 km) to the fault trace on the footwall side of the White Wolf fault. described in a later section). The results should provide an important analog for similar effects expected for a large thrust fault earthquake in the Los Angeles basin. Ground-Motion Modeling Using Complex and Simple Models High-Frequency Motions We have carried out preliminary calculations to estimate synthetic seismograms at two locations near the White Wolf fault (Fig. 1). Assuming 2 m average slip to be consistent with the geodetic data and the seismic moment (about dyne-cm), we carried out the calculations using the Zeng et al. (1994) composite source model, and specifying a range of subevent stress drops, rupture velocities, and shallow slip distributions. The rupture front initiates at the hypocenter and propagates outward (up and to the northeast) along the fault plane with a circular rupture front, which determines the approximate initiation times for a complex set of subevents with a specified stress drop. We carried out calculations for two sites on the hanging wall. The synthetic waveforms shown in Figure 7 correspond to sites 1 and 2 marked off in Figure 1. In dislocation theory the absolute ground motion on both the hanging wall and footwall are the same, but this is probably not realistic, as indicated by dynamic and physical models and precarious rock evidence cited earlier. At any rate, the calculated high-frequency ground motions on the footwall are too high to be consistent with precarious rocks, so we focus on the hanging wall. On the hanging-wall side of the fault, the peak ground accelerations for an intermediate stress drop case (subevent stress drop 50 bars, rupture velocity 2.7 km/sec, and tapered dynamic stress drop to the surface) were calculated for two sites shown in Figure 1. One site is closer to the epicenter, where directivity effects would be expected to be lower, and the other site is further to the northeast, where directivity effects would be expected to be higher. At the site closer to the source, the peak accelerations are about 0.4 g, and at the other site about 0.8 g (Fig. 8), clearly showing the effect of

7 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake 1999 a) b) Figure 4. Examples of hanging-wall shattered rock in the White Wolf fault system. (a) Looking north toward Bakersfield from the hanging wall of the White Wolf fault, showing a large area of the shattered rock, the fault outcrops at the intersection of the valley and the hills. (b) A close-up of the shattered rock along the Comanche Point Road. The approximate location where these photos were taken is shown with a circle in Figure 1. Figure 5. A photograph of toppled transformers at Weedpatch substation during the 1952 Kern County earthquake (Peers, 1955). directivity and consistent with the distribution of precarious rocks and estimates of upper-bound ground motions shown in Figure 1. For the southwest half of the White Wolf fault, which is covered with thick sediments, we modified the structure to approximately correspond to thick sedimentary cover and carried out similar calculations. In this case, because of sediment amplification and rupture directivity directly up the fault toward the site, the peak accelerations were much higher, of the order of 1 g, even for the case where the shallow slip was strongly tapered. This is consistent with the observations of overturned ground transformers near the trace of the fault in this region. Figure 6. Dynamic toppling amplitudes of fullsine wave pulses of acceleration as a function of frequency that topple the transformers shown in Figure 5. Lattice Modeling of the Longer Period Effect of Sediment Blanketing of Thrust Faults We have carried out 2D studies with the lattice model to compute particle velocity for three types of thrust fault, corresponding to the northeast, central, and southwest sections of the White Wolf fault (see Fig. 1). The thrust fault model consisted of lattice points with a dip

8 2000 J. N. Brune, A. Anooshehpoor, B. Shi, and Y. Zeng Figure 7. Synthetic acceleration at the precarious rock site (number 1 in Fig. 1), and at the shaken-down site (number 2 in Fig. 1) for a dynamic stress drop of 50 bars. Peak ground acceleration is 0.4 g at site 1, and 0.8 g at site 2, a clear indication of directivity. Figure 9. A schematic of a thrust fault. The boundary between the hanging wall and the footwall (B) is defined as the intersection of the fault plane and the free surface, whether or not the rupture reaches the surface. Figure 8. Intensity map for the 1952 Kern County earthquake. (From United States Earthquakes, 1952, U.S. Department of Commerce, serial number 773.) angle of 42. Since on the southwest end of the fault the rupture did not reach the surface, we modeled the fault as a blind thrust (rupture not reaching the surface) covered with a layer of sediments. The thickness of the sediment is 30 particle units with a shear-wave velocity 50% of that of the underlying material (Fig. 9). As an approximate realistic case we used a cell length of 20 m with shear-wave velocity of 3000 m/sec and 1500 m/sec for the bedrock and the sedimentary layer, respectively. Here, our intention is not to model the White Wolf fault exactly, which would require a fully 3-D calculation including effects of directivity, but rather to understand physical processes that might control behavior of different sections of the fault. Since we do not know the actual sedimentary structure in the region, nor the attenuation in the sediments, the scaling of the results of this calculation to the real situation is uncertain. The best equivalent thickness of the lower velocity layer might be considerably larger, increasing the horizontal scale in Figures 10 and 11. Figure 10 shows snap shots of the particle velocities in a cross section perpendicular to the fault trace in the lattice model simulation of three different segments of the White Wolf fault. In the northeast section, where there is no overlying sedimentary layer, the rupture reaches the surface with a strong decoupling of the hanging wall from the footwall, just as is observed in the foam rubber model and the corresponding numerical model (Shi et al., 1998). However, for the southwest segment of the fault with an overlying layer of sediments, the intense motion on the hanging wall side couples into the shallow sediments overlying the tip of the blind thrust and transmits energy across the fault to the footwall side. This explains the fact that overturned transformers were observed to distances of at least a few kilometers on both sides of the projected surface trace of the fault. In the central section the trapped energy in the sedimentary layer over the footwall results in only slightly higher ground motion in the footwall. We also simulated a hypothetical rupture that might break through the sediments to the surface of the southwest section (discussed later in this section). Figure 11 shows plots of the peak horizontal velocity on the free surface, along a line perpendicular to the fault trace, for exposed and buried thrust faults shown in Figure 10. In the case without the sedimentary layer (northeast segment of the White Wolf fault), in which the fault breaks the surface at distances more than 10 km from the fault trace, ground-motion amplitudes are nearly the same on both the hanging-wall and the footwall sides of the fault trace. But increasingly closer to the fault trace the amplitude on the hanging-wall side becomes much greater than the amplitude on the footwall side. At the fault trace the peak velocity on the hanging wall is twice that on the footwall. In the case with layers of sediments on both the footwall and hanging wall (blind thrust fault corresponding to the southwest segment), due to trapped energy in the sedimentary layer, the peak ground velocity near the fault trace is nearly the same on both the footwall and hanging wall. However, if the rupture were allowed to break through the sedimentary layer and reach the free surface (hypothetical case), the ground velocity on the hanging wall would be about 3 times of that on the footwall. The results would be similar

9 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake 2001 Figure 10. Snapshots of the particle velocities in a crosssection perpendicular to the fault trace in the lattice model simulations of the northeast, central and southwest segments of the White Wolf fault. For the northeast segment the velocities in the hanging wall are higher than in the footwall. On the contrary, in the central section, where the footwall is covered with sediment, the ground motion is slightly higher on the footwall side. For the southwest segment (blind thrust) the seismic energy generated in the hanging-wall side is trapped in the sedimentary layer and propagates to the footwall side, resulting in ground motions comparable to those on the hanging-wall side. We also simulated hypothetical surface rupture in the southwest segment. The results are similar to those of the northeast segment, but larger by almost a factor of 2. to those of the northeast segment (no sedimentary layer), but larger by almost a factor of 2 (because of the lower impedance of the sediments, which amplifies the motion). In the central section the trapped energy in the sedimentary layer over the footwall results in only slightly higher ground motion in the footwall. Implications for Ground Motions from a Possible Large Thrust Fault in the Los Angeles Region Estimates of damage and loss of life from a large thrust earthquake in a large metropolitan area such as Los Angeles are hampered by the lack of instrumental ground-motion data from such earthquakes. Extrapolations from smaller well-recorded thrust earthquakes such as Loma Prieta and Northridge are very uncertain, and there is no guarantee that the ground-motion data from the one well-recorded large earthquake, the Chi Chi, Taiwan, earthquake, are typical of thrust faults in general or of any specific future thrust fault earthquake. Ground-motion data from the Chi Chi earthquake were nearly one standard deviation below the extrapolated mean (Boore et al., 2001; Anderson et al., 2002), and the soil and shallow geologic structure may have been quite different from that for particular earthquakes in the Los Angeles area, and specifically from the type of earthquakes postulated by Dolan et al. (2003). In this uncertain situation, estimates of ground motion for any large thrust fault earthquake, such as the Kern County earthquake studied here, are critically important. Some of the large thrust faults postulated to be capable of producing large earthquakes in the Los Angeles area are blind thrusts similar to the southwest section of the White Wolf fault and thus may transmit damaging strong ground motion to the footwall side of the fault, into the densely populated center of the Los Angeles Basin. Other faults have higher-velocity material on the hanging-wall side, but sediments on the footwall side, similar to the central section of the White Wolf fault. No cases are exactly similar to the northeast part of the White Wolf fault, with crystalline basement rocks on both the hanging wall and footwall, but some of the faults have relatively high velocity material on the footwall. At any rate, constraints on ground motion for the Kern County earthquake are critically important to understanding thrust faults in general. Conclusions We use the distribution of precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault to provide estimates of ground motion during the 1952 Kern County earthquake. Preliminary estimates of

10 2002 J. N. Brune, A. Anooshehpoor, B. Shi, and Y. Zeng numerous transformers indicates significant transfer of energy from the hanging wall to the footwall. Transformers near the projected fault outcrop were damaged or overturned on both the hanging-wall and footwall sides of the fault during the 1952 earthquake. Numerical prediction of the minimum amplitude of a full-sine pulse of acceleration that could have toppled one of the transformers suggests a minimum peak amplitude of about 0.5 g at 1.5 hz, consistent with Intensity VIII estimated for this region. The results of this study may have important bearing on the expected hazard from large thrust faults that might occur in the Los Angeles basin and in other parts of the world. Acknowledgments Figure 11. Plots of the peak horizontal velocity on the free surface, along a line perpendicular to the fault trace, for exposed and buried thrust faults shown in Figure 10. In the case without the sedimentary layer (northeast segment of the White Wolf fault) in which the fault breaks the surface, at distances more than 10 km from the fault trace, ground-motion amplitudes are nearly the same on both the hanging-wall and the footwall sides of the fault trace. But, increasingly closer to the fault trace, the amplitude on the hangingwall side becomes much greater than the amplitude on the footwall side. At the fault trace the peak velocity on the hanging wall is twice that on the footwall. In the case with layers of sediments on both the footwall and hanging wall (blind thrust fault corresponding to the southwest segment), due to trapped energy in the sedimentary layer, the peak ground velocity near the fault trace is nearly the same on both the footwall and hanging wall. However, if the rupture were allowed to break through the sedimentary layer and reach the free surface (hypothetical case), the ground velocity on the hanging wall would be about 3 times of that on the footwall. The results would be similar to those of the northeast segment (no sedimentary layer), but larger by almost a factor of 2 (because of the lower impedance of the sediments, which amplifies the motion). In the central section the trapped energy in the sedimentary layer over the footwall results in only slightly higher ground motion in ground motion on the footwall give low peak accelerations of about 0.3 g at about 10 km and 0.5 g at less than 1 km from the fault trace. In the case of the hanging wall, there is evidence of intense ground shattering and lack of precarious rocks, consistent with the intense hanging-wall accelerations (near 1 g) suggested by foam rubber modeling, numerical modeling, and previous thrust fault earthquakes. There is clear evidence of rupture directivity in high-frequency ground motions on the hanging-wall side of the fault, also clearly reproduced in numerical simulations. On the southwest part of the fault, which is blind thrust fault covered by sediments, evidence from the overturning and damage of We thank Jim Dolan and an anonymous reviewer for their constructive comments. Special thanks are due to members of the first author s (J.N.B.) family and to Nate Robison (1999 SCEC intern) and his wife who helped in the field reconnaissance surveys. We wish to acknowledge the joint support of the U.S. Geological Survey (USGS) and the SCEC for this research project (by USGS, Department of the Interior, under the award number 00HQGR0074, and by SCEC through the Cooperative Agreement EAR and USGS Cooperative Agreement 02HQAG0008). The SCEC contribution number for this paper is 779. References Aagaard, B. T., J. F. Hall, and T. H. Heaton (2004). Effects of fault dip and slip rake angles on near-source ground motions: why rupture directivity was minimal in the 1999 Chi-Chi, Taiwan, earthquake, Bull. Seism. Soc. Am. 94, Anderson, J. G., J. N. Brune, and S. G. Wesnousky (2002). Physical phenomena controlling high-frequency seismic wave generation in earthquakes, in Proceedings of the Seventh US National Conference of Earthquake Engineering, Boston, July, CD-ROM, Paper 00272, Earthquake Engineering Research Institute, Palo Alto, California. Anooshehpoor, A., T. H. Heaton, B. Shi, and J. N. Brune (1999). Estimates of the ground accelerations at Point Reyes Station during the 1906 San Francisco earthquake, Bull. Seism. Soc. Am. 89, Anooshehpoor, A., T. H. Heaton, B. Shi, and J. N. Brune (2000). Reply to Comment on Estimates of ground accelerations at Point Reyes, California During the 1906 San Francisco earthquake by Zhang, J., and N. Makris Bull. Seism. Soc. Am. 90, Anooshehpoor, A., J. N. Brune, and Y. Zeng (2004). Methodology for obtaining constraints on ground motion from precariously balanced rocks, Bull. Seism. Soc. Am. 94, Allen, C. R., J. N. Brune, L. S. Cluff, and A. G. Barrows, Jr. (1998). Evidence for unusually strong near-field motion on the hanging wall of the San Fernando fault during the 1971 earthquake, Seism. Res. Lett. 69, Bawden, G. W. (2001). Source parameters for the 1952 Kern County earthquake, California: a joint inversion of leveling and triangulation observations, J. Geophys. Res. 106, Bawden, G. W., A. Donnellan, L. H. Kellogg, D. Dong, and J. B. Rundle (1997). Geodetic measurements of horizontal strain near the White Wolf fault, Kern County, California, , J. Geophys. Res. 102, no. 3, Bell, J. W., J. N. Brune, T. Liu, M. Zerda, and J. C. Yount (1998). Dating the precariously balanced rocks in seismically active parts of California and Nevada, Geology 26, Boore, D. M. (2001). Comparisons of ground motions from the 1999 Chi-

11 the footwall.

12 Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the M s 7.7 Kern County Earthquake 2003 Chi earthquake with empirical predictions largely based on data from California, Bull. Seism. Soc. Am. 91, Brune, J. N. (1996). Precariously balanced rocks and ground-motion maps for southern California, Bull. Seism. Soc. Am. 86, Brune, J. N. (2001). Shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting, Bull. Seism. Soc. Am. 91, Brune, J. N. (2002). Precarious rock constraints on ground motion from historic and recent earthquakes in southern California, Bull. Seism. Soc. Am. 92, Buwalda, J. P., and P. St. Amand (1955). Geological effects of the Arvin- Tehachapi earthquake, in Earthquakes in Kern County, California during 1952, Bulletin 171 of the California Division of Mines, G.B. Oakeshott (Editor), California Department of Natural Resources. San Francisco, California, Dolan, J. F., S. A. Christofferson, and J. H. Shaw (2003). Recognition of paleoearthquakes on the Puente Hills blind thrust fault, California, Science 300, Dunbar, W. S., D. M. Boore, and W. Thacher (1980). Pre, co-, and postseismic strain changes associated with the 1952 M L 7.2 Tehachapi, California, earthquake, Bull. Seism. Soc. Am. 70, Neumann, F., and W. K. Cloud (1955). Strong-motion records of the Kern County earthquake, in Bulletin 171 of the California Division of Mines, G. B. Oakeshott (Editor), G. B. Oakeshott (Editor) (1955). Earthquakes in Kern County, California during 1952, Bulletin 171 of the California Division of Mines, 283 pp. Oglesby, D. D., R. J. Archuleta, and S. B. Nielson (1998). Earthquakes on dipping faults: the effects of broken symmetry, Science 280, Oglesby, D. D., R. J. Archuleta, and S. B. Nielson (2000). Threedimensional dynamics of dipping faults, Bull. Seism. Soc. Am. 90, Peers, G. A. (1955). Damage to electrical equipment caused by Arvin- Tehachapi earthquake, in Bulletin 171 of the California Division of Mines, G. B. Oakeshott (Editor), Robison, N. E. (1999). Geologic constraints on the seismic hazard of the White Wolf fault earthquake zone, SCEC Intern Final Report, September Shaw, J. H., A. Plesch, J. F. Dolan, T. L. Pratt, and P. Fiore (2002). Puente Hills blind-thrust system, Los Angeles, California, Bull. Seism. Soc. Am. 92, Shi, B., A. Anooshehpoor, Y. Zeng, and J. N. Brune (1996). Rocking and overturning of precariously balanced rocks by earthquakes, Bull. Seism. Soc. Am. 86, Shi, B., A. Anooshehpoor, J. N. Brune, and Y. Zeng (1998). Dynamics of thrust faulting: 2D lattice model, Bull. Seism. Soc. Am. 88, Stein, R. S., and W. Thatcher (1981). Seismic and aseismic deformation associated with the 1952 Kern County, California, earthquake and relationship to the quaternary history of the White Wolf fault, J. Geophys. Res. 86, Wald, D. J., V. Quitoriano, T. H. Heaton, and H. Kanamori (1999). Relationships between peak ground acceleration, peak ground velocity, and modified Mercalli intensity in California, Earthquake Spectra 15, Zeng, Y., and C. Chen (2001). Fault rupture process of the September 20, 1999 Chi Chi, Taiwan, earthquake, Bull. Seism. Soc. Am. 91, Zeng, Y., J. G. Anderson, and G. Yu (1994). A composite source model for computing realistic synthetic strong ground motions, Geophys. Res. Lett. 21, Zhang, J., and N. Makris (2000). Comment on Estimates of ground accelerations at Point Reyes, California, during the 1906 San Francisco earthquake, by Anooshehpoor, A., T. H. Heaton, B. Shi, and J. N. Brune Bull. Seism. Soc. Am. 90, Zhang, J., and N. Makris (2001). Rocking response of free-standing blocks under cycloidal pulses, J. Eng. Mech. 127, Seismological Laboratory MS 174 University of Nevada, Reno Reno, Nevada rasool@seismo.unr.edu (J.N.B., A.A., Y.Z.) Kentucky Geological Survey University of Kentucky Lexington, Kentucky (B.S.) Manuscript received 13 January 2004.