The 1906 San Francisco Earthquake a Century Later: Introduction to the Special Section

Transcription

1 Bulletin of the Seismological Society of America, Vol. 98, No. 2, pp , April 2008, doi: / The 1906 San Francisco Earthquake a Century Later: Introduction to the Special Section by Brad T. Aagaard and Gregory C. Beroza The great 1906 San Francisco earthquake is perhaps the landmark event in the history of earthquake science. It began with a foreshock at 5:12 a.m. local time in the morning of 18 April Some 30 sec later, the main event initiated on the San Andreas fault, just off the San Francisco coast (Lawson, 1908). Within 90 sec, nearly 480 km of the San Andreas fault ruptured (see Fig. 1), extending south to the northern end of the creeping section near San Juan Bautista and north to the terminus of the fault at the triple junction near Cape Mendocino (Song et al., 2008). As it ruptured, it generated powerful seismic waves over the entire rupture length and set in motion a chain of events that led to the destruction of most of San Francisco, the largest city of the western United States at the time. The earthquake occurred in the early days of instrumental seismology, which renders the data difficult to analyze, but our best estimate is that the moment magnitude was 7.9 (Song et al., 2008), about 26 times the size of the 1989 magnitude 6.9 Loma Prieta earthquake as measured by seismic moment (Hanks and Krawinkler, 1991; Song et al., 2008). The strong shaking caused widespread damage along the entire length of the rupture (see Figs. 2 4). Masonry walls were reduced to rubble, houses were knocked off their foundations, chimneys fell down, and pipes were severed (Lawson, 1908). Ground failures in the form of landslides and liquefaction were also prevalent, especially in the man-made fill along the San Francisco waterfront (Lawson, 1908). The city of San Francisco, a bustling city of 400,000, suffered damage from both the shaking and well as the firestorm that followed. How much of the devastation in San Francisco was caused directly by the earthquake, rather than indirectly by the many fires it triggered, has been a controversial issue. Certain interests may have sought to attribute the damage to fire rather than the shaking (Hansen and Condon, 1989), and others contend that the way in which the fires were fought added to the destruction (Smith, 2005). Based on eyewitness accounts and photographs taken shortly after the quake, it appears that many areas of the city were largely intact after the earthquake and were subsequently consumed by the fire (Tobriner, 2006). A number of the buildings that survived the earthquake and fire remain in use today (Klett et al., 2006). Arguably, the most concentrated damage from shaking occurred in Santa Rosa, where the downtown was almost completely destroyed (Lawson, 1908). San Jose, just an agricultural hub at the time, suffered only moderate damage, but partial collapse of the Agnew State Hospital outside San Jose caused about 100 deaths (Greely, 1906). The death toll in the earthquake is another source of controversy. U.S. Army accounts tallied a total of about 700. Of this total, some 498 deaths occurred in San Francisco, with 64 in Santa Rosa and 102 in the vicinity of San Jose (Greely, 1906); however, research by Gladys Hansen suggests the death toll in San Francisco alone may have exceeded 3000 (Hansen and Condon, 1989). The destruction, confusion, and dislocation during and after the earthquake ensure that the full extent of the tragedy will never be known. The Seismological Society of America, and by extension this journal, was founded in response to the devastation wrought by this earthquake (see Fig 5; Byerly, 1964). The centenary of the earthquake provided an important opportunity to remind ourselves, and the public, of the hazards posed by earthquakes and what can be done to reduce the threat they pose. Only a very few survivors of the event 100 yr ago remain alive today. Furthermore, rapid growth in the San Francisco Bay region due to influx of people from other areas means a significant fraction of the current residents did not experience the most recent destructive event in the region, the 1989 Loma Prieta earthquake. Thus, while San Francisco remains in the popular consciousness as an earthquake-threatened city, the threat becomes less tangible each year, as the memory of 1906 fades. Most of what we know about the earthquake from the scientific perspective comes directly from the careful research plan, data collection, and archiving performed by the earthquake investigation commission led by Andrew Lawson. Local historical societies and educational institutions also preserved maps, photographs, letters, and other documents that have proven to be useful in better understanding the earthquake and its effects. The earthquake commission collected the seismograms recorded throughout the world and documented the damaging effects of the shaking, how it was perceived by people, and the geomorphic features of the rupture (Lawson, 1908). They also undertook research to advance scientific theories to explain the observations (Reid, 1910). For example, deformation in the vicinity of the San Andreas fault in the 1906 earthquake convinced geologists that fault slip released stored elastic energy, but it was not clear at the time what caused the strain to accumulate. They hypothesized that elastic strain slowly accumulates until it raises stress sufficiently to overcome the friction holding the fault in place. The fault then slips, the deformed crust rebounds, and in doing so suddenly releases the slowly ac- 817

2 818 B. T. Aagaard and G. C. Beroza Figure 3. Liquefaction inducing ground failure and collapse of the four-story Valencia Street Hotel in San Francisco (U.S. Geological Survey Photographic Library, Bear 1). Figure 1. The thick dark line shows the extent of rupture along the San Andreas fault in the 1906 earthquake, and the star denotes the epicenter. cumulated strain energy as seismic waves (Reid, 1910). A half-century later, the theory of plate tectonics explained the forces driving the deformation. Our understanding of the link between strain accumulation and slip on faults continued to progress even before the theory of plate tectonics was formulated. Scientists recognized that geologic features across the San Andreas fault were offset by hundreds of kilometers. Early estimates of the long-term slip rate on the fault ranged from 5 to 50 mm per year (Hill and Dibblee, 1953). Modern geodetic data indicates that total motion across the plate boundary is close to the higher of these numbers; however, not all of this motion occurs as slip on the San Andreas. Other faults such as the San Gregorio fault offshore, the Hayward fault in the East Bay, as well as active faults as far to the east as Utah, accommodate the total motion across the Pacific North America plate boundary. This plate motion drives seismic hazard in the region. A recent synthesis of earthquake hazards in the greater San Francisco Bay Area found a 62% chance of at least one magnitude 6.7 or larger earthquake during the period (Working Group on California Earthquake Probabilities, 2003). The hazard is dominated by the larger high slip-rate faults such as the San Andreas, Hayward, and Rodgers Creek faults, but other faults contribute, and it is Figure 2. Fence half a mile northwest of Woodville (east of Point Reyes), offset by approximately 2.6 m of right-lateral strike-slip motion on the San Andreas fault in the 1906 San Francisco earthquake (U.S. Geological Survey Photographic Library, Gilbert, G. K. 2845). Figure 4. Aftermath of the 1906 earthquake in downtown Santa Rosa (U.S. Geological Survey Photographic Library, Townley, S. D. 7). Damage in the earthquake was not confined to San Francisco. Strong shaking and a subsequent fire devastated Santa Rosa as well.

3 The 1906 San Francisco Earthquake a Century Later: Introduction to the Special Section 819 Figure 5. The ruins of San Francisco City Hall (U.S. Geological Survey Photographic Library, Degenkolb, H. 17). The devastation caused by the 1906 earthquake led to the founding of the Seismological Society of America. These ruins serve as the emblem of the Seismological Society of America. quite possible that the next significant earthquake will occur on lesser-known, or even as yet unidentified, faults. The extensive data collection and analysis by the earthquake commission (Lawson, 1908; Reid, 1910) was unique in its time and, as a result, the 1906 earthquake remains today as one of the most carefully and exhaustively studied and documented earthquakes in history. A comprehensive understanding of this earthquake and its effects is critical to characterizing hazards from future earthquakes in northern California. Large, continental, strike-slip earthquakes are rare, and lessons learned from the 1906 earthquake should apply to similar faults and earthquakes elsewhere. Moreover, as our understanding of earthquakes evolves and the technology to increase our knowledge develops, there is much to be gained by revisiting older events. Traces of the earthquake and its effects can be discerned from previously unrecognized sources of information, from tombstones to turbidites, and new insights can be gleaned from them. To this end, 102 yr after the 1906 earthquake, new research on this event is the focus of this special section of the Bulletin of the Seismological Society of America. The Seismological Society of America and the Earthquake Engineering Research Institute convened a joint meeting, cosponsored by the California Office of Emergency Services, on April 2006, the centenary of the earthquake. In addition to the science and engineering professionals, the conference was attended by political figures representing federal, state, and local governments. Many of these organizations also participated in the 1906 Earthquake Centennial Alliance, a group of over 200 members representing over 100 organizations, whose objective was to use the centennial of the 1906 earthquake to highlight a century of progress in understanding earthquake hazards and reducing risks as well as to commemorate the cultural and social response to this historic event. The Earthquake Alliance promoted many activities highlighting the earthquake and its effects, including exhibits displayed in museums, universities, and libraries; a symphony; a ballet; and various television specials, films, documentaries, walking tours, and public lectures. The highlights of the joint meeting included the latest findings on the 1906 earthquake, its consequences, and lessons it held for future earthquakes. Most of the work described in the 13 articles that follow in this issue were presented at that meeting. Three of the articles characterize the earthquake source using state-of-the-art analysis techniques. Song et al. (2008) performed a unified, seismic, and geodetic kinematic source inversion using a Bayesian approach. Applying a projection method developed by Yu and Segall (1996), which greatly expands the number of geodetic observations, Song et al. are able to constrain the spatial variation of slip over the entire rupture length. Based on the teleseismic recordings, Song et al. propose that supershear rupture in the first 100 km of rupture north of the hypocenter resolves discrepancies between the seismic inversion by Wald et al. (1993) and the geodetic inversion by Thatcher et al. (1997). This has important ramifications for the ground motions as discussed in Boatwright and Bundock (2008) and Aagaard, Brocher, Dolenc, Dreger, Graves, Harmsen, Hartzell, Larsen, McCandless, et al. (2008). Lorito et al. (2008) also use a Bayesian approach to find an ensemble of faulting models that reproduce the marigram of the small tsunami recorded at Fort Point, beneath the present-day Golden Gate bridge. This analysis complements the work by Lomax (2008), which uses a probabilistic global-search algorithm coupled with an equal-differentialtime likelihood function and a 3D seismic velocity model (Brocher, 2008) to obtain accurate absolute and relative

4 820 B. T. Aagaard and G. C. Beroza locations of microseismicity. From the microseismicity patterns, Lomax infers the hypocenter and the basic geometry of the fault surfaces involved in the 1906 rupture. These two studies of the faulting in the hypocentral region of the 1906 earthquake suggest that the faulting offshore of San Francisco is quite complex and that the 1906 event could have involved rupture on secondary faults as well as the San Andreas fault. One of the articles in this special section focuses directly on the important question of earthquake recurrence. Goldfinger et al. (2008) find that turbidite deposits recovered from sediment cores offshore northern California appear to be synchronous and thus are likely to have been triggered by large earthquakes. This conclusion is supported by onshore paleoseismology along the San Andreas fault. Both datasets show similar mean recurrence intervals of yr for large events on the San Andreas fault (Kelson et al., 2006). Goldfinger et al. also find a possible link between large events on the southern Cascadia megathrust and subsequent San Andreas earthquakes. The documented damage and eyewitness accounts of shaking at over 600 locations compiled by the earthquake commission (Lawson, 1908) served as the foundation for construction of a ShakeMap of the earthquake by Boatwright and Bundock (2005). Boatwright and Bundock augmented the earthquake commission information (Lawson, 1908) with newspaper accounts and analysis of overturned headstones in cemeteries. Instead of simply creating a contour map from the intensity values, they applied ShakeMap interpolation techniques to fill in intensity values between sites, resulting in a much sharper image of shaking intensity compared with previous efforts (Lawson, 1908; Toppozada and Parke, 1982; Stover and Coffman, 1993). Boatwright and Bundock (2008) review their own interpretations of the MMI intensity scale in testing the correlation of cemetery damage with MMI intensity and explore possible explanations for the intensity distributions in four areas with dense observations. Shostak (2008) examined Sanborn Map Company fire insurance maps along with detailed inspection reports to construct the first comprehensive analysis of damage in the San Jose area. Her work shows that the shaking intensities are remarkably consistent over census blocks, whereas variation across larger areas of several kilometers appears consistent with recent studies of amplification due to basin effects (Fletcher et al., 2003; Hartzell et al., 2006). Dengler (2008) combined the relatively limited information from the earthquake commission with that from local historical societies to reexamine the intensity of shaking at the northern end of the 1906 rupture along the northern coast of California. This reassessment supplies intensities at a finer scale consistent with the pattern described by previous studies (Toppozada and Parke, 1982; Stover and Coffman, 1993). It also supports the higher intensities ascribed by Boatwright and Bundock (2008) to this area compared with their previous study (Boatwright and Bundock, 2005). The ShakeMap produced by Boatwright and Bundock (2005) also permits reexamination of other historical events through comparison of the intensity values. Hough and Hutton (2008) revisit the intensities of the 1872 Owens Valley, California, earthquake and find that at a given distance from rupture, the intensities from the Owens Valley earthquake tend to cluster near highest values from the 1906 earthquake and are systematically higher than those from the 1906 earthquake at distances greater than about 200 km. Hough and Hutton conclude that the Owens Valley earthquake likely had a moment magnitude in the range of , which is significantly larger than the previous preferred estimate of 7.4. Ground-motion simulations of the 1906 earthquake by Aagaard, Brocher, Dolenc, Dreger, Graves, Harmsen, Hartzell, Larsen, McCandless, et al. (2008) provide a more thorough spatial and temporal characterization of the shaking compared with the intensity studies. These simulations use a new 3D seismic velocity model constructed from empirical velocity versus depth relationships for P and S wavespeeds as a function of depth and lithology for northern California developed by Brocher (2008). A combination of borehole, laboratory, seismic refraction and tomography, and density measurements formed the basis for these empirical regressions. Two studies test this new 3D seismic velocity model by comparing synthetic waveforms against recorded motions. Rodgers et al. (2008) calculated synthetic seismograms by a finite-difference method with the 3D velocity model for a set of 12 recent moderate earthquakes in the San Francisco Bay Area. For these events, they found that at periods of less than 10 sec, 3D effects were important and suggested that although the model is a good first step, several improvements should be made. Specifically, the true S-wave velocities at shallow depths may be 5% 6% lower than those predicted by the empirical relations developed by Brocher (2008). Aagaard, Brocher, Dolenc, Dreger, Graves, Harmsen, Hartzell, Larsen, and Zoback (2008) tested the seismic velocity model by computing ground motions for two finite-source models (Beroza, 1991; Wald et al., 1991) of the 1989 magnitude 6.9 Loma Prieta earthquake. The study included four different ground-motion modeling groups, each with a different wave-propagation code, simulation domain, and discretization procedure. They found that all of the simulations produced ground motions similar to those recorded during the Loma Prieta earthquake. Small variations among the results are attributable to differing simplifying assumptions made by the five modeling groups. They noted a tendency to slightly under predict observed intensities. They conclude that the 3D seismic velocity model and the different wavepropagation codes provide a suitable basis for modeling the 1906 earthquake, as well as other possible scenario events in the San Francisco Bay region. Aagaard, Brocher, Dolenc, Dreger, Graves, Harmsen, Hartzell, Larsen, McCandless, et al. (2008) used the validated 3D seismic velocity model and five wave-propagation codes together with the source model for the 1906 earth-

5 The 1906 San Francisco Earthquake a Century Later: Introduction to the Special Section 821 quake developed by Song et al. (2008) to estimate ground motions produced by the 1906 San Francisco earthquake. The five ground-motion modeling groups involved found that their simulations reproduce the main features of the improved intensity distributions for the earthquake as determined by Boatwright and Bundock (2005). Furthermore, the simulations illustrate the spatial variation in amplitude and duration associated with slip variability, directivity, and geologic structure, especially sedimentary basins. By considering other possible rupture scenarios, the authors conclude that future San Andreas fault earthquakes may subject the urbanized San Francisco Bay Area to stronger shaking than occurred in the 1906 earthquake. Using ground motions provided by Aagaard, Brocher, Dolenc, Dreger, Graves, Harmsen, Hartzell, Larsen, McCandless, et al. (2008), Olsen et al., (2008) studied the possible effects of a repeat of the 1906 earthquake, or similar-sized earthquakes on the San Andreas fault. They considered scenarios with hypocenters either north or south of San Francisco on two types of structures that are sensitive to long-period ground motion: a 20-story moment-resisting frame structure and base-isolated structures. They found that a 1906-size earthquake could cause substantial damage to each building type over much of the San Francisco Bay urban area. What would a repeat of the 1906 earthquake mean for the Bay Area? Much has changed since In 1906, the population of San Francisco was about 400,000 with the total population of the Bay Area at about 650,000. The 1906 quake occurred in the early days of the automobile, and those who wanted to cross the bay did so by boat. High-rise buildings were a rarity. Today, automobile and air travel are commonplace, and five major bridges span different parts of the bay. Many more high-rise buildings are present in San Francisco and elsewhere than were present in Estimates of the consequences of a repeat of the 1906 are uncertain, but most experts believe it would result in thousands of fatalities, perhaps more than 5,000 if it were to occur when large commercial buildings are occupied, and that it would cause over $100 billion in damage (Kircher et al., 2006). Although this number of fatalities and economic loss remains much greater than we would like, the number of fatalities is comparable to those in 1906, when the population residing in the Bay Area was roughly 10% of what it is today. Thus, while we have not eliminated the earthquake risk, we have made important progress in making our communities earthquake resistant. Acknowledgments Many thanks to Mary Lou Zoback who led the 1906 Earthquake Centennial Alliance and spearheaded the renewed interest in studying the 1906 earthquake. Reviews by William Ellsworth, Ruth Harris, and Mary Lou Zoback led to improvements in the manuscript. We also thank all of the authors for contributing to this special section and the reviewers for their constructive comments and recommendations that improved the manuscripts. The historical photographs in Figures 2 5 are available through the U.S. Geological Survey Photographic Library (U.S. Geological Survey, 2007). References Aagaard, B. T., T. M. Brocher, D. Dolenc, D. Dreger, R. W. Graves, S. Harmsen, S. Hartzell, S. Larsen, and M. L. Zoback (2008). Groundmotion modeling of the 1906 San Francisco earthquake, part I: Validation using the 1989 Loma Prieta earthquake, Bull. Seismol. Soc. Am. 98, no. 2, Aagaard, B. T., T. M. Brocher, D. Dolenc, D. Dreger, R. W. Graves, S. Harmsen, S. Hartzell, S. Larsen, K. McCandless, S. Nilsson, N. A. Petersson, A. Rodgers, B. Sjögreen, and M. L. Zoback (2008). Ground-motion modeling of the 1906 San Francisco earthquake, part II: Ground-motion estimates for the 1906 earthquake and scenario events, Bull. Seismol. Soc. Am. 98, no. 2, Beroza, G. C. (1991). Near-source modeling of the Loma Prieta earthquake: evidence for heterogeneous slip and implications for earthquake hazard, Bull. Seismol. Soc. Am. 81, no. 5, Boatwright, J., and H. Bundock (2005). Modified Mercalli intensity maps for the 1906 San Francisco earthquake plotted in ShakeMap format, U.S. Geol. Surv. Open-File Rept , available at (last accessed February 2007). Boatwright, J., and H. Bundock (2008). The distribution of modified Mercalli intensity in the 18 April 1906 San Francisco earthquake, Bull. Seismol. Soc. Am. 98, no. 2, Brocher, T. M. (2008). Compressional and shear-wave velocity versus depth relations for common rock types in northern California, Bull. Seismol. Soc. Am. 98, no. 2, Byerly, P. (1964). History of the Seismological Society of America, Bull. Seismol. Soc. Am. 54, no. 6, Dengler, L. (2008). The 1906 earthquake on California s north coast, Bull. Seismol. Soc. Am. 98, no. 2, Fletcher, J. B., J. Boatwright, and A. G. Lindh (2003). Wave propagation and site response in the Santa Clara Valley, Bull. Seismol. Soc. Am. 93, no. 1, Goldfinger, C., K. Grijalva, R. Bürgmann, A. E. Morey, J. E. Johnson, C. H. Nielsen, J. Gutiérrez-Pastor, A. Ericsson, E. Karabanov, J. D. Chaytor, J. Patton, and E. Gràcia (2008). Late Holocene rupture of the northern San Andreas fault and possible stress linkage to the Cascadia subduction zone, Bull. Seismol. Soc. Am. 98, no. 2, Greely, A. W. (1906). Special report of Major General Adolphus W. Greely, U.S.A., commanding the Pacific Division, on the relief operations conducted by the military authorities of the United States at San Francisco and other points, Bancroft Library, University of California, Berkeley, California. Hanks, T. C., and H. Krawinkler (1991). The 1989 Loma Prieta earthquake and its effects: introduction to the special issue, Bull. Seismol. Soc. Am. 81, no. 5, Hansen, G., and E. Condon (1989). Denial of Disaster, Cameron and Company, San Francisco, 150 pp. Hartzell, S., S. Harmsen, R. A. Williams, D. Carver, A. Frankel, G. Choy, R. C. Jachens, T. M. Brocher, and C. M. Wentworth (2006). Modeling and validation of a 3D velocity structure for the Santa Clara Valley, California, for seismic-wave simulations, Bull. Seismol. Soc. Am. 96, no. 5, Hill, M. W., and T. W. Dibblee (1953). San Andreas, Garlock, and Big Pine faults, California: a study of the character, history, and tectonic significance of their displacements, Bull. Seismol. Soc. Am. 64, no. 4, Hough, S. E., and K. Hutton (2008). Revisiting the 1872 Owens Valley, California, earthquake, Bull. Seismol. Soc. Am. 98, no. 2, Kelson, K., A. Strieg, R. Koehler, and K. Kang (2006). Timing of the late Holocene paleoearthquakes on the northern San Andreas fault at the Fort Ross orchard site, Sonoma County, California, Bull. Seismol. Soc. Am. 96, no. 3,

6 822 B. T. Aagaard and G. C. Beroza Kircher, C. A., H. A. Seligson, J. Bouabid, and G. C. Morrow (2006). When the Big One strikes again: estimated losses due to a repeat of the 1906 San Francisco earthquake, Earthq. Spectra 22 (special issue II), S297 S339. Klett, M., P. L. Fradkin, R. Solnit, K. Breuer, and M. Lundgren (2006). After the Ruins, 1906 and 2006 Rephotographing the San Francisco Earthquake and Fire, University of California Press, Berkeley, California, 134 pp. Lawson, A. (Editor) (1908). The California Earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission, Carnegie Institution of Washington, Washington, D.C.. Lomax, A. (2008). Location of the focus and tectonics of the focal region of the California earthquake of 18 April 1906, Bull. Seismol. Soc. Am. 98, no. 2, Lorito, S., A. Piatanesi, and A. Lomax (2008). Rupture process of the 18 April 1906 California earthquake from near-field tsunami waveform inversion, Bull. Seismol. Soc. Am. 98, no. 2, Olsen, A. H., B. T. Aagaard, and T. H. Heaton (2008). Long-period building response to earthquakes in the San Francisco Bay Area, Bull. Seismol. Soc. Am. 98, no. 2, Reid, H. (Editor) (1910). The mechanics of the earthquake, The California Earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission, Vol. 2, Carnegie Institution of Washington, Washington, D.C.. Rodgers, A., N. A. Petersson, S. Nilsson, B. Sjogreen, and K. McCandless (2008). Broadband waveform modeling of moderate earthquakes in the San Francisco Bay Area and preliminary assessment of the USGS 3D seismic velocity model, Bull. Seismol. Soc. Am. 98, no. 2, Shostak, N. C. (2008). A high-resolution intensity study for the 1906 San Francisco earthquake in the vicinity of San José, California, Bull. Seismol. Soc. Am. 98, no. 2, Smith, D. (2005). San Francisco is Burning, Penguin Books, London, 294 pp. Song, S., G. C. Beroza, and P. Segall (2008). A unified source model for the 1906 San Francisco earthquake, Bull. Seismol. Soc. Am. 98, no. 2, Stover, C. W., and J. L. Coffman (1993). Seismicity of the United States, (revised), U.S. Geol. Surv. Profess. Pap Thatcher, W., M. Grant, and M. Lisowski (1997). Resolution of fault slip along the 470-km-long rupture of the great 1906 San Francisco earthquake and its implications, J. Geophys. Res. Solid Earth 102, no. B3, Tobriner, S. (2006). Bracing for Disaster: Earthquake-Resistant Architecture and Engineering in San Francisco, , Heyday Books and the Bancroft Library, University of California, Berkeley, California, 351 pp. Toppozada, T. R., and D. L. Parke (1982). Areas damaged by California earthquakes, , Calif. Div. Mines Geol. Open-File Rept SAC. U.S. Geological Survey (2007). U.S. Geological Survey Photographic Library, available at (last accessed October 2007). Wald, D. J., D. V. Helmberger, and T. H. Heaton (1991). Rupture model of the 1989 Loma Prieta earthquake from the inversion of strong-motion and broadband teleseismic data, Bull. Seismol. Soc. Am. 81, no. 5, Wald, D. J., H. Kanamori, D. V. Helmberger, and T. H. Heaton (1993). Source study of the 1906 San Francisco earthquake, Bull. Seismol. Soc. Am. 83, no. 4, Working Group on California Earthquake Probabilities (2003). Earthquake probabilities in the San Francisco Bay region: , U.S. Geol. Surv. Open-File Rept Yu, E., and P. Segall (1996). Slip in the 1868 Hayward earthquake from the analysis of historical triangulation data, J. Geophys. Res. Solid Earth 101, no. B7, 16,101 16,118. U.S. Geological Survey MS977, 345 Middlefield Rd. Menlo Park, California (B.T.A.) Department of Geophysics Stanford University 397 Panama Mall Stanford, California (G.C.B.) Manuscript received 9 November 2007