Study of the 1906 San Francisco Earthquake

All quality teleseismic recordings of the great 1906 San Francisco earthquake archived in the 1908 Carnegie Report by the State Earthquake Investigation Commission were scanned and digitized. First order results were obtained by comparing complexity and amplitudes of teleseismic waveforms from the 1906 earthquake with well calibrated, similarly located, more recent earthquakes (1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta earthquakes) at nearly co-located modern stations. Peak amplitude ratios for calibration events indicated that a localized moment release of about 1 to 1.5 × 1027 dyne-cm was responsible for producing the peak the teleseismic body wave arrivals. At longer periods (50 to 80 sec), we found spectral amplitude ratios of the surface waves require a total moment release between 4 and 6 × 1027 dyne-cm for the 1906 earthquake, comparable to previous geodetic and surface wave estimates (Thatcher, 1975). We then made a more detailed source analysis using Morgan Hill S body waves as empirical Green's Functions in a finite fault subevent summation. The Morgan Hill earthquake was deemed most appropriate for this purpose as its mechanism is that of the 1906 earthquake in the central portion of the rupture. From forward and inverse empirical summations of Morgan Hill Green's functions, we obtained a good fit to the best quality teleseismic waveforms with a relatively simple source model having two regions of localized strong radiation separated spatially by about 110 km. Assuming the 1906 epicenter determined by Bolt (1968), this corresponds with a large asperity (on the order of the Loma Prieta earthquake) in the Golden Gate/San Francisco region and one about three times larger located northwest along strike between Point Reyes and Fort Ross. This model implies that much of the 1906 rupture zone may have occurred with relatively little 10 to 20 sec radiation. Consideration of the amplitude and frequency content of the 1906 teleseismic data allowed us to estimate the scale length of the largest asperity to be less than about 40 km. With rough constraints on the largest asperity (size and magnitude) we produced a suite of estimated synthetic ground velocities assuming a slip distribution similar to that of the Loma Prieta earthquake but with three times as much slip. For purposes of comparison with the recent, abundant Loma Prieta strong motion data set, we "moved" the largest 1906 asperity into Loma Prieta region. Peak ground velocity amplitudes are substantially greater than those recorded during the Loma Prieta earthquake, and are comparable to those predicted by the attenuation relationship of Joyner and Boore (1988) for a magnitude M w = 7.7 earthquake. INTRODUCTION The great 1906 San Francisco earthquake began an era in earthquake seismology. Following this earthquake, direct observations of surface displacement combined with the analysis of the surrounding crustal deformation led Reid (1910) to formulate the elastic rebound theory. Although much has been learned from the numerous studies of the 1906 earthquake, a systematic analysis of the recorded teleseismic body and surface waveforms has not been made. Yet, the *Now at U.S. Geological Survey, 525 S. Wilson Avenue, Pasadena, CA 91106. 981 982 D. J. WALD E T AL. seismic recordings of the 1906 earthquake have been well preserved in the Atlas of the 1908 Carnegie Report by the State Earthquake Investigation Commission (Lawson, 1908), hereafter referred to as the Atlas or the Report. It is the authoritative reference and summary of the 1906 earthquake, including geological observations, the effects of ground shaking, and all the data collected following the earthquake. In this study, we revisit the waveform data set contained in the Atlas and analyze the data in the context of modern source analysis. The need to understand the ground motion hazard potential from earthquakes in the San Francisco area has been rekindled by the occurrence of and damage from the 1989 Loma Prieta earthquake. The Loma Prieta event has provided a valuable strong motion data set for analysis of source complexity and ground motion damage from a magnitude 7 earthquake. Unfortunately, local strong ground motion data from the (much larger) 1906 earthquake was limited to one off-scale, partial recording on the Ewing three-component seismograph at Mt. Hamilton (Boore, 1977). Few strong motion recordings have been made from any large strike-slip earthquakes. However, it is possible to obtain source information relevant to understanding the local strong motions through analysis of the teleseismic data. In a separate study of the Loma Prieta earthquake, Wald et al. (1991) inverted the broadband teleseismic and local strong motion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods 3 to 30 sec) and strong motion data (periods 1 to 5 sec) resulted in similar rupture models. Hence, the broadband teleseismic data has the capability of providing important constraints on the nature of the strong motions at ]ong periods, independent of the strong motion recordings. In the study that follows, we apply this insight to the 1906 earthquake, although clearly the quality and bandwidth of the historic data are not as impressive as the modern digital, broadband data. Our study focuses on several important unresolved issues relevant to the 1906 rupture. Was the 1906 rupture complex or were there large portions of the fault where rupture was fairly uniform? What was the nature and location of fault asperities? As we will show, the body waveforms are fairly simple considering the rupture duration expected for such a large rupture length (at least 300, and likely 430 km). Did the Loma Prieta section of the fault have a dip-slip component? The geodetic study of Segall and Lisowski (1990) requires a few meters of strike-slip motion for 1906 along the Loma Prieta segment of the fault, but their data does not rule out a thrust component comparable to the Loma Prieta earthquake at greater depths. Is there evidence for a dip-slip component in this or other portions of the fault? We address these issues in this study. Processing and interpreting the turn-of-the-century seismic data recorded presented many challenges. However, we believe that the historic data are valuable despite their limitations, and thus, it is desirable to try and obtain as much information from them as possible considering the importance of the 1906 San Francisco earthquake. Hence, we have revisited the data available for the 1906 earthquake in an effort to place constraints on the nature of that rupture, relate the radiated seismic energy to fault breakage and geodetic offset measurements, and to determine its relationship to the Loma Prieta rupture. Although the records of the 1906 earthquake alone may be insufficient to SOURCE STUDY OF THE 1906 EARTHQUAKE 983 resolve the above questions, the use of records from the Loma Prieta, Morgan Hill, and Coyote Lake earthquakes first as calibration events, and then as empirical Green's functions assists in extracting important information from this unique data set. The analysis of the teleseismic data proves useful in answering questions about fault rupture style on the San Andreas Fault and asperity positions in addition to allowing an estimation of strong ground motions likely experienced during the 1906 earthquake. OVERVIEW OF PREVIOUS STUDIES The significance of the 1906 earthquake resulted in careful collection and mapping of the geologic, geodetic, seismic, and sociological data. A wealth of investigations have been made, and scientific studies of this event still appear in the geophysical literature occasionally. The occurrence of the 1989 Loma Prieta earthquake rejuvenated interest in previous San Francisco Bay area earthquakes, particularly the 1906 event. Most recently, reanalysis has been made of both the geodetic (Segall and Lisowski, 1990) and surface offset data (Prentice and Schwartz, 1991). Constraints on the rupture characteristics are provided by previous studies of the epicenter, surface offset, geodetic slip, and mapped isoseismal distributions. Surface Offset and Geodetic Observations The 1906 surface rupture is known to have ruptured about 300 km from San Juan Bautista (or Chittenden) to Point Arena (Fig. 1). It is commonly assumed that the rupture continued offshore for 140 km to Cape Mendocino. This was initially based on an observation of surface rupture at Shelter Cover (near Point Delgada, Fig. 1), although the amount of slip at Point Delgada was never documented, and it might not be of tectonic origin. Other equivocal evidence for offshore rupture is suggested by other observations, but questions about the offshore extension of rupture have not been eliminated. Observations that support the extension of rupture offshore include the impressive ground shaking and damage in the Cape Mendocino region as shown by the 8 to 9 Modified Mercalli isoseismal values (Fig. 2). In addition, a linear zone of strong shaking (X on the Rossi-Forel scale), narrower but similar to regions along the fault trace further south, is evident in the Atlas map of apparent intensities but is not so clear because of the limited number of data points in Figure 2. The Report also documents many strongly felt aftershocks within the same region, many of which occurred locally considering they were not reported at locations toward the southeast. Furthermore, geodetic modeling by Thatcher and Lisowski (1987a) favors about 4 to 6 m of displacement to a depth of 10 km on the offshore region to satisfy the distortion of the geodetic network onshore to the east. Alternatively, the strong shaking along the extension of the northwest terminus of a rupture propagating over 200 km in that direction would be expected from the effects of source directivity. Likewise, aftershocks commonly occur well off the end of the rupture zone (i.e., the 1992 Landers earthquake) and thus do not necessarily reflect the true source dimension. Concerning the geodetic evidence for large 1906 offsets offshore, the data of Thatcher and Lisowski (1987a) spanned a very long duration from about 1880 to 1940 and were, of course, limited to a one-sided, onshore network well east of the rupture. Therefore, their resolution is not good, and any displacement observed was not 9 8 4 D . J . WALD E T AL.