14Toppozada.qxd 509 ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004 Key words California – historical earthquakes 1. Introduction California’s written history is very short. It started in 1769 with the diaries of the first Spanish explorers. That year they felt earth- quakes in the vicinity of Los Angeles during their travel from San Diego northward. The Spanish priests established 21 Catholic mis- sions near the coast, from San Diego in 1769 to San Francisco de Solano in 1823 (fig. 1). Many of the missions’ annual reports are available in the Santa Barbara Mission Archives. These reports documented the num- bers of births, deaths, baptisms, livestock, agricultural harvest, and mentioned earth- quakes when they damaged the buildings. The Missions were secularized in 1834, ending the annual reports and leading to general aban- donment and disrepair. For the next 15 years, until the 1849 Gold Rush and United States statehood, the sources of information were mostly the diaries of trav- elers and the records of ships’ captains. In 1849 regular newspaper publishing started in San Francisco and in the gold country east of Sacramento. Felt earthquakes were noted in the newspapers, which continued to spread across California with time as the population increased. The descriptions of the earthquakes were cataloged by various contemporary re- searchers, and were later interpreted in terms California earthquake history Tousson Toppozada and David Branum California Geological Survey, Sacramento, CA, U.S.A. Abstract This paper presents an overview of the advancement in our knowledge of California’s earthquake history since ~ 1800, and especially during the last 30 years. We first review the basic statewide research on earthquake occur- rences that was published from 1928 through 2002, to show how the current catalogs and their levels of com- pleteness have evolved with time. Then we review some of the significant new results in specific regions of Cali- fornia, and some of what remains to be done. Since 1850, 167 potentially damaging earthquakes of M ~ 6 or larg- er have been identified in California and its border regions, indicating an average rate of 1.1 such events per year. Table I lists the earthquakes of M ~ 6 to 6.5 that were also destructive since 1812 in California and its border re- gions, indicating an average rate of one such event every ~ 5 years. Many of these occurred before 1932 when epi- centers and magnitudes started to be determined routinely using seismographs in California. The number of these early earthquakes is probably incomplete in sparsely populated remote parts of California before ~ 1870. For ex- ample, 6 of the 7 pre-1873 events in table I are of M ≥ 7, suggesting that other earthquakes of M 6.5 to 6.9 occurred but were not properly identified, or were not destructive. The epicenters and magnitudes (M) of the pre-instru- mental earthquakes were determined from isoseismal maps that were based on the Modified Mercalli Intensity of shaking (MMI) at the communities that reported feeling the earthquakes. The epicenters were estimated to be in the regions of most intense shaking, and values of M were estimated from the extent of the areas shaken at various MMI levels. MMI VII or greater shaking is the threshold of damage to weak buildings. Certain areas in the regions of Los Angeles, San Francisco, and Eureka were each shaken repeatedly at MMI VII or greater at least six times since ~ 1812, as depicted by Toppozada and Branum (2002, fig. 19). Mailing address: Dr. Tousson Toppozada, California Geological Survey, 801 K Street, Sacramento, CA 95814, U.S.A.; e-mail: ttoppoza@consrv.ca.gov 510 Tousson Toppozada and David Branum of shaking intensities, and ultimately earth- quake epicenters and magnitudes (table I). The routine seismographic determinations of earthquake epicenters and magnitudes (M) started in southern California in 1932. By com- paring the shaking intensities of the modern in- strumentally determined earthquakes to those of pre-1932 events, the epicenters and magni- tudes of the pre-instrumental earthquakes could be estimated with some consistency. The short earthquake history has shown that most regions in California are suscepti- ble at various levels to earthquake damage. This information has been applied to earth- quake hazard assessment and mitigation at critical structures such as nuclear power Fig. 1. Index map showing the California Missions and the dates the missions were established. All missions were secularized in 1834 (modified from Toppozada et al., 2002). 511 California earthquake history Table I. Significant California earthquakes (M ~ 6 to 6.5 and/or destructive). Date (*) Latitude Longitude M Region Loss of life and property 1812.12.08 34.37 (?) –117.65 (?) ~ 7.3 Orange County, Los Angeles, Wrightwood 40 deaths at San Juan Capistrano 1812.12.21(**) 34.75 (?) –118.60 (?) ~ 7.1 Los Angeles, Ventura, Santa Barbara 1 death 1838.06.00 37.30 (?) –122.15 (?) ~ 7.4 San Francisco to San Juan Bautista Damage from San Francisco to Monterey 1857.01.09 36.20 –120.80 7.9 Great Fort Tejon earthquake 1 death, damage from Monterey Co. to San Bernardino Co. 1865.10.08 37.20 –121.90 6.5 Santa Cruz Mountains $500 000 in property loss 1868.10.21 37.70 –122.10 7.0 Hayward Fault 30 deaths, $350 000 loss 1872.03.26 36.70 –118.10 7.4 Owens Valley 27 deaths, 56 injuries, $250 000 loss 1873.11.23 42.00 (?) –124.20 (?) 6.9 Crescent City regoin Damage in California - Oregon border area 1890.02.09 33.40 –116.30 6.8 San Jacinto fault? Little damage 1892.02.24 32.55 –115.65 7.3 Laguna Salada, Baja California Damage San Diego to Imperial Valley 1892.04.19 38.40 –122.00 6.6 Vacaville 1 death, $225 000 loss 1898.03.31 38.20 –122.50 6.4 Mare Island $350 000 loss 1898.04.15 39.20 –123.80 ~ 6.7 Fort Bragg - Mendocino Damage from Fort Bragg to Mendocino 1899.12.25 33.80 –117.00 6.7 San Jacinto and Hemet 6 deaths, $50 000 loss 1906.04.18 37.70 –122.50 7.8 Great 1906 earthquake 3 000 deaths, $524 million loss (counting fire damage) 1918.04.21 33.75 –117.00 6.8 San Jacinto 1 death, several injuries, $200 000 loss 1923.01.22 40.40 –124.90 7.2 Off Cape Mendocino Destructive in Humbolt Co., strongly felt to Reno 1925.06.29 34.30 –119.80 6.8 Santa Barbara 13 deaths, $8 million loss 1927.11.04 34.60 –120.90 7.1 40 km west of Lompoc Damage in Santa Barbara and San Luis Obispo counties 1933.03.11 33.70 –118.00 6.4 Long Beach 115 deaths, $40 million loss 1940.05.19 32.73 –115.50 7.0 Imperial Valley 9 deaths, $6 million loss 1952.07.21 35.00 –119.02 7.3 Kern County earthquake 12 deaths, $60 million loss 1954.12.21 40.93 –123.78 6.6 East of Arcata 1 death, several injuries, $2.1 million loss 1971.02.09 34.41 –118.40 6.6 San Fernando 65 deaths, 2,000 + injuries, $505 million loss 1979.10.15 32.61 –115.32 6.5 Imperial Valley 91 injuries, $30 million loss 1980.11.08 41.12 –124.67 7.4 West of Eureka 6 injuries, $2 million loss 1983.05.02 36.23 –120.31 6.4 Coalinga $31 million loss, 1 death, 47 injuries 1984.04.24 37.31 –121.68 6.2 Morgan Hill $8 million loss 1987.10.01 34.07 –118.08 6.0 Whittier Narrows 8 deaths, $358 million loss 1987.11.24 33.01 –115.85 6.6 Superstition Hills $3 million loss 1989.10.18 37.04 –121.88 6.9 Loma Prieta 63 deaths, 3,757 injuries, $6 billion loss 1992.04.25 40.33 –124.23 7.2 Cape Mendocino area 356 injuries, $48.3 million loss. Two M 6.6 aftershocks next day 1992.06.28 34.20 –116.44 7.3 Landers 1 death, 402 injuries, $91.1 million (losses include Big Bear, M 6.5, earthquake 3 hours later) 1994.01.17 34.21 –118.54 6.7 Northridge 57 deaths, 9000 + injuries, ~$40 billion loss 1999.10.16 34.60 –116.27 7.1 Hector Mine Minimal injuries and damage due to sparse population Loss from Stover and Coffman (1993). (*) The date is given as year, month, day (Greenwich). (**) Proposed epicenter on or near San Andreas Fault (Toppozada et al., 2002). 512 Tousson Toppozada and David Branum plants, dams, schools and hospitals. The his- torical earthquake information is also an im- portant component of the probabilistic seis- mic hazards assessments that are part of the modern building codes. 2. Earthquake catalogs in California 2.1. Before 1970 The most comprehensive catalog of histor- ical California earthquakes available before 1970 was that of Townley and Allen (1939). This provided mostly narrative descriptions of historically reported felt earthquakes, from the 1769 Spanish exploration of California to 1928. It did not generally provide epicenter coordinates or magnitudes for the earthquakes. The sources of information were mostly Catholic Mission documents before the 1849 Gold Rush, and Newspapers and varied corre- spondence thereafter. The Bulletin of the Seismological Society of America (BSSA) started publication in 1910, and has included many articles on various Cal- ifornia earthquakes. The BSSA section titled «Seismological Notes» describes the shaking and felt effects of contemporary earthquakes, with some seismographic information generally for post-1932 events. Since 1928, the annual publication United States Earthquakes (USE) has provided de- scriptions of felt effects for earthquakes in the United States, and available seismographic in- formation, such as epicenter coordinates and magnitude. Since 1950 USE has included rough isoseismal maps of some of the most widely felt earthquakes. Special publications were also available for numerous significant events, including the ma- jor San Andreas Fault earthquakes of 1857 (Wood, 1955) and 1906 (Lawson, 1908). 2.2. The 1970s Hileman et al. (1973) published the Califor- nia Institute of Technology (CIT) seismograph- ic catalog of earthquakes from 1932 to 1972 for Southern California, roughly south of the 36th parallel. This catalog lists instrumentally deter- mined epicenter coordinates and magnitudes, and provides annual and multi-year epicenter maps of earthquakes of various magnitudes, generally of M > 3. Online updates of the CIT catalog are available at: http://www.scec dc.scec.org/catalogsearch.html Bolt and Miller (1975) published the Uni- versity of California, Berkeley (UCB) catalog of earthquakes from 1910 to 1972, for northern California, roughly north of the 36th parallel. Their estimates of pre-1942 earthquake location and size were approximate, and generally not determined from seismographs. These esti- mates were largely based on Townley and Allen’s (1939) descriptive catalog, except for a few seismographic values of epicenter and magnitude (M) quoted from Gutenberg and Richter (1949) and CIT. Starting in 1942, UCB seismographic records were used to determine epicenters and M for generally M > 3 earth- quakes. Online updates of the UCB catalog are available at: http://quake.geo.berkeley.edu/nced c/catalog-search.html Toppozada (1975) analyzed the size of the ar- eas shaken by earthquakes in California and Western Nevada at various Modified Mercalli In- tensity levels (MMI), and related them to Richter local magnitude. These relations were subse- quently used in the 1970s and 1980s to estimate the magnitudes of preinstrumental earthquakes. Toppozada et al. (1978) interpreted the felt effects and estimated epicenters and magni- tudes for the most strongly felt California earth- quakes occurring from 1900 through 1931. They based their estimates on the felt effects and size of areas shaken at various levels of in- tensity as reported by Townley and Allen (1939) and USE. Supplemental information came from felt shaking effects described in pre- 1932 newspapers, BSSA, and available seismo- graphic information including the UCB Bul- letin of the Seismographic Stations. They se- lected 517 events of probable M ~ 4 or larger based on Toppozada’s 1975 relations, having Modified Mercalli Intensity (MMI) of V or larger or felt area ~7000 square km or larger. They determined that at least 229 earthquakes were of M ≥ 4, but the available MMI and felt 513 California earthquake history area data were not sufficient to allow robust es- timates of magnitude for the remaining events, that were probably of M ~ 4. Real et al. (1978) combined this pre-instru- mental 1900-1931 catalog with a 1932-1975 catalog that they created by merging the CIT and UCB catalogs, and the USGS catalog for Central Coastal California since 1969. Their resulting 1900-1975 catalog was available on computer tape, and on an epicenter wall map of 3600 earthquakes of M~4 or larger, or MMI V or greater if M was unknown. The catalog lists the event dates, epicenter coordinates, and M, and includes 452 events of M ≥ 5. This catalog is essentially pre-instrumental before 1932 in Southern California, and before 1942 in North- ern California. 2.3. The 1980s Toppozada et al. (1981) analyzed in detail the felt effects of the most strongly felt California earthquakes between 1769 and 1900. They se- lected earthquakes from Townley and Allen (1939) of probable M ~ 5 or larger based on Top- pozada’s 1975 relations, having MMI at least VI or felt area at least 30 000 square km. They con- structed isoseismal maps for 100 earthquakes, showing the reporting points and their MMI val- ues. They supplemented the published intensity information by researching the reports of Spanish Missionaries from 1769 to 1840, mainly in the Santa Barbara Mission archives. After the advent of regular newspaper publishing with the 1849 Gold Rush, reports of feeling earthquakes be- came available beyond the coastal zone where the Missions were located (fig. 1), and extended in- land to the goldfields East of Sacramento (fig. 2a). The newspaper coverage gradually expanded to cover most of California by the 1880s (fig. 2b). Pre-1932 earthquakes were best known in the areas that had Catholic Missions before the 1840s, and newspaper coverage after 1849. The State epicenter map (fig. 3) employs three sym- bol shapes, Xs, circles and triangles, to identify Fig. 2a. Locations of newspapers in print, 1850- 1859 (modified from Toppozada et al., 1981). Fig. 2b. Locations of newspapers in print, 1880- 1889 (modified from Toppozada et al., 1981). Eureka Sacramento San Francisco Los Angeles Eureka Sacramento San Francisco Los Angeles 514 Tousson Toppozada and David Branum the three approximate thirds of the 200-year record. It shows that before 1869, earthquakes indicated by Xs were relatively well known in the central and southern coast and in the gold and silver fields northeast of Sacramento, but not in the northwestern or southeastern regions of California. After 1868 when the newspaper coverage expanded to these remote regions their high seismicity became apparent, as indi- cated in fig. 3 by the circles and triangles in these areas. The San Andreas Fault region be- tween the 36th and 37th parallels was much more active before than after 1932, as illustrat- ed by the Xs and circles greatly outnumbering the triangles in this region (fig. 3). This will be discussed below under the «Parkfield region». Fig. 3. Magnitude greater or equal to 5.5 California earthquakes, 1800-2000 (modified from Toppozada and Branum, 2002). 515 California earthquake history Most of Toppozada et al. (1981)’s data re- sulted from searching about 12 000 newspaper issues for towns that might have felt the earth- quakes, for several issues after each event. About a quarter of the issues searched provided some earthquake descriptions, although many descriptions were duplicated in various news- papers. A bibliography of the newspapers searched for each earthquake was appended to the 1981 report. Also, a summary of the report- ed felt effects for each earthquake and the cor- responding MMI value at each reporting town was appended to the 1981 report. Toppozada and Parke (1982) completed a similar study of the areas damaged by California earthquakes that caused MMI VI to VII or greater effects in California, from 1900 to 1949. This study provided isoseismal maps and sum- maries of reported effects, which were not avail- able in USE (1928-1949) or in Toppozada et al. (1978)’s study of pre-1932 events. A newspaper bibliography and a summary of the reported ef- fects were also appended to the 1982 report. 2.4. 1990 to 2003 Ellsworth (1990) reviewed the history of major California and Nevada earthquakes from 1769 to 1989, and compiled a comprehensive catalog of M ≥ 6 earthquakes. This included the known values of the various magnitude types for each earthquake, derived from MMI, short and long period seismic records, seismic mo- ment, and a summary or preferred M. Stover and Coffman (1993) summarized the information from USE, Toppozada et al. (1981), and other available sources, and provid- ed a catalog and felt descriptions for historical earthquakes of M ≥ 4.5. They also provided iso- seismal maps for some of the most significant California events, generally of M ≥ 6.5. Toppozada et al. (2000) compiled a wall map of epicenters of M ≥ 5 earthquakes from 1800 to 1999. This map lists the date, co-ordinates, and magnitude of the 383 events of M ≥ 5.5. An inset map shows the areas damaged at MMI VII or greater by these earthquakes, which is the thresh- old of damage to weak buildings, and the number of times the various areas have been damaged. Certain areas in the regions of Los Angeles, San Francisco, and Eureka were shaken repeat- edly at MMI VII or greater at least six times each since ~1800. A generalized subset of this wall map is available on the web at: http://www.con- srv.ca.gov/cgs/rghm/quakes/MS49.htm Toppozada and Branum (2002) summarized the information on the M ≥ 5.5 historical earth- quakes, and the areas that they damaged. For each earthquake they listed all known values of M, and identified a preferred M. They obtained instru- mental magnitudes generally for post-1932 events from the earthquake catalogs of CIT and UCB, Ellsworth (1990) for M ≥ 6.0 events, Hut- ton and Jones (1993), Stein and Hanks (1998), and Bakun (1999, 2000). The preferred magni- tude was moment M when available. Otherwise, the order of preference was generally Mg-r (Gutenberg and Richter, from 20-sec surface waves); Ms (from surface waves, before 1935 generally from un-damped Milne seismographs according to Abe, 1988); ML (local M of Richter, 1935); Ma (based on the size of the areas shaken at or above MMI V, VI, VII); Mi (from Bakun 1999, 2000, using Toppozada et al., 1981, MMI data). They also showed the spatial and temporal evolution of the seismicity and the areas damaged at MMI VII or greater from 1800 to 2000. 2.4.1. Updated relations between areas shaken and M Toppozada and Branum (2002) developed new relations between the moment magnitude of mod- ern California earthquakes of M 5.5 to 7.3 and the areas in square km that they shook at or greater than MMI V, VI, or VII (fig. 4). The standard devi- ations of the three linear least squares fits range from 0.11 magnitude units for M derived from MMI3V, to 0.17 for M derived from MMI3VII: MV = 0.83Log10 AreaV + 2.43 St. Dev. = 0.11 MVI = 0.73Log10 AreaVI + 3.44 St. Dev. = 0.13 MVII = 0.67Log10 AreaVII + 4.29 St. Dev. = 0.17 516 Tousson Toppozada and David Branum Toppozada and Branum (2002) and Toppozada et al. (2002) used these relations to derive estimates of magnitude of pre-instrumental earthquakes from the areas shaken at these intensities or greater. For example the area shaken at MMI V or greater by the 12 April 1885 earthquake is that enclosed by the innermost two contours in fig. 5. That is the whole area from Visalia to Santa Cruz (E to W) and from Oakdale to Santa Maria (N to S). The values of MMI are determined at the points that described the shaking effects, and the contours are drawn to distinguish between areas having different values of MMI. The new equations result in magnitudes that are approximately 0.3 to 0.4 units higher than those resulting from Toppozada (1975)’s rela- tions, which were based on local magnitude val- ues and partly on Nevada earthquakes. Ellsworth (1990) independently found that Toppozada (1975)’s relations underestimated M by about 0.2 to 0.3 units for modern California earth- quakes. The 2002 relations are superior to the 1975 relations because: i) they are based on Cal- ifornia earthquakes only whereas the 1975 rela- tions included data from several Nevada events; ii) they are based on modern events up to the year 2000 whereas the 1975 relations included no data after 1971; and iii) they are based on moment M whereas the 1975 relations were based on local M. 2.4.2. Selected regions The earthquake history of selected regions of California has also been studied. For example, Agnew (1991) studied the completeness of the historical earthquake record in Southern Califor- nia, in light of the distribution of newspapers and other available sources of information. Bakun (1999, 2000) studied the seismicity of the San Francisco Bay region, and of California’s North Coast, using the method of Bakun and Wentworth (1997) to determine epicenters and magnitudes for pre-instrumental earthquakes. Uhrhammer (2003) is currently studying the early seismographic record (1910-1950) in the San Francisco Bay region, using the UCB seismic records and notebooks to revise the early epicenter and M determinations. This should distinctly improve the earthquake record before 1942 when UCB started to rou- tinely determine epicenters and magnitudes using seismic records. Fig. 4. Moment magnitude as a function of the areas, in km2, shaken at or greater than MMI V, VI, VII (mod- ified from Toppozada and Branum, 2002). 517 California earthquake history Fig. 5. 12 April 1885 isoseismal map. Numbers at towns are the MMI values based on the felt effects. When felt effects did not indicate a MMI value we use a letter: F, felt; S, severe; L, light; H, heavy. The triangle repre- sents the epicenter. The three dashed lines represent the areas of MMI greater than or equal to II-IV, V or VI used in the magnitude-area relations of Toppozada and Branum (2002) (modified from Toppozada et al., 2002). 3. Significant results 3.1. Parkfield region The Parkfield Earthquake prediction by Bakun and Lindh (1985) followed, and was based largely on the historical earthquake cata- log work available at that time. The prediction was based on the observation that M ~ 6 events had occurred quasi-regularly near Parkfield in 1857 (identified by Sieh, 1978), 1881 (identi- fied by Toppozada et al., 1981), 1901, 1922, 1934, and 1966 (Bakun and McEvilly, 1979, 1984). This simple model predicted that the next M ~ 6 earthquake would occur before 1993. The predicted earthquake has not yet oc- curred as of this writing (September 2003). Toppozada et al. (2002) undertook a de- tailed study of the earthquake history of the Parkfield region and adjoining segment of the San Andreas fault, based on exhaustive re- search of newspapers and local diaries. This study showed that the pre-1932 seismicity of Parkfield and the adjoining segment of the San Andreas fault to the north was much greater than previously thought, and has been decreas- ing steadily from 1857 to the present time. The simple 1985 model of Bakun and Lindh does not include the high pre-1900 San Andreas Fault seismicity within 70 km NW of Park- field. This seismicity brackets the northern end of the major 1857 San Andreas fault rup- ture, which is at the largest 1857 epicenter symbol (X) in fig. 6a. Epicenters of the strong 1885 earthquake of M ~ 6.5 (fig. 5) and other pre-1932 M ~ 6 earth- quakes in this zone are shown in fig. 6a. The epicenters were estimated at the centers of strongest MMI. Relative locations of neighbor- ing earthquakes were determined by comparing their MMI values at the same towns (e.g., Park- field, Salinas, etc.). The earthquakes in fig. 6a have isoseismal maps or descriptions of MMI in Toppozada et al. (2002). Their rate of occur- rence indicates a decaying earthquake sequence (fig. 6b). This suggests post-earthquake stress decay at the northern end of the 1857 fault rup- ture, and helps explain the delay of the predict- ed earthquake. Figures 6a and 6b show that M ≥ 5.5 earthquakes have occurred in the San Andreas fault zone within 70 km northwest of Parkfield ten times from 1857 to 1932, but on- ly twice after 1932. Ben-Zion et al. (1993) modeled a decreas- ing stress at the terminus of the 1857 rupture that is generally compatible with that indicated by our observed decrease in earthquake rate. They concluded that the decreasing stress could delay the predicted Parkfield earthquake from ~1988 to ~1995. 3.2. December 1812 Southern California earthquakes Toppozada et al. (2002) re-evaluated the ma- jor earthquakes that occurred on 8 and 21 De- cember 1812, damaging the Missions from San Juan Capistrano to Purisima Concepcion (fig. 7), in Orange, Los Angeles, Ventura and Santa Bar- bara counties. They found that the 1812 San An- dreas faulting indicated by Jacoby et al. (1988), extending ~170 km from near Cajon Pass to near Fort Tejon, could have ruptured in two events and generated both earthquakes (fig. 7). Tree ring studies by Jacoby et al. (1988) placed the first event that damaged mostly San Juan Capistrano and San Gabriel, on the Wrightwood segment that includes Cajon Pass and Pallett Creek. Toppozada et al. (2002) concluded that the second faulting event 13 days later could have started from the west end of the first, with rupture directed to the west toward Fort Tejon and Purisima Concepcion. It damaged mostly Santa Barbara, Santa Inez and Purisima. Missions San Fernando and San Bue- naventura were damaged by both the 8 and 21 De- cember 1812 earthquakes (fig. 7). The damage at Purisima Concepcion was stronger than that at Santa Inez. This was appar- ently due to Mission Purisima’s location on sloping ground at the edge of a marsh (Toppoza- da et al., 1981). Also, Rancho de San Antonio, 17 km north of Purisima, was not damaged (fig. 7). The adverse site effects at Mission Purisima were recognized by the padres and noted in their 1813 written communications. They conse- quently rebuilt Mission Purisima on flat dry ground, 5 km north of the 1812 ruins. If the 1812 faulting extends westward to Fort Tejon, or beyond to Mil Potrero as suggested by 518 Tousson Toppozada and David Branum 519 California earthquake history Fig. 6a,b. a) Epicenters of M ≥ 5.5 earthquakes in the Parkfield-Bitterwater region and surrounding areas. Sym- bol size is proportional to M and shape indicates time period. The dashed region is that considered in fig. 6b (Modified from Toppozada et al., 2002). b) Cumulative seismic moment released in the Parkfield-Bitterwater zone of the San Andreas fault from 1857 to 2001 (Modified from Toppozada et al., 2002). a b 1 8 5 0 1 8 6 0 6 C u m u la ti v e M o m e n t (1 0 2 6 d y n e -c m ) Date 5.5 5 4.5 4 3.5 3 2.5 1 8 7 0 1 8 8 0 1 8 9 0 1 9 0 0 1 9 1 0 1 9 2 0 1 9 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 9 9 0 2 0 0 0 520 Tousson Toppozada and David Branum Fig. 7. Missions damaged by the December 1812 earthquakes. We divide the 1812 rupture proposed by Jaco- by et al. (1988) into 2 segments to account for the damage observed on 8 and 21 December, and on both dates (modified from Toppozada et al., 2002). Davis (1983), it cannot all have ruptured on 8 December. Faulting that extends that far west- ward would damage Santa Barbara, which was not damaged on December 8th but on the 21st. Thus the total 1812 faulting suggested by Jaco- by et al. (1988) and depicted in fig. 7 would sug- gest that the western segment ruptured on the 21st. Definition of the western extent of the total 1812 faulting is complicated by the major 1857 earthquake rupture that extended from near Parkfield to Cajon Pass, and re-ruptured the 1812 faulting after only 44 years. The conventional Santa Barbara Channel epi- center of the 21 December earthquake was based largely on a tsunami that resulted from the event. However, it is becoming clear that tsunamis are often generated by submarine landslides. Lander et al. (1993) interpreted the description of the 1812 sea wave as «a submarine landslide-gener- ated wave and not one generated by a (subma- rine) fault movement source». Such landslides are mapped frequently in the Santa Barbara Channel (Greene and Kennedy, 1989), and could have been triggered by the 1812 earthquake shaking (Greene et al., 2000). 3.3. 1830s San Francisco Bay area earthquakes The destructive 1989 Loma Prieta earth- quake provided the impetus for Toppozada et al. (1996) to study the San Francisco bay area earthquake history in detail. This lead to a bet- 521 California earthquake history ter understanding of the major 1838 San An- dreas fault event and to a radical re-evaluation of the 1836 «Hayward Fault earthquake» by Toppozada and Borchardt (1998). They concluded that the 1838 earthquake was of M ~ 7.4, significantly larger than previ- ously thought. The 1906 San Francisco earth- quake of M 7.8 occurred 68 years later as a re- sult of major San Andreas faulting that over- lapped and reruptured the 1838 faulting. This pair of events is similar to the 1857 Central San Andreas Fault earthquake of M 7.9 whose major faulting overlapped and reruptured the faulting associated with the1812 earthquakes of M ~ 7.3 and ~ 7.1. Thus the 1857 and 1906 M 7.8 and 7.9 San Andreas faulting events have each overlapped a M ~ 7.4 faulting event that occurred 44 to 68 years earlier. Earthquake ruptures that occur 44 to 68 years apart are difficult to differentiate in fault trenches because of the limitations in stratigraphic resolution and paleoseismic dat- ing. This suggests that overlapping ruptures oc- curring a few decades apart can be more com- mon than paleoseismology would indicate. Toppozada and Borchardt (1998) also found that the 1836 event was not a M ~ 7 event on the Hayward fault as was previously believed, but rather a M ~ 6 1/2 event ~ 100 km to the south, in the general vicinity of Monterey Bay. The idea of an 1836 Hayward Fault earthquake had been based on successive false assumptions and misinterpretations of information, which Top- pozada and Borchardt summarized in their table 2 : ‘Evolution of the Myth concerning the 1836 «Hayward Fault earthquake»’. 4. Current work We are building a database of the felt effects of M > 5 historical earthquakes. For each earth- quake there are various towns that reported felt effects. The database will have a separate entry for each earthquake and each town that report- ed the event. We have digitized the data previ- ously analyzed by Toppozada et al. (1981) and Toppozada and Parke (1982). We will supple- ment this with unpublished data that we have analyzed, and also add information for post- 1950 events from USE. The database will be searchable for event dates, towns, geographic regions, and levels of MMI shaking. This will allow us to determine how many times specific towns, as well as regional areas, have felt different levels of MMI shaking. The database will be linked to digital isoseismal maps of the corresponding earthquakes. These maps and data will be published on a website to allow comparing the felt effects of current earthquakes to those of neighboring historical events. This will permit a rapid assessment of the potential MMI shaking and damage from a current strong earthquake from the shaking of neighboring historical events of similar magni- tude. It will also allow recalibrating the epicen- ters and magnitudes of preinstrumental earth- quakes by comparing their intensities of shak- ing to those of nearby well-instrumented mod- ern earthquakes. REFERENCES ABE, K. (1988): Magnitudes and origin times from Milne seismograph data: earthquakes in China and California, 1898-1912, in Historical Seismograms and Earth- quakes of the World, edited by W.H.K. LEE, H. MEYERS and K. 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