Open-File Report 98-465
Total Page：16
File Type：pdf, Size：1020Kb
UNITED STATES DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY FORESHOCKS AND AFTERSHOCKS OF THE GREAT 1857 CALIFORNIA EARTHQUAKE Aron J. Meltzner and David J. Wald 39°N NEVADA 38°N 37°N 36°N 35°N 34°N 33°N Open-File Report 98-465 123°W 122°W 121°W 120°W 119°W 118°W 117°W 116°W Pasadena, California 1998 Foreshocks and Aftershocks of the Great 1857 California Earthquake By Aron J. Meltzner Caltech / U.S.G.S. and David J. Wald U. S. Geological Survey 525 S. Wilson Ave Pasadena, California, 91106 U.S.G.S. Open-File Report 98-465 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes and does not imply endorsement by the U.S. Government. Page 1 CONTENTS Abstract and Paper 3 Tables 17 Figure Captions 24 Figures 26 Appendix 1 A: Felt Foreshocks 38 Appendix IB: Felt Aftershocks 41 Appendix 2A: List of Primary Sources 67 Appendix 2B: Primary Documents 69 Appendix 3A: Modified Mercalli Scale 107 Appendix 3B: Altered Modified Mercalli Scale (Toppozada et al, 1981) 111 Appendix 4: A discussion on Confidence Parameters 113 Appendix 5: Calendar of 1857 115 Page 2 Foreshocks and Aftershocks of the Great 1857 California Earthquake Aron J. Meltzner, David J. Wald Abstract. As part of a larger effort to understand more about the behavior of the San Andreas Fault through analysis of aftershocks to major San Andreas earthquakes, we have attempted to "map out" the largest foreshocks and aftershocks of the 1857 Tort Tejon" earthquake on the central and southern segments of the fault. We searched carefully and systematically through archived first-hand accounts from 1857 through 1862, associated felt reports temporally, and assigned Modified Mercalli Intensities to each site. We then used the grid-search algorithm of Bakun and Wentworth (1997), derived from empirical analysis of modern earthquakes, to find the location and magnitude most consistent with these estimated intensities. Our analysis confirms the conclusions of Sieh (1978b) that at least two foreshocks ("dawn" and "sunrise") on or near the Parkfield segment shortly preceded the mainshock, and we estimate their magnitudes to be M = 6.1 and M « 5.6, respectively. The aftershock sequence was energetically low to average, compared with the number of aftershocks expected based on statistics of modern southern California mainshock- aftershock sequences. The most significant aftershocks included two events within just over a week of the mainshock, M « 6.25 and M = 6.7, near the southern half of the rupture, as well as a M ~ 6 event near San Bernardino nearly two years later, and a M « 6.25 event near the Parkfield segment in 1860. INTRODUCTION stress loading and redistribution as well as fault The San Andreas Fault (SAP) is the longest property heterogeneity. fault in California and one of the longest strike-slip Although a handful of aftershocks of the great faults anywhere in the world, yet we know little 1857 quake are mentioned in various catalogs and about many aspects of its behavior before, during, and articles (Townley and Alien, 1939, which was drawn after large earthquakes, because the two great historic largely from Holden, 1898; Agnew and Sieh, 1978; earthquakes (1857 and 1906) occurred before the Sieh, 1978b; Agnew et al, 1979; and Toppozada, advent of many of the modern seismological tools we 1981), no systematic search for aftershocks has take for granted today. This is particularly true of the heretofore been carried out, and none of the existing 1857 "Fort Tejon" (FT) earthquake, for which no catalogs are complete, or as complete as they can be. seismometric recordings were made. (In contrast, the The data for our study has been extracted mostly from 1906 "San Francisco" (SF) earthquake was recorded preserved newspapers, diaries, and meteorological at over one hundred seismic stations worldwide.) In reports, with occasional supplements from the this paper, we attempt to "map out" the foreshocks catalogs above. A number of the documents we used and aftershocks to the last major event on the central were taken from the appendix to Agnew and Sieh and southern segments, the FT earthquake of January (1978), which contains, in addition to first-hand 9, 1857; in a separate study we will do a similar accounts describing the mainshock, references to analysis for the SF quake of April 18, 1906, the last foreshocks and to aftershocks in the first few weeks significant event on the northern segment. The following the mainshock. A series of foreshocks were spatial and temporal distribution of aftershocks for identified and discussed by Sieh (1978b), as were two both earthquakes will then be compared, and analyzed large aftershocks within the first week; we started in the context of modern seismological insights into with a list of these events and expanded upon it. Page 3 DATA SOURCES AND appear to exist for dates after May 14, 1857,1 and no COMPLETENESS OF DATA other regular source exists for the Santa Barbara area. Information from the California Newspaper Project at Rupture during the great FT earthquake has U.C. Riverside (written communication, 1997) been documented to extend from near Parkfield to indicates that a paper under the title San Luis Obispo near San Bernardino by Sieh (1978a). In looking for Gazette was published in San Luis Obispo from 1858 aftershocks, we searched any documents we could find to 1860, although no preserved copies are known to in and around that geographical region. (Agnew, exist for that paper either. Visalia had a regular paper 1991, discusses newspapers in California in the latter beginning in June, 1859 (which underwent a series of half of the nineteenth century, and the Library of name changes), but had no regular source before that. Congress, 1984, provides a useful index to any Mission records were also checked, but as a result of newspapers available in microform today.) The most secularization in the 1830s, few records were kept, valuable sources of information for our study came and none of what was kept was helpful in our search. from Fort Tejon, located north of Los Angeles and As for northern California, newspapers were 5-6 km from the "SAP (in 1857, this was the closest checked from Santa Cruz, San Jose, San Francisco, population center to the fault rupture itself) these and Stockton, although in these cases they were only included a daily log of weather phenomena and checked on dates which corresponded to earthquakes disturbances (which included earthquakes), and the in southern or central California, and far the weeks monthly reports to the superintendent, which often which followed. (It was assumed that an earthquake included a list of felt earthquakes. These lists felt in northern California, but not anywhere in continue until the abandonment of the Fort in June of southern or central California, was not an aftershock 1861, and do not resume again until August of 1863. of the 1857 FT mainshock.) Records for Fort Miller Los Angeles also provided a regular source in (near Fresno) were also checked, although nothing the Los Angeles Star, which was published on a was reported besides the January 9th mainshock and a weekly basis uninterruptedly from 1851 through foreshock. 1864, and other newspapers and diaries were checked We chose May 1862 as a cutoff for the end of on occasion. The letters to the U.S. Coast Survey our search for aftershocks; this date (May 27) superintendent from the Survey stations at Sycamore corresponds to a large earthquake which occurred in Valley and San Fernando, dated January and San Diego, and which was followed by many widely- February, 1857, respectively (which were included in felt and widely-reported aftershocks. In looking over the appendix to Agnew and Sieh, 1978), do not the existing catalogs for the time, it was observed appear to have been repeated, as no further earthquake that no earthquakes were listed for the remainder of reports appear in letters held in the Records of the 1862 for southern California that were not felt in San Superintendent. Although San Bernardino had a Diego (with the exception of one undated event in newspaper beginning in 1860, no known copies of Santa Barbara and Goleta). With Fort Tejon having that newspaper still exist; nevertheless, there was a been abandoned in June of 1861, it is believed that if regular correspondent from San Bernardino to the Los there were any aftershocks to the FT quake this late Angeles Star who often reported on felt earthquakes in the sequence, they would have been buried in the in the area, and two diaries, both covering the year San Diego sequence, and if that were the case, we 1857, were checked. For San Diego, we used the list would be unable to properly identify them. of earthquakes catalogued by Agnew et al. (1979), A location map, showing the extent of which is believed to be as complete as can be for the mainshock rupture and all locations used as data period. points, is given as Figure 1. All primary documents Santa Barbara had a regular newspaper in the relevant to our study are reprinted as Appendix 2B to Santa Barbara Gazette, although no preserved copies 1 Only one additional isolated issue, for February 11,1858, has been located and microfilmed. Page 4 this article. A list of these sources appears as identify (on our list of "noteworthy aftershocks," Appendix 2A. A table cataloging all reports of Table 8B), at the .least, all of the following earthquakes in southern and central California (with aftershocks: some reports added for northern California) for January 8, 1857 through May 26, 1862 is given as all aftershocks SE of the Carrizo Plain, Jan.