Aron Meltzner * B.S. in Geology, Class of 2000 * California Institute of Technology * Pasadena, CA
Research and Publications
Study of the Aftershocks and Triggered Events of the 1906 San Francisco Earthquake
My present work involves an attempt to learn about the aftershock sequence of the 1906 San Francisco earthquake -- which has never been studied in a comprehensive manner -- and to answer basic questions such as when and where the largest aftershocks occurred, and how large they were. Because instrumental data from the period is extremely limited, the best source of information lies in the historic record. I began this study by looking through existing earthquake catalogs, and I have supplemented these lists with information found in newspapers, diaries, and letters written by people who felt the aftershocks. The abstract below outlines the methodology and preliminary conclusions.
My latest findings will be presented at the annual meeting of the Seismological Society of America, on 18 April 2001, in San Francisco.
The San Andreas fault is the longest fault in California and one of the longest strike-slip faults in the world, yet little is known about the behavior of aftershocks following the most recent event on the San Andreas, the M~7.8 San Francisco earthquake, on 18 Apr 1906. We conducted a study to locate and to estimate magnitudes for the largest aftershocks and triggered events of this earthquake. We examined existing catalogs and historical documents for the period Apr 1906 to Dec 1907, compiling data on the first twenty months of the aftershock sequence. We grouped felt reports temporally, and assigned Modified Mercalli intensities for the larger events based on the perceived most reliable descriptions. For onshore and near-shore events, a grid-search algorithm (derived from empirical analysis of modern earthquakes; modified from an algorithm of Bakun and Wentworth) was used to find the epicentral location and magnitude most consistent with the assigned intensities. For events identified as far offshore, the event's intensity distribution was compared with those of modern events, in order to constrain the event's location and magnitude.
The two largest aftershocks, which occurred on 23 Apr 1906 and 11 Aug 1907, were located off the Humboldt County coast. These were estimated to be M~6.7, about 100 km west of Eureka, and M~6.5, about 60 km west of Cape Mendocino, respectively. South of Humboldt County, the largest aftershock was a M~5.6 event near San Juan Bautista on 17 May 1906. Other significant aftershocks included a M~5.0 event on 6 Jul 1906 on the creeping segment of the San Andreas fault (southeast of the mainshock rupture), a M~5.1 event on 5 Jun 1907 on the San Andreas fault in San Mateo County, and a M~5.1 event on 8 Aug 1907 near Punta Gorda. The 1906 San Francisco earthquake also triggered events in southern California (including separate events in or near the Imperial Valley, the Pomona Valley, and Santa Monica Bay), in western Nevada, in southern central Oregon, and in western Arizona, all within a 40-hour period following the mainshock. Of these triggered events, the largest were a M~6.1 earthquake near Brawley, CA, and a M~5.0 under or near Santa Monica Bay, ~11 and ~31 hours after the San Francisco mainshock, respectively.
Study of the Foreshocks and Aftershocks of the 1857 Fort Tejon Earthquake
During the summer of 1997, I had the opportunity to begin my research with the U.S. Geological Survey (USGS) in Pasadena, through a grant from Caltech's Summer Undergraduate Research Fellowships program, or SURF. My research advisor was (and still is) Dr. David Wald of the USGS. I spent the summer analyzing the foreshocks and aftershocks of the 1857 Fort Tejon earthquake, which was the last major earthquake on the central and southern segments of the San Andreas Fault. (This was supposed to be a small project before I started work on my 1906 aftershock research, but it turned into a major publishable project in its own right!) The procedure for this project was similar to my 1906 project, and it is outlined in another abstract below.
During the following school year, I continued my work for research credits, and during Summer 1998 I completed a paper on the study, which was published as a U.S. Geological Survey Open-File Report. Since then, a more polished version of that paper has been published in the Bulletin of the Seismological Society of America, a leading scientific journal in seismological research. I also wrote an article briefly summarizing the effects of the Fort Tejon earthquake, and links to further information on this topic can be found on the SeismoLinks page.
The San Andreas fault is the longest fault in California and one of the longest strike-slip faults anywhere in the world, yet we know little about many aspects of its behavior before, during, and after large earthquakes. We conducted a study to locate and to estimate magnitudes for the largest foreshocks and aftershocks of the 1857 M= 7.9 "Fort Tejon" earthquake on the central and southern segments of the fault. We began by searching archived first-hand accounts from 1857 through 1862, by grouping felt reports temporally, and by assigning Modified Mercalli Intensities to each site. We then used a modified form of 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 the assigned intensities for each of the largest events. The result confirms a conclusion of Sieh (1978b) that at least two foreshocks ("dawn" and "sunrise") located on or near the Parkfield segment of the San Andreas fault preceded the mainshock. We estimate their magnitudes to be M~ 6.1 and M~ 5.6, respectively. The aftershock rate was below average, but within one standard deviation of the number of aftershocks expected based on statistics of modern southern California mainshock-aftershock sequences. The aftershocks included two significant events during the first eight days of the sequence, with magnitudes M~ 6.25 and M~ 6.7, near the southern half of the rupture; later aftershocks included a M~ 6 event near San Bernardino in December 1858 and a M~ 6.3 event near the Parkfield segment in April 1860. From earthquake logs at Fort Tejon, we conclude that the aftershock sequence lasted a minimum of 3.75 years.
A Stratigraphic Study of the 610 A.D. Eruption of the Mono Craters
During the summer of 1998, I began research in a field known as volcanic stratigraphy. My project involved a field investigation of the stratigraphic relations and lithologies of the rock beds laid down in the 610 A.D. eruption of the Mono Craters in eastern-central California. I worked with fellow student Matthew Dawson, under the guidance of Professor of Geology Dr. Kerry Sieh. This project was again funded by SURF, along with several private donors. Over the course of the summer, I spent time both in Mono Craters and in the Spatial Interpretation Laboratory at Caltech, analyzing and mapping the data. An abstract, written in August 1998, is below, and my original proposal for this project may also be viewed online. Once again, we were not able to finish the research in one summer (there are locations for which we still need to collect data), although our preliminary findings were quite interesting, and I hope to pick up where I left off at some point in the future.
Although much attention has been focused recently on the Long Valley caldera resurgent dome in eastern-central California and on associated volcanic hazards at Mammoth Mountain, little is known about the details of the most recent (latest Pleistocene to Holocene) eruptions in the greater Long Valley caldera complex, specifically those in the Mono and Inyo Craters chain. Our project involves several weeks of field work, digging and logging a series of pits through the air-fall and pyroclastic flow deposits from a sequence of eruptions around 610 A.D., and the subsequent analysis of collected data. By studying lithological notes recorded in situ and corresponding photographs of pit walls, we are attempting to identify the depositional nature of each stratigraphic layer, and to correlate layers from pit to pit. Upon completion of these correlations, we plan to contour the thickness of each layer, to identify the layer's source vent and dispersal axis, and to interpolate to calculate the layer's total volume. We ultimately hope to gain a detailed understanding of the timing and nature of the overall sequence of the eruption.
We made the cover of Caltech's On Campus news magazine!
Check out my page of SeismoLinks!
Check out some photos I took of Mono Craters!
Check out an article I wrote on the Fort Tejon quake!
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This page initiated 10 September 1997.
Last modified 14 April 2001.
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