2001 Research Projects

Late Holocene Earthquake History of the Anza Seismic Gap, Central San Jacinto Fault Zone, Southern California: A Work in Progress

Project Description: Ongoing paleoseismic investigations across the central (Clark) strand of the San Jacinto fault at Hog Lake near Anza reveal evidence for seven past surface ruptures. Several new exposures reconfirm the original work by Klinger and Rockwell (1987) that demonstrated three surface ruptures during the past 1100 years. Current age data indicate that the most recent event occurred between A.D. 1660 and 1770, and there is no evidence for rupture in the 1918 earthquake, which apparently terminated northwest of Hog Lake. If correct, then the Anza Gap may be nearing the end of its seismic cycle. We also have exposed evidence in multiple trenches for four earlier events (prior to about A.D. 900, with radiocarbon dates pending. Evidence for individual events includes: 1) abrupt upward truncation of fault splays; 2) folding overlain by unfolded sediments; 3) angular unconformities; and 4) increases in vertical separation with depth. These new observations will provide a complete rupture history of the Anza Seismic Gap for the past two millennia to compare with large earthquakes on the San Andreas fault.
Intern(s): Danielle Verdugo

Susan Owen, University of Southern California

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Analysis of Updated Seismicity Catalogues for Systematic Variation

Project Description: Throughout the last 68 years seismicity in the Southern California area has been recorded and compiled in a catalogued database by the Southern California Seismic Network (SCSN). However, in two recent studies, Richards-Dinger & Shearer (2000) and Hauksson (2000) have derived improved seismicity catalogues by relocating nearly 300,000 events recorded since 1975. This project will entail using these two relocated catalogues to study Southern California seismicity with the aim of answering several key questions: (a) How well do the improved locations delineate faults? (b) How do frequency-magnitude statistics vary spatially? (c) In particular, can systematic variations in event statistics be related to distance from major active faults? (d) Are there systematic variations in seismicity associated with lithologic heterogeneities in Southern California, as indicated in the study by Magistrale and Zhou (1996)? Associated with answering these questions I will delve into modeling the data into a format that accurately portrays the results to the above questions. I will work with professor Tom Jordan of USC in pursuit to interpret the modern catalogues and present the findings in a successful manner.
Intern(s): Keegan Delaney

Thomas Jordan, University of Southern California

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Strong Ground Motions Generated By the 2001 Bhuj, Indida Earthquake, Mw 7.6

Project Description: This project is intended to access the hazard posed by the strong ground motions generated by large intraplate earthquakes. We have collected, compiled and assigned Modified Mercalli intensities to first hand and news accounts of the 2001 Bhuj, India earthquake (Mw7.6) from various locations in South Asia, namely in India. We have used a slip model derived from teleseismic data and a program for simulating the horizontal component of shear waves from a finite fault will be used to generate synthetic accelerograms for these locations.
Intern(s): Stacey Martin Servito

Susan Hough, United States Geological Survey

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