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Seismic-reflection evidence that the Hayward Fault extends into the lower crust of the San Francisco Bay area, California

Tom E. Parsons
Seismic-reflection evidence that the Hayward Fault extends into the lower crust of the San Francisco Bay area, California
Bulletin of the Seismological Society of America (October 1998) 88 (5): 1212-1223

Abstract

This article presents deep seismic-reflection data from an experiment across San Francisco Peninsula in 1995 using large (125 to 500 kg) explosive sources. Shot gathers show a mostly nonreflective upper crust in both the Franciscan and Salinian terranes (juxtaposed across the San Andreas fault), an onset of weak lower-crustal reflectivity beginning at about 6-sec two-way travel time (TWTT) and bright southwest-dipping reflections between 11 and 13 sec TWTT. Previous studies have shown that the Moho in this area is no deeper than 25 km ( approximately 8 to 9 sec TWTT). Three-dimensional reflection travel-time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle, northeast of the San Andreas fault. However, upper mantle reflections from these depths were not observed on marine-reflection profiles collected in San Francisco Bay, nor were they reported from a refraction profile on San Francisco Peninsula. The most consistent interpretation of these events from 2D raytracing and 3D travel-time modeling is that they are out-of-plane reflections from a high-angle (dipping approximately 70 degrees to the southwest) impedance contrast in the lower crust that corresponds with the surface trace of the Hayward fault. These results suggest that the Hayward fault truncates the horizontal detachment fault suggested to be active beneath San Francisco Bay.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 88
Serial Issue: 5
Title: Seismic-reflection evidence that the Hayward Fault extends into the lower crust of the San Francisco Bay area, California
Author(s): Parsons, Tom E.
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Pages: 1212-1223
Published: 199810
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 51
Accession Number: 1999-011976
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
N37°30'00" - N37°30'00", W122°30'00" - W122°30'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199905
Program Name: USGSOPNon-USGS publications with USGS authors
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