|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
INSTITUTE FOR THE STUDY OF THE CONTINENTS (INSTOC) AND DEPARTMENT OF GEOLOGICAL SCIENCES CORNELL UNIVERSITY, ITHACA, NEW YORK 14853
Abstract
In 1977, COCORP recorded a short, seismic reflection test line across the San Andreas fault through Parkfield, California, where the Salinian crustal block is juxtaposed against a contrasting terrane of Franciscan and overlying material. Although analysis of this seismic line has been limited by crooked geometry, lack of cross-lines, complex terrain, possible recording noise, and perhaps most of all, its short length, gross crustal features are discernible after extensive reprocessing and reanalysis. The Salinian block southwest of the San Andreas consists of a maximum of 2 km of relatively underformed sediments overlying a reflective and inhomogeneous basement which exhibits deep, subhorizontal layering. At 8 to 10 km depth, density and amplitude of reflections markedly decay, perhaps corresponding to the base of a Salinian "granitic" crust which overlies Franciscan or other contrasting material. Northeast of the San Andreas, subhorizontal reflections suggest that Franciscan material may extend as deeply as 13 km, below which is a nonreflective zone which might represent relict oceanic crust. Reflections from Moho depths differ across the San Andreas fault. Beneath the Salinian block is a thick zone of reflections beginning at about 22 km depth, while less layered and weaker reflections seem to define the Moho discontinuity at 29 km beneath the Franciscan terrane. The San Andreas fault zone appears to be traceable as a distinct feature on the reflection section throughout the entire crust, forming a near-vertical zone about 3 km wide. In the seismogenic upper crust (down to 15 km), the fault zone is marked by sharp structural truncations and short reflection or diffraction segments. At depth, it seems to be distinguished by a nonreflective column. This vertical change in reflection character may correspond to the transition from a brittle to a ductile deformation regime.
This article has been cited by other articles:
|
|
 |
|
  L. R. Johnson and R. M. Nadeau Asperity Model of an Earthquake: Dynamic Problem Bulletin of the Seismological Society of America, February 1, 2005; 95(1): 75 - 108. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y.-G. Li, W. L. Ellsworth, C. H. Thurber, P. E. Malin, and K. Aki Fault-zone guided waves from explosions in the San Andreas fault at Parkfield and Cienega Valley, California Bulletin of the Seismological Society of America, February 1, 1997; 87(1): 210 - 221. [Abstract] [PDF]
|
|
|
|
|
|
D. Jongmans and P. E. Malin Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California Bulletin of the Seismological Society of America, December 1, 1995; 85(6): 1805 - 1820. [Abstract] [PDF]
|
|
|
|
|
|
A. MICHELINI and T. V. MCEVILLY Seismological studies at Parkfield. I. Simultaneous inversion for velocity structure and hypocenters using cubic B-splines parameterization Bulletin of the Seismological Society of America, April 1, 1991; 81(2): 524 - 552. [Abstract] [PDF]
|
|
|
|
|
|
T. M. DALEY and T. V. MCEVILLY Shear-wave anisotropy in the Parkfield Varian well VSP Bulletin of the Seismological Society of America, August 1, 1990; 80(4): 857 - 869. [Abstract] [PDF]
|
|
|
|
 |
|
 S. McGEARY Reflection seismic evidence for a Moho offset beneath the Walls Boundary strike-slip fault Journal of the Geological Society, April 1, 1989; 146(2): 261 - 269. [Abstract] [PDF]
|
|
|
|
|
|
C. M. POLEY The San Ardo, California, earthquake of 24 November 1985 Bulletin of the Seismological Society of America, June 1, 1988; 78(3): 1360 - 1366. [PDF]
|
|
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
