CaltechAUTHORS
  A Caltech Library Service

A change in fault-plane orientation between foreshocks and aftershocks of the Galway Lake earthquake, M_L = 5.2, 1975, Mojave desert, California

Fuis, G. S. and Lindh, A. G. (1979) A change in fault-plane orientation between foreshocks and aftershocks of the Galway Lake earthquake, M_L = 5.2, 1975, Mojave desert, California. Tectonophysics, 52 (1-4). pp. 601-602. ISSN 0040-1951. https://resolver.caltech.edu/CaltechAUTHORS:20191127-135257545

Full text is not posted in this repository. Consult Related URLs below.

Use this Persistent URL to link to this item: https://resolver.caltech.edu/CaltechAUTHORS:20191127-135257545

Abstract

A marked change is observed in P/SV amplitude ratios, measured at station TPC, from foreshocks to aftershocks of the Galway Lake earthquake. This change is interpreted to be the result of a change in fault-plane orientation occurring between foreshocks and aftershocks. The Galway Lake earthquake, M_L= 5.2, occurred on June 1, 1975. The first-motion fault-plane solutions for the main shock and most foreshocks and aftershocks indicate chiefly right-lateral strike-slip on NNW-striking planes that dip steeply, 70–90°, to the WSW. The main event was preceded by nine located foreshocks, ranging in magnitude from 1.9 to 3.4, over a period of 12 weeks, starting on March 9, 1975. All of the foreshocks form a tight cluster approximately 1 km in diameter. This cluster includes the main shock. Aftershocks are distributed over a 6-km-long fault zone, but only those that occurred inside the foreshock cluster are used in this study. Seismograms recorded at TPC (Δ = 61 km), PEC (Δ = 93 km), and CSP (Δ = 83 km) are the data used here. The seismograms recorded at TPC show very consistent P/SV amplitude ratios for foreshocks. For aftershocks the P/SV ratios are scattered, but generally quite different from foreshock ratios. Most of the scatter for the aftershocks is confined to the two days following the main shock. Thereafter, however, the P/SV ratios are consistently half as large as for foreshocks. More subtle (and questionable) changes in the P/SV ratios are observed at PEC and CSP. Using theoretical P/SV amplitude ratios, one can reproduce the observations at TPC, PEC and CSP by invoking a 5–12° counterclockwise change in fault strike between foreshocks and aftershocks. This interpretation is not unique, but it fits the data better than invoking, for example, changes in dip or slip angle. First-motion data cannot resolve this small change, but they permit it. Attenuation changes would appear to be ruled out by the fact that changes in the amplitude ratios, P_(TPC)/P_(PEC) and p_(TPC)/p_(CSP), are observed, and these changes accompany the changes in P/SV. Observations for the Galway Lake earthquake are similar to observations for the Oroville, California, earthquake (M_L = 5.7) of August 1, 1975, and the Brianes Hills, California, earthquake (M_L = 4.3) of January 8, 1977 (Lindh et al., Science Vol. 201, pp. 56–59). A change in fault-plane orientation between foreshocks and aftershocks may be understandable in terms of early en-echelon cracking (foreshocks) giving way to shear on the main fault plane (main shock plus aftershocks). Recent laboratory data (Byerlee et al., Tectonophysics, Vol. 44, pp. 161–171) tend to support this view.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/0040-1951(79)90276-2DOIArticle
Additional Information:© 1979 Published by Elsevier B.V. Accepted 8 June 1978.
Group:Seismological Laboratory
Issue or Number:1-4
Record Number:CaltechAUTHORS:20191127-135257545
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20191127-135257545
Official Citation:G.S. Fuis, A.G. Lindh, A change in fault-plane orientation between foreshocks and aftershocks of the Galway Lake earthquake, ML = 5.2, 1975, Mojave desert, California, Tectonophysics, Volume 52, Issues 1–4, 1979, Pages 601-602, ISSN 0040-1951, https://doi.org/10.1016/0040-1951(79)90276-2. (http://www.sciencedirect.com/science/article/pii/0040195179902762)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100114
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:28 Nov 2019 01:57
Last Modified:28 Nov 2019 01:57

Repository Staff Only: item control page