A Caltech Library Service

Seismic Study of an Oceanic Ridge Earthquake Swarm in the Gulf of California

Thatcher, Wayne and Brune, James N. (1971) Seismic Study of an Oceanic Ridge Earthquake Swarm in the Gulf of California. Geophysical Journal of the Royal Astronomical Society, 22 (5). pp. 473-489. ISSN 0016-8009. doi:10.1111/j.1365-246X.1971.tb03615.x.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


Detailed seismic investigation of an unusually intense earthquake swarm which occurred in the northern Gulf of California during March 1969 has provided new information about seismic processes which occur on actively spreading oceanic ridges and has placed some constraints on the elastic wave velocities beneath them. Activity during this swarm was similar to that of a foreshock-mainshock-aftershock sequence, but with a ‘mainshock’ composed of over 70 events with magnitudes between 4 and 5.5 occurring in a 6-hr period about a day after swarm activity was initiated. ‘Aftershocks’, including many events greater than magnitude 5, continued for over two weeks. Near-source travel-time data indicate all sources located are within 5–10 km of each other and that hypocentres are confined to the upper crust. Teleseismic P-delays for rays travelling beneath this ridge may be interpreted in terms of an upper mantle with compressional velocities 5–10 per cent less than normal mantle to a depth of 200 km. Average apparent stresses for all swarm events studied are very similar, show no consistent pattern as a function of time, and are close to values obtained from other ridges. The focal mechanism solution shows a large component of normal faulting. An apparent non-orthogonality of nodal planes common to this mechanism solution and to normal faulting events on other ridges disappears when the indicated low upper mantle velocities beneath the source are taken into account. A survey of recent seismicity (post 1962) in the northern Gulf suggests seismic coupling across about 200 km between adjacent inferred spreading ridge segments. Surface waves from these Gulf Swarm earthquakes have amplitudes from one to two orders of magnitude greater than Northern Baja California events with similar short period body wave excitation.

Item Type:Article
Related URLs:
URLURL TypeDescription
Additional Information:Copyright © 1971 The Royal Astronomical Society. In original form 1970 October 6. Received 1970 December 29. The research reported in this paper was made possible through a cooperative research program with the National University of Mexico initiated in January 1969. The aid and cooperation of the National University of Mexico (UNAM) and the Comission Federale de Electricidad (CFE) in carrying out field investigations is gratefully acknowledged. In particular, Dr Cinna Lomnitz of UNAM provided crucial aid and support in the installation of the Rio Hardy station. This station was attended and serviced by CFE personnel and the records were processed and read at UNAM. lnformal discussions with Dr L. R. Sykes, P. Molnar, and D. P. Hill were helpful. Dr Sykes kindly furnished a preprint of his paper in advance of publication. Dr C. R. Allen, Mr T. C. Hanks, and Dr Cinna Lomnitz read the manuscript and made helpful suggestions. One of the portable seismographs used in this study was purchased from funds associated with the Grove Karl Gilbert Award in Seismic Geology (H. O. Wood Fund) awarded to one of us (J.N.B.). The research was supported by NSF grants GA 11133 and GA 12868.
Funding AgencyGrant Number
Grove Karl Gilbert Award in Seismic GeologyUNSPECIFIED
NSFGA 11133
NSFGA 12868
Issue or Number:5
Record Number:CaltechAUTHORS:20141118-135845716
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51916
Deposited By: George Porter
Deposited On:18 Nov 2014 22:23
Last Modified:10 Nov 2021 19:17

Repository Staff Only: item control page