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A possible seismomagnetic observation on the Garlock fault, California

NATIONAL CENTER FOR EARTHQUAKE RESEARCH U.S. GEOLOGICAL SURVEY, MENLO PARK, CALIFORNIA 94025
EARTH AND PLANETARY SCIENCE DIVISION NAVAL WEAPONS CENTER, CHINA LAKE, CALIFORNIA 93555

Abstract

Simultaneous measurements of geomagnetic fields have been made at pairs of sites approximately 12 km apart along the Garlock fault. On June 9 and 10, 1974, several minor earthquakes (M = 2.6 to 4.3) occurred near one of these sites 3 weeks after the first measurements were made. Two repeated data sets were taken 2 weeks and 3 months after the earthquakes. The differenced data indicate that, relative to pre-earthquake values, the local magnetic field at this site had increased by about 2. A piezomagnetic source for this change implies a stress change that would exceed 10 bars. After 3 months the same form of anomaly still remained although larger in spatial scale. This might indicate dispersion of the stress discontinuities produced by the earthquake.