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Science Frontiers ONLINE No. 24: Nov-Dec 1982 |
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Seismic Ghost Slithers Under California
From 1973 into 1978, instruments monitoring the San Andreas Fault told geophysicists that north-south distances between observing stations along the fault line were contracting. The Pacific Coast was moving north while the rest of North America was heading south! Then, in 1978, the strain eased in the far south of California only to reassert itself in a few weeks. This strange relaxation of strain seemed to propagate slowly across southern California, as seen by other instruments farther north.
As the wave of relaxation moved ponderously along, like a slow flexure of the earth's strata, microearthquakes almost disappeared while the flow of radon from the ground increased, as one might expect. When the strain reappeared, radon flow diminished and a rash of microearthquakes were detected. It took about a year for the strain wave, called a "seismic ghost" by the geophysicists, to flow north from the Imperial Valley into the Los Angeles area.
(Alexander, George; "Quakewatch," Science 82, 3:38, September 1982.)
Comment. Something had to cause this curious disturbance; and where is it located now?