Transcript for Computer Animation of Loma Prieta Aftershocks, segment 03 of 12


The earthquakes on this map were detected by a network of more than three hundred seismic stations in central California. The network is operated and analyzed by the U.S. Geological Survey. The earthquakes were timed by computer and later were hand-checked for accuracy. The earthquakes in yellow occurred in the nine months before the Loma Prieta earthquake. The earthquakes in red are the first month of Loma Prieta aftershocks. The San Andreas Fault runs most of the length of California. The Loma Prieta earthquake only ruptured a forty-five kilometer segment of the fault. The San Andreas Fault north of the Loma Prieta earthquake is locked. It slips infrequently in large earthquakes and produces relatively few small earthquakes. The San Andreas south of Loma Prieta is creeping and is not locked. The fault slips continually with many small earthquakes and does not produce earthquakes larger than magnitude five or six.

What follows is a time lapse animation of earthquakes on this map. The animation runs from January through November, nineteen eighty-nine. There are several areas that were active during this period. The geothermal power plants at the geysers induce many small earthquakes per day by the extraction of steam and reinjection of the condensate. The Alum Rock section of the Calaveras Fault was very active in nineteen eighty-nine. Aftershocks of the magnitude six point seven Coalinga earthquake in nineteen eighty-three still occur. In May a swarm of earthquakes began beneath Mammoth Mountain along the edge of the Long Valley Caldera. The seismicity is of interest because Long Valley Caldera is an ancient volcano that has become increasingly restless in recent years. Each frame of the animation steps ahead eight hours. An earthquake is plotted as a solid symbol for two frames and as an open symbol for four frames. Each event thus persists on the screen for two days before being erased. All times and dates use universal time.

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Watch the Calaveras Fault east of San Jose for the magnitude five Alum Rock earthquake on April third.

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The swarm at Mammoth Mountain at the west edge of Long Valley Caldera started in May and lasted through the year.

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