Transcript for Computer Animation of Loma Prieta Aftershocks, segment 07 of 12

It is useful to compare the seismicity before and immediately after the Loma Prieta earthquake. The yellow dots show earthquakes from the previous fourteen years and thus define the active faults in the Bay area. The red dots represent the first month and a half of aftershocks. Notice that the Loma Prieta aftershock zone was quiet and was a seismic gap before the earthquake. Most of the earthquakes within the gap before Loma Prieta were the Lake Ellsman preshocks from June, nineteen eighty-eight and August, nineteen eighty-nine.

This is the same set of earthquakes plotted in both maps and cross sections. The cross section below center uses red dots to show Loma Prieta aftershocks and thus the rupture area that slipped in the main shock. Note that the U-shaped patch of fault that slipped in the earthquake was quiet before the earthquake as seen by the lack of yellow dots. The Loma Prieta earthquake thus filled a seismic gap just as other large earthquakes will fill gaps to the north along the fault.

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