The 1989 Loma Prieta Earthquake occurred at 5:04 PM on Tuesday, October 17, 1989. It was measured as a magnitude 7.1 on the Richter scale, a device utilized for measuring earthquake strength. It lasted about 20 seconds and was centered approximately 60 miles south of San Francisco. The tremor could be felt as far south as San Diego and as far east as Western Nevada. This earthquake was pinpointed along the San Andreas Fault line, long considered one of the most dangerous and active portions of the fault.
The famed San Andreas fault line runs along a northwesterly direction about 800 miles from the Gulf of California up to Cape Mendocino just north of San Francisco. This particular fault has been the source of many large quakes, including the 7.0 1838 quake on the Peninsula of San Francisco, a previous Santa Cruz quake in 1865 and the well known and recorded 8.3 magnitude San Francisco earthquake. The Loma Prieta quake is a continuation in the cycle of quakes that include the 1865 Santa Cruz event. Other quakes at this epicenterial area include a 5.2 on November 15, 1964, a 5.3 magnitude in June 1988 and the recent 5.4 in August of 1999.
The earthquake was created by a slip in the Andreas fault. The epicenter, for which this earthquake received the name, is a peak in the Santa Cruz Mountains about 10 miles northeast of the California beach city of Santa Cruz. A strong motion seismograph located near the epicenter source gave an acceleration reading of 0.6 G there.
This fault rupture is an interesting case study because it was not a surface quake, but rather measured a focal depth of 11 miles. For a California earthquake, a focal depth of 11 miles is unusually deep. Typical or average California quake focal depths range only from 4-6 miles deep at the most.
A side issue related to earthquakes is the occurrence of Aftershocks. Some aftershocks can be recorded as being greater than the actual ear
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