Loma Prieta 1989 magnitude

The San Francisco area and smaller cities to the south, especially Santa Cruz, were hit by a moderate-sized earthquake (magnitude 7.1) at 5:04 p.m. on Tuesday, October 17, 1989, during live broadcast of the World Series baseball game. Sixty-seven people died, 3,757 people were injured, and 12,000 left homeless. Tens of millions of people watched on television as the earthquake struck just before the beginning of game three, and the news coverage that followed was unprecedented in the history of earthquakes.

The earthquake was caused by a rupture along a 26-mile- (42-km-) long segment of the San Andreas fault near Loma Prieta peak in the Santa Cruz Mountains south of San Francisco. The segment of the fault that had ruptured was the southern part of the same segment that ruptured in the 1906 earthquake, but this rupture occurred at greater depths and involved some vertical motion as well as horizontal motion. The actual rupturing lasted only 11 seconds, during which time the western (Pacific) plate slid almost six feet (1.9 m) to the northwest, and parts of the Santa Cruz Mountains were uplifted by up to four feet (1.3 m). The rupture propagated at 1.24 miles per second (2 km/sec) and was a relatively short-duration earthquake for one of this magnitude. Had it been much longer, the damage would have been much more extensive. As it was, the damage totals amounted to more than 6 billion dollars.

The actual fault plane did not rupture the surface, although many cracks appeared and slumps formed along steep slopes. The Loma Prieta earthquake had been predicted by seismologists, because the segment of the fault that slipped had a noticeable paucity of seismic events since the 1906 earthquake and was identified as a seismic gap with a high potential for slipping and causing a significant earthquake. The magnitude 7.1 event and the numerous aftershocks filled in this seismic gap, and the potential for large earthquakes along this segment of the San Andreas fault is now significantly lower. There are, however, other seismic gaps along the San Andreas fault in heavily populated areas that should be monitored closely.

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