1989 Loma Prieta Quake Measured 6.9
On Tuesday, October 17, 1989, at 5:04 p.m., the San Andreas Fault ruptured 11 miles below the earth's surface. The west side of the fault, the Pacific Plate, uplifted almost 14 inches. The quake epicenter, near Loma Prieta Peak, is located in the upper section of the park.
The Loma Prieta earthquake was measured at 7.1 local magnitude (in California also referred to as Richter Scale). The United States Geological Survey, however, publishes its estimate of the Loma Prieta earthquake as a 6.9 Moment Magnitude. Geologists consider "Moment Magnitude" a better measurement of seismic energy instead of the more commonly known local magnitude or Richter Scale.
Johnston, Malcom J.S.; Diagram by Michael Rymer, USGS, published as frontspiece.
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