The probability of a repeat of the 1906 magnitude 8.3 earthquake is still significant. .
Surface displacements with offsets of up to 3 or 4 feet along a zone about 20 miles long would normally be expected to accompany an earthquake of this magnitude. Instead many cracks have been found over several discontinuous and indistinct zones. There are several possible explanations for this lack of clear surface expression. The earthquake was unusually deep, making it difficult for the bedrock rupture to propagate to the ground surface. The State Commission report of the 1906 earthquake described very similar surface rupture characteristics along the Santa Cruz Mountains portion of the San Andreas fault. .
The Loma Prieta quake basically repeats an 1865 event that occurred on the San Andreas, northeast of Santa Cruz. The lack of activity on this section of the fault, as well as the occurrence of several magnitude 5.0+ events in the area had prompted geologists to forecast this event. .
The Loma Prieta rupture is unusual for earthquakes on the San Andreas fault. Past earthquakes have been the result of horizontal slip of the ground on the two sides of the fault, with the southwest side moving northwest relative to the northeast side. The Loma Prieta earthquake includes not only this horizontal component of slip, but also a significant thrusting of the southwest side up and over the northeast side. The fault has steep dip to the southwest in this area. .
Strong motion records were obtained from hundreds of instruments, allowing a detailed understanding of the ground motion throughout the affected region. The pattern of damage related to liquefaction and soil consolidation in San Francisco and Oakland is similar to that of 1906. .
The number of aftershocks for the Loma Prieta earthquake is typical for earthquakes in California. .
In addition to being felt by millions of people, the earthquake was recorded by 93 strong-motion accelerographs operated by the Division of Mines and Geology Strong Motion Instrumentation Program.
