The Loma Prieta earthquake occurred on October 18, 1989 in the Santa Cruz Mountains of Central California. The quake occurred just after 5:00pm, a very busy time of day for the bay area. Game three of the World Series was just underway when the 15-second earthquake hit. The earthquake was centered about 60 miles south of San Francisco and could be felt as far away as San Diego. This earthquake was the first major event to occur along the San Andreas Fault zone since 1906. At the time this was the most costly natural disaster in the United States causing about $ 5,900,000,000 in property damage. This earthquake was also responsible for 3,757 injuries and 63 deaths. The earthquake caused several structural collapses and landslides causing the residents of Santa Cruz and several other bay area communities to go into a state of shock for several days. The magnitude of the Loma Prieta earthquake was considered "moderate" however the damage was major. .
The Loma Prieta earthquake was unique because the type of seismic movement wasn't typical of the San Andres Fault system. "The Loma Prieta earthquake ruptured the southernmost 40 km of the 1906 break, in comparable amounts of right-lateral strike slip and reverse slip motion. The average strike-slip displacement was 1.2 meters while the average reverse-slip displacement was 1.6 meters" (UC Berkeley 1). This type of motion is not typical of the San Andreas Fault and suggests that the earthquake occurred on a sub-parallel fault and not on the San Andreas itself. The potential for a damaging earthquake on the San Andreas Fault system in the Santa Cruz Mountains may still exist because the stress along this fault may not have been relieved during the Loma Prieta earthquake.
Damage from the Loma Prieta earthquake to public and private structures was quite severe. The severity of the damage caused officials to change several structure codes for buildings and bridges.