The 1989 Loma Prieta earthquake occurred in Northern California on October 17 at 5:04 p.m. local time. The shock was centered in The Forest of Nisene Marks State Park approximately 10 mi (16 km) northeast of Santa Cruz on a section of the San Andreas Fault System and was named for the nearby Loma Prieta peak in the Santa Cruz Mountains. With a moment magnitude of 6.9 and a maximum Mercalli intensity of IX (Violent), the shock was responsible for 63 deaths and 3,757 injuries.
The Loma Prieta earthquake was named for the Loma Prieta peak in the Santa Cruz Mountains that lies just to the east of the main shock epicenter. The duration of the heaviest shaking in the Santa Cruz Mountains was about 15 seconds, but strong ground motion recordings revealed that the duration of shaking was not uniform throughout the affected area (due to different types and thicknesses of soil). At sites with rocky terrain, the duration was shorter and the shaking was much less intense, and at locations with unconsolidated soil (like the Marina District or the Cypress Street Viaduct) the intensity of the shaking was more severe and lasted longer. The strong motion records also allowed for the causative fault to be determined – the rupture was related to the San Andreas Fault System |
Ground Effects:
While the effects of a four-year drought limited the potential of landslides, the steep terrain near the epicenter was prone to movement, and up to 4,000 landslides may have occurred during the event. The majority of landslides occurred to the southwest of the epicenter, especially along road cuts in the Santa Cruz Mountains and in the Summit Road area, but also along the bluffs of the Pacific Coast, and as far north as the Marin Peninsula. Highway 17 was blocked for several weeks due to a large slide and one person was killed by a rockfall along the coast. Other areas with certain soil conditions were susceptible to site amplification due to the effects of liquefaction, especially near the shore of San Francisco Bay (where its effects were severe in the Marina District) and to the west of the epicenter near rivers and other bodies of water. Minor lateral spreading was also seen along the shores of San Francisco Bay and to the south near Monterey Bay. Other ground effects included downslope movement, slumps, and ground cracks. Injuries and Fatalities: Fifty-seven of the deaths were directly caused by the earthquake; six further fatalities were ruled to have been caused indirectly. In addition, there were 3,757 injuries as a result of the earthquake, 400 of which were serious. The highest number of fatalities, 42, occurred in Oakland because of the Cypress Street Viaduct collapse on the Nimitz Freeway (Interstate 880), where the upper level of a double-deck portion of the freeway collapsed, crushing the cars on the lower deck. One 50-foot (15 m) section of the San-Francisco-Oakland Bay Bridge also collapsed, leading to a single fatality, a 23-year-old woman named Anamafi Moala. three people were killed in the collapse of buildings along the Pacific Garden Mall in Santa Cruz, and five people were killed in the collapse of a brick wall on Bluxome Street in San Francisco. When the earthquake hit, the third game of the 1989 World Series baseball championship (image on left) was just beginning. Because of the unusual circumstance that both of the World Series teams (the San Francisco Giants and Oakland Athletics) were based in the affected area, many people had left work early or were staying late to participate in after work group viewings and parties. As a consequence the normally crowded freeways contained unusually light traffic. If traffic had been normal for a Tuesday rush hour, injuries and deaths would certainly have been higher. The initial media reports failed to take into account the game's effect on traffic and initially estimated the death toll at 300, a number that was corrected to 63 in the days after the earthquake. |