1987 Whittier Narrows earthquake

The 7:42 a.m. shock was the strongest in the Los Angeles area since the 1971 San Fernando earthquake and was felt as far as San Diego and San Luis Obispo, California and Las Vegas, Nevada. Communication systems and local media were temporarily impaired and power was cut, leaving numerous early morning workers stranded in disabled elevators. Other minor disruptions included a number of water and gas main breaks, shattered windows and some ceiling collapses. Like the San Fernando earthquake, transportation systems were again affected, but this time it was only the Santa Ana Freeway and San Gabriel River Freeways that were closed near Santa Fe Springs after pieces of concrete were dislodged and cracks were observed in the roadway. Los Angeles County+USC Medical Center took many of the injured, whose injuries were summarized by an emergency room spokesman as very bad to minor, and three people died as a direct result.[7]

While total casualties amounted to eight, the destruction of homes was significant. Throughout Los Angeles, Orange, and Ventura counties, 123 homes and 1,347 apartments were destroyed, and an additional 513 homes and 2,040 apartments were damaged. An inspection of a highway bridge on Interstate 605 revealed that there were fractures on the support columns, which resulted in a temporary closure, and minor damage affected 28 other bridges. Other typical failures included more than 1,000 gas leaks, with many resulting in fire, ceramic elements on high-voltage substation equipment breaking, and phone systems becoming overwhelmed.[8]

Caltech scientists recorded the events on a cluster of twelve strong motion sensors that were placed throughout the region with a total of 87 channels of recorded data. Nine of these instruments were located on the Caltech campus, two were at the nine story Jet Propulsion Laboratory building 180 (ten miles northwest of the campus) and the final device was placed on a hillside 5 km to the west. Investigation of the accelerograms from these units revealed the strongest shaking lasted 4–5 seconds. The vertical accelerations were considered relatively high and early analysis (pre-digitilization) indicated that the mainshock was complex, with a double train of P-waves arriving with a 1.4–1.8 second interval.[9]

The National Strong-Motion Instrumentation Network (NSMIN) (a cooperative effort including the United States Geological Survey and other organizations) also monitored a set of 52 strong motion stations in the Los Angeles area. Most of the stations successfully captured the event, and the closest unit to the mainshock, a rock site at Garvey Reservoir, recorded a peak horizontal acceleration of .47g. A twelve-story steel frame building in Alhambra was outfitted with accelerographs in the basement, at mid-level, and at the top of the structure. The top floor instruments recorded a peak acceleration of .18g during the 1971 San Fernando earthquake and instruments on the sixth floor recorded a peak acceleration of .47g at the time of the Whittier mainshock. A ten-story reinforced concrete building in Whittier (7215 Bright Ave.) saw a peak horizontal reading of .63g in the basement.[4]

A magnitude 5.2 event occurred three days later on October 4, causing additional damage in Alhambra, Pico Rivera, Los Angeles, and Whittier. The shock's effects were assessed at VII (Very strong) on the Mercalli Intensity Scale with damaged chimneys, broken windows, and the collapse of two bell towers at the San Gabriel Civic Auditorium. This event was also responsible for several injuries and one additional death. On February 11 of the following year another small aftershock again damaged chimneys, broke windows, cracked drywall and some homes' foundations in Pico Rivera, Pasadena, and Whittier.[8]

The October 4 aftershock struck 3 kilometers (1.9 mi) to the northwest of the mainshock, and was primarily a strike-slip event on a steeply dipping, northwest-striking fault. The origin of faulting for this event was at a depth of 12 km (7.5 mi), which places it within the hanging wall of the thrust fault that was responsible for the main shock.[6]

This aftershock was recorded on thirty of the NSMIN stations at distances of up to 57 kilometers (35 mi). The majority of the stations were located in buildings, but nine were located at dams or reservoirs, and four were at Veterans Administration buildings. Lighter accelerations were observed than the main shock, with peak accelerations in the range of .15g–.33g, all occurring at six stations that were within 12 kilometers (7.5 mi) of the epicenter.[10]