San Simeon, California Earthquake

Update 1 | Summary
Posting Date: December 23, 2003, 1:45:00 PM

More reports of damage have come in after yesterday’s magnitude (Mw) 6.5 earthquake, which was centered about 5 miles (8 km) northeast of San Simeon in the central California county of San Luis Obispo. The epicentral location of the earthquake and the distribution of its aftershocks, suggest that the rupture propagated from a northwest to southeast direction. Officials at the U.S. Geological Survey (USGS) suggest that it is too early to be certain which fault in the Santa Lucia Mountains is responsible for this event. According to the scientist in charge at the U.S. Geological Survey office in Pasadena, the quake likely ruptured along roughly 20 miles of an as yet unknown fault, parallel to the San Andreas Fault. The largest historical earthquake in the vicinity of Monday's earthquake, about 10 km northwest, was the magnitude 6.2 Bryson earthquake of November 1952. Monday’s quake was the strongest to hit California since 1999, when a 7.1 magnitude temblor struck in the southern Mojave Desert. While the earthquake was felt as far as Orange County, south of Los Angeles, and San Francisco in the north, the hardest hit area was the city of Paso Robles, about some 24 miles (39 km) southeast of the epicenter. The clock tower and roof of the 1892 Mastagni Building (also called the Acorn Building) slid off the collapsing structure and onto the sidewalk, resulting in the quake’s two confirmed fatalities. In all, more than 80 commercial buildings in Paso Robles were damaged, many in the historic downtown area. According to police reports, some buildings were damaged so severely that they may need to be demolished. Most damage seems to have affected buildings constructed before 1971 that do not meet the strict present-day seismic building codes and that lack steel reinforcements to help them withstand violent ground shaking. Local news showed an essentially unscathed retrofitted masonry building adjacent to collapsed ones of the same vintage. A number of other areas in San Luis Obispo country suffered significant damage to older buildings. Nevertheless, while residents were shocked by the quake, structural damage was scattered and fairly light. The city hall in Atascadero, built in 1918, was evacuated and closed due to holes in the rotunda. Four county bridges, including one in Templeton, were closed. There was extensive contents damage to stores and part of a residence was destroyed by fire in Cambria and Morro Bay. The airport at Oceano in South County was closed due to cracks in the runway. Landslides closed many side roads. According to Pacific Gas and Electric (PG&E), about 75,000 people lost electricity for some hours after the quake. The utility reported no major damage to its Diablo Canyon nuclear power plant. Another utility, Duke Energy, said there was no substantive damage at its two coastal power plants, Morro Bay and Moss Landing. The AIR earthquake team has assembled the available information on this event and has run simulations based on estimated epicentral location, magnitude, and ground motion data. Maps of ZIP Code level loss and intensity representing five selected scenarios for the current event may be viewed on the Scenarios page of this website. Clients can also download event sets representing all scenarios. Again, though the magnitude and focal depth make this a significant event from a seismological point of view, losses are expected to be only moderate given that the earthquake occurred in a relatively sparsely-populated stretch of the California coast.

Update 1 | Downloads
Posting Date: December 23, 2003, 1:45:00 PM