Map shows estimated epicenters of quakes

The Associated Press
Saturday February 10, 2001

LOS ANGELES — Los Angeles and San Francisco have been damaged at least six times by major earthquakes in the past 200 years but the mighty San Andreas Fault has been relatively quiet in recent decades, according to a new map of historical temblors. 

The map shows the estimated epicenters of more than 800 quakes of magnitude-5 or above, along with areas that were shaken strongly enough to suffer building damage. 

The quakes occurred between 1800 and 1999. The map cannot be used to predict earthquakes but it does point out areas that have been historically active, said Tousson Toppozada, a senior seismologist with the state Department of Conservation’s Division of Mines and Geology. 

It “tells you where the risk is highest,” he said. “It can be used by planners, city and county officials who are concerned. It can be used in site studies for critical structures like hospitals and schools,” he said. 

However, quake damage can occur far from the epicenter depending on factors such as the temblor’s magnitude and how its energy is released. 

For example, the 1989 Loma Prieta earthquake in the Santa Cruz Mountains caused a freeway to collapse 55 miles away. 

“A map that just shows epicenters wouldn’t tell you whether other areas had been damaged, or how often they have been damaged. This map provides that information,” Jim Davis, a state geologist who heads the mines division, said in a news release. 

The map was released a day before the anniversary of the Feb. 9, 1971, San Fernando quake. The 6.7-magnitude temblor killed 65 people. 

More than 20 years in the making, the map used earthquake information recorded by instruments. For older quakes, estimates of the epicenters and damage were made based on reports in travelers’ journals, mission records and Gold Rush-era newspapers, Toppozada said. 

The map shows that, on average, California has been jarred by a quake of magnitude-6 or higher every year since 1850, Toppozada said. 

Since 1800, earthquakes capable of damaging unreinforced buildings have occurred at least six times in the Los Angeles area north to San Fernando, San Francisco Bay area to Santa Cruz, and the region from Eureka to Cape Mendocino. 

A 1999 U.S. Geological Survey study concluded that there is a 70 percent chance that a large earthquake will shake the Bay area in the next 30 years. 

The study found that the rate of large quakes in the region dropped after the 1906 San Francisco earthquake, probably because it released strain on local faults.  

But the study said strain on those faults has been slowly building and strong quakes began to occur in the 1980s. 

The new map shows that the San Andreas Fault system has been relatively quiet in the past 90 years compared to a century earlier. 


That “raises the red flag” that the fault could be building up pressure and could be helpful as scientists determine whether a major fault will erupt on the fault in the next 30 years, Toppozada said. 


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