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The earthquakes of the late 1980s and '90s in California are still fresh in many memories: The terrifying shaking from Loma Prieta in the Bay Area 19 years ago; the fires, the bridge collapse, the high loss of life. Then five years later, Northridge, where 72 people died and thousands were injured as apartment buildings and freeways instantly turned to rubble.
With those destructive quakes in mind, earth scientists Monday unveiled their first comprehensive earthquake forecasting model for the entire state. They've mapped the highest risk areas. The L.A. basin and San Francisco are both highly vulnerable.
"According to the new study, during the next 30 years, the chances of one or more magnitude-6.7 earthquakes somewhere in California, are nearly 100 percent," said Tom Jordan, Southern California Earthquake Center.
This is the computer model of the shaking of a magnitude-7-plus quake centered in Puente Hills. The shock waves quickly spread through the entire basin. Estimated result: $130 billion in damage; death toll: 3,000 to 18,000 people. Scientists hope their model helps people prepare for the big one.
"With each earthquake, we're reminded how important it is to be prepared, not only for our persons, but for policy-makers and for the building codes to be up to date," said David Jackson, UCLA seismologist. "And so they're improving all the time, and this will be a significant step toward making them even better and more realistic."
An even larger risk of destruction lies north and east of L.A., on the giant San Andreas Fault. The model shows a 37 percent chance of a magnitude 7 or larger earthquake in the next 30 years.
Paleoseismologist Ray Weldon has dug down into the fault and says quakes in 1857 and 1812 on the southern San Andreas were huge.
"This part of the San Andreas ruptured in 1812. It knocked down a mission at San Juan Capistrano and was strongly felt as far away as San Diego," said Weldon. "So when we have a 7.5 or greater, which most of the earthquakes are on the southern San Andreas, it's going to be a very challenging event for Southern California."
The scientists compiled their model from the location and patterns of past earthquakes, from GPS satellites that track tectonic movement and from geologic mapping of California's thousands of faults. It's not perfect, but better than anything done in the past.