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WAITING FOR THE QUAKE : San Andreas Fault : GEOLOGISTS PLOT A SCENARIO FOR A 8.3 MAGNITUDE QUAKE

November 05, 1989

This map, from a 1982 scenario prepared by the state Division of Mines and Geology, provides a hypothetical look at the degree of ground shaking and ground failure that might result from an 8.3 magnitude temblor along the San Andreas Fault. It would be equivalent to the greatest earthquake ever recorded in California.

The scenario's authors chose the San Andreas Fault because it is the longest, most famous fault in the state. It also produced the last 8 magnitude earthquake in Southern California, centered at Fort Tejon, between Los Angeles and Bakersfield, in 1857.

In putting the epicenter of the scenario's 8.3 earthquake also at Ft. Tejon,

the scientists at the Division of Mines and Geology chose a spot on the fault close to the Los Angeles metropolitan area that would presumably cause the greatest shaking in that area. But they may have opted for something less than a most-likely outcome, according to two leading Caltech seismologists.

Hiroo Kanamori notes that most earthquake scientists believe the next "Big One" on the San Andreas in Southern California is likely to occur further east, in the Coachella Valley. The epicenter's greater distance from--and different orientation to--the Los Angeles area would likely produce less shaking intensities than shown in the map.

And a second scientist, Kerry Sieh, said it is "very unlikely" such a temblor would occur in the Ft. Tejon area soon.

Scientists have calculated a theoretical average interval of about 130 years for quakes in the area. But Sieh found that since about 700 AD only one quake had actually occurred according to that timetable. That's because the quakes have tended to be bunched in twos or threes fairly close in time, with intervals of 250 or 300 years separating the clusters, Sieh said.

If an 8 magnitude earthquake were to occur on the fault segment which is the focus of the San Andreas scenario, it would unquestionably be a serious event in the Los Angeles area, resulting in "significant interruption of ground transportation, communications, water supply, sewage treatment, electricity and pipeline distribution of natural gas and petroleum," according to the scenario.

Legend for the San Andreas map showing three levels of intensity, slightest first:

Areas in white--Little or no structural damage to buildings of good design, considerable damage to poorly built or badly designed buildings. Considerable fall of plaster and window breakage.

Areas in gray--Considerable structural damage in ordinary substantial brick buildings,

partial collapse of wooden houses.

Areas in dark gray--More serious damage; fall of walls, chimneys, monuments, also factory stacks and towers.

Liquefaction zones:

Soil is subject to liquefaction. Under intense shaking, it becomes like gelatin or quicksand, often resulting in collapse of structures above or breakage of underground pipes.

Source: U.S. Geological Survey

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