Quake’s Direction as Crucial as Force

Why did the earthquake on the Central Coast do so little damage in towns close to the epicenter -- like San Simeon and Cambria -- but cause serious damage 20 miles away to the southeast in Paso Robles and Atascadero?

The answer has to do with a seismic concept known as “directivity,” under which the rupture builds up away from the epicenter in a particular direction and the strongest shaking doesn’t necessarily occur right next to the quake’s point of origin.

A quake rupture is like tearing a piece of paper, said Lucy Jones, scientist in charge of the U.S. Geological Survey office in Pasadena. The beginning point of the tear is the epicenter, she said, but the force of the tear, its energy, may increase farther along.

Seismic recording instruments showed that Monday’s magnitude 6.5 quake produced stronger shaking in the Paso Robles area than in San Simeon near the epicenter, said acting State Geologist Michael Reichle.

The largest ground motion recorded was 50% of the force of gravity at a hospital in Templeton, on U.S. 101 between Paso Robles and Atascadero, he said. Meantime, another instrument, on California 1 at the bridge over San Simeon Creek, just six miles southwest of the epicenter, showed ground motion of 18% of gravity.

This experience of stronger shaking away from the epicenter followed the pattern of the 1994 Northridge earthquake. The epicenter then was on the Reseda-Northridge community line. But 70% of the energy of the earthquake was expended north of the San Fernando Valley in the Santa Susana Mountains.

Quake scientists said they believe that in both quakes, the aftershocks continued along the same direction as the original rupture, demonstrating directivity. In the Northridge quake, the aftershocks were to the north. In Monday’s quake, they were to the southeast.

In fact, the U.S. Geological Survey’s Internet site showed that the apex of scores of aftershocks Monday night and Tuesday morning was pointed directly at Atascadero, where at least one home slid off its foundation and ended up at the bottom of a hill. Paso Robles and Atascadero were outside, or in front of, the rupture zone, but the instrument readings did not necessarily mean they were shaken harder than the southeastern-most points in the zone, since there were no instruments at those points.

However, computerized models of the quake by seismologist Doug Dreger of UC Berkeley and by research scientist Chen Ji of Caltech showed the greatest ground motion and maximum slip took place well southeast of the epicenter.

Jones and Bill Ellsworth, the scientist who directs earthquake studies at the U.S. Geological Survey facility in Menlo Park in the Bay Area, said Tuesday that the relative paucity of the most modern recording instruments on the Central Coast shows a need to develop a more comprehensive seismic network.

To some extent, Ellsworth said, the San Simeon quake constituted a lost opportunity for research because the full variability of ground motion could not be measured by the scattered instruments.

In fact, Jones said, instruments dating back to the 1950s actually misplaced some quakes.

On Monday afternoon, for instance, the U.S. Geological Survey’s Internet site showed two aftershocks directly on the San Andreas fault southeast of Parkfield. That could have been viewed as disquieting had it not been soon verified that the quakes had not occurred there.

There has been study recently of triggered earthquakes, and scientists are always on the lookout for precursors of a huge quake on the San Andreas, although most believe a 6.5 magnitude quake such as Monday’s is too small to be an immediate trigger.