Science/Medicine : Quake Clues : Loma Prieta Temblor Helps Science Turn Corner Toward Understanding

Although the San Andreas Fault is probably the most intensely studied earthquake system in the world, the Loma Prieta earthquake that struck last Oct. 17 told scientists something they didn’t want to hear: There’s still a lot they don’t understand.

“We learned there are some surprises,” said William Bakun of the U.S. Geological Survey’s Menlo Park office. “We don’t understand as much about earthquakes as we thought we did.”

Why was the Loma Prieta quake deeper in the Earth than it should have been, larger than had been expected, and a few miles away from where scientists had thought it would hit? Why didn’t the quake break the ground at the surface, as earthquakes do nearly everywhere else along the San Andreas? Why did it cause the ground to move quite differently than had been anticipated? And, as the greatest puzzlement, why does the Earth seem to have moved in the “wrong” direction in some areas?

If there is a good side to the quake it is that in trying to answer all those questions, scientists are getting much closer to understanding a baffling seismological mystery. The San Andreas bends where it passes through the area where the quake hit. But earthquakes tend to follow straight lines. So scientists have been perplexed over what happens when an earthquake turns a corner.

Loma Prieta has helped them make considerable progress toward understanding that.

The Loma Prieta quake shows that when an earthquake bends along a fault line, it does so very awkwardly, shifting and lifting great blocks of ground along the fault in ways that had not been anticipated. Scientists, who were astonished to learn soon after the quake that the temblor caused a vertical displacement as well as horizontal movement, now believe that was because of the bend in the fault.

“The fact that the fault did not break through the surface was a surprise. The fact that it occurred on an inclined plane (and thus had a vertical component) was a surprise. But in hindsight it all makes sense,” Bakun said.

The San Andreas Fault marks the border between the North American and Pacific tectonic plates, the huge slabs of the Earth’s crust that support the continent and the ocean. The San Andreas is known as a right lateral shift fault, which means simply that during an earthquake, someone standing on one side of the fault would see the other side shift laterally to the right. That is the direction of the plate movement.

Here is what Bakun believes happened at Loma Prieta:

When the quake struck, the bend in the fault caused a complex series of stress patterns and giant blocks of ground were rotated slightly. That caused movement in some areas in the opposite direction of the fault’s normal movement.

In addition, the movement of the Pacific Plate into the curvature of the bend caused strain to build up there, but the curve prevented the plate from moving forward. So the San Andreas handled that by “sliding up and over.”

Thus a fault that normally moves right laterally also moved vertically, and in some cases to the left, because of the bend in the fault.

All of that might have been easier to predict, Bakun said, if the record from the great San Francisco quake in 1906 had been more complete.

“Most of the break in 1906 was well documented,” he said. “But that particular section was not. They had an inexperienced graduate student assigned to that section, and he got lost.”

So scientists are now closer to understanding how the San Andreas turns a corner, but not all the lessons from Loma Prieta are encouraging.

One of the biggest surprises from the October quake was that it caused so much damage at such a distance, collapsing freeway structures and houses more than 50 miles away.

“It was surprising that an earthquake of that size that far away could do that much damage,” Bakun said.

“What we learned is that areas that have suffered damage in historic quakes are susceptible to damage in future quakes, even if the quakes are far away.” And, as Loma Prieta demonstrated, even if quakes are not very big.

SHIFTING PLATES Scientists now believe they understand why the San Andreas Fault, which normally moves horizontally, also moves vertically during the Loma Prieta quake last October. The quake hit in an area where the San Andreas bends slightly to the west, and that kink in the fault caused the Pacific side to slip up over the continental side. The focus of the quake, called hypocenter, was unusually deep, 11.5 miles below surface. The fault slipped 6.2 feet horizontally, and 14 inches vertically.