Earthquake Theory May Be Rattled

An early analysis of last month’s magnitude 6 earthquake in Parkfield has led state geologists to reconsider prevailing theories of how shaking radiates from a quake’s epicenter.

Geologists studying how the ground shook during the Parkfield temblor found that the shuddering died off quickly just a few miles from the fault.

The finding also raises the possibility that building codes for structures far from faults may be much safer than previously believed.

“Future building codes may have less stringent requirements if you’re some distance, say 30 or 40 miles, from a fault,” Michael Reichle, acting state geologist, said at a press conference Wednesday.

Reichle said ground shaking 10 kilometers, or 6.25 miles, from the fault was far less severe than had been predicted. Similar patterns were seen in the magnitude 6.5 San Simeon earthquake in December and in the magnitude 7.3 Landers quake in 1992, he said, leading scientists to believe they may have discovered an important trend and not just a quirk.

Despite being a moderate shaker of magnitude 6 in a rural area, the Sept. 28 Parkfield earthquake is scientifically important because it was so well recorded.

The earthquake-prone stretch of the San Andreas had been thoroughly wired with sensors by geologists in order to “catch” an earthquake and glean as much data as possible about how one starts and grows.

It would take years of analysis, debate and legislative work to revise building codes, and state geologists are not suggesting any change should be made yet.

But they said information from Parkfield and other recent quakes may reverse the trend of radically strengthening building codes after every major earthquake.

“There’s a glimmer of positive development in this area,” said Tony Shakal, who heads the state’s strong motion instrumentation program. “It’s not like after every earthquake we’re going to have to keep raising and raising and raising the building codes.”

Shakal said residents of the Los Angeles Basin would probably see little benefit because the region is crisscrossed by faults. Thus, virtually every place is close to a fault.

A large part of the Bay Area also is close to two major faults, the San Andreas and the Hayward.

The finding also appears to apply only to “strike-slip faults” such as the San Andreas, where one side slips past the other with little up-and-down motion.

The finding probably does not hold for “thrust faults,” which produce vertical motion, the source of the most violent shaking. The magnitude 6.7 Northridge quake in 1994 was on a type of thrust fault hidden beneath the surface.

Tom Heaton, a structural engineer and geophysicist at Caltech, said the new data from Parkfield weren’t enough to convince him that hard-won safety improvements in building codes should be changed.

“It would be premature for us to say now that we’ve got one earthquake, we can change all our codes,” he said. “In Parkfield, there were no buildings to be tested -- and it wasn’t that big of an earthquake.”

It is much more economical to make a building strong while designing and building it than to retrofit it later, Heaton added.

Scientists are still puzzling over how the ground trembled after the Parkfield quake. They found a strange lull in shaking on one section of the fault in the town of Parkfield, about 6 miles from the epicenter. Shaking there was one-third as intense as it was just northwest and southeast of the town.

The intense shaking to the northwest was expected because the rupture was occurring in that direction.

“It’s sort of like the difference between looking down a barrel of a gun and being behind it,” Shakal said.

But scientists are still perplexed by the strong shaking southeast of the epicenter -- opposite the direction of the rupture.

The differences in shaking intensity may have been a product of the soil and rock underlying the fault or some still undetermined quake dynamic, scientists said.

“What the Parkfield earthquake is going to tell us is that things aren’t as simple as we had hoped,” Reichle said. “They don’t rupture in nice, smooth processes. They have slowdowns and speed-ups, and the pattern of shaking is much more complicated than we anticipated.”