Scientists Find New Troubles in Predicting Quake Effects
Since the costly Northridge quake of 1994, seismologists have been scrambling to understand “site effects”--how sediments and structures deep beneath the ground can dangerously amplify or dampen earthquake waves.
But scientists gathered here Tuesday at a conference on earthquake science now say that such site effects may be far harder to decipher than once anticipated.
Earthquakes don’t always follow rules--at least not the rules seismologists understand. For instance, in the Northridge quake, houses were destroyed in Santa Monica, 17 miles from the epicenter. And although some houses there collapsed, others a block away stood intact. In another mystery, it is not known why areas 50 miles from the epicenter shook so hard after the Hector Mine quake last October, when areas closer to the fault barely trembled.
A widely used government hazard map created in 1996 doesn’t help answer such questions. Based on faults that crisscross the region, it assumes a California made of solid rock. The problem is, it’s not. Rather, the state is a complex amalgam of different sediments, basins and valleys.
Most cities sit on sediments considered far less stable than rock, said Edward Field, a USC research geologist. Field is heading a team at the Southern California Earthquake Center charged with improving the state’s hazard maps by including various site effects. Just a few weeks ago, Field and his team were ready to toss out the long-held belief that geological details--how old sediment layers are and how large the particles are within them--can predict the intensity of shaking.
“We’re changing our story now,” Field said.
A man responsible for that change is Chris Wills, a senior engineering geologist with the California Division of Mines and Geology in San Francisco. Since 1996, Wills and his team have been piecing together a map of the state’s various sediment types, using geological data and hundreds of experiments that measure how quickly waves travel through the ground in different areas.
Less than two months ago, Wills sent the new map to Field for review. Field immediately tested whether areas with different sediments showed different responses to earthquake waves. Much to the surprise of both men, they found a close connection between sediment types and shaking that had not been seen using previous maps. “It’s astounding,” said Wills.
While the new map is an advance that should help improve hazard prediction, Wills remains humble about earthquake damage prediction. The intensity of a quake depends not only on some of the site conditions he has identified, but on conditions at the source of an earthquake and also along the path earthquake waves travel.
Structures deep underground can also play a role. A group of UCLA scientists is currently testing a theory that a bowl-shaped dip in bedrock two miles below ground is responsible for focusing earthquake shock waves from the Northridge quake that damaged Santa Monica. Studies have provided more evidence for the theory but have not conclusively proved it, said Paul Davis, a UCLA seismologist who presented new findings on the question Tuesday.
Since 1996, Kim Olsen, a seismologist at the Institute for Crustal Studies at UC Santa Barbara, has used computer models to predict how earthquakes on different faults would affect the Los Angeles Basin.
Olsen was hoping to find patterns highlighting which areas of the basin were consistently most vulnerable to quakes. What he found instead was more chaotic: Quakes on each fault affected the area in entirely different ways. Even quakes on the same fault could affect the region differently, depending on the direction of the rupture.
“This was surprising to me and kind of discouraging,” he said. “Nobody expected this.”
But Olsen did find one factor common to all quake scenarios. The deepest parts of basins are affected the most. In the Los Angeles area, this would include the heavily populated area south and west of downtown Los Angeles. It would also include parts of the San Fernando Valley near the Golden State and parts of the San Gabriel Valley near the Foothill Freeway, he said.
A number of studies presented here Tuesday confirm the idea that areas over deep basins shake the hardest during and after quakes. But even that conclusion remains a bit murky. It might be the depth of sediment layers that affect stability. But it might also be distance from the edge of the basin that influences earthquake waves.
The group does believe the data enough to confidently predict that areas in the middle of basins may experience shaking twice as strong as areas at the edge of basins. But other geologists warn that edges have their own instability, because they can redirect earthquake waves in unexpected ways. As to nailing down exactly where future earthquake hot spots lie, Field is realistically pessimistic--and declines to predict when the science might improve.
“This intrinsic uncertainty, it looks like we’re going to have to live with it,” he said. “That may be true forever.”
