Scientists from the Southern California Earthquake Center (SCEC) have released a comprehensive new study that identifies hot spots of seismic shaking in Southern California.
“Considering all possible earthquakes, these are areas that we believe are going to shake more than other areas,” said Tom Henyey, SCEC director and professor of earth sciences at the USC. “Every earthquake is unique and has its own hot spots that won’t necessarily be identical to these average hot spots.”
The hot spots are Southern California’s large valleys and basins where the ground is flatter, the soil softer and the area more populous.
“If you live up in the mountains you can get away from the amplification of seismic waves that occur down in the valleys,” said Lucy Jones, scientist-in-charge for the United States Geological Service (USGS). “This is another piece of information that all of us in Southern California can use to manage the risks in our lives. Up in the mountains, you risk wildfires and mudslides. And, if you live on America’s southeast or Gulf coasts, you risk hurricanes.”
Jones added that she considered the new study as important as studies that have revealed new faults.
“Most faults have repeat times of thousands of years, but seismic waves are amplified with every earthquake,” she said.
Ned Field, a seismologist with USGS and the SCEC working group leader, said the study, released Jan. 16, focuses on how local geologic conditions in crease earthquake shaking.
“We found that the two most important factors are the softness of the rock or soil near the earth’s surface and the thickness of the sedimentary rock under the site,” said Field. “We looked at many other site attributes, but these were the two that most affect shaking.”
While it has been known for at least 200 years that sediment-filled valleys usually shake more than rock-hard mountain slopes during earthquakes, Field said the study quantifies and refines how local Southern California geology affects the waves generated by earthquakes.
“When seismic waves hit softer material, they slow down and they get bigger because they are still transmitting the same amount of energy,” explained Field. “The bowl shape of valleys tends to focus those waves on the centers of the valleys, where sediments tend to be the deepest.”
Field reiterated Henyey’s observation that every earthquake is unique and said that the intensity of shaking at any given location is determined by:
• the location’s distance from the epicenter, the strength of the quake and the direction in which a fault ruptures; and
• local geologic conditions near the surface of the earth, which can amplify seismic waves.
Field added that Santa Monica, which is near the edge of the Los Angeles Basin, was hit harder by the Northridge quake because the geometry of the bedrock at the edge of the basin focused the seismic waves on Santa Monica.
“Santa Monica was a local hot spot in the Northridge quake, although it isn’t located in a very dangerous area in our new study,” said Field.
The more than 20 scientists who participated in the study compared real measurements of shaking taken during earthquakes with a variety of existing geological data. They found that a new geological map by the California Division of Mines and Geology (CDMG) correlated better with seismic shaking measurements than other more detailed maps.
“This map covers the entire state, and it should prove useful in performing the same kind of studies in other regions,” said Field.
The SCEC study will be published this month in a special volume of the Bulletin of the Seismological Society of America (BSSA).
“We are presenting a scientific study, not a policy recommendation,” said USC’s Henyey. “It is up to public officials to take this new scientific in formation and implement any changes in policy that may be warranted.”
One of 28 National Science Foundation science and technology centers and co-funded by USGS, SCEC is a community of scientists from eight core academic institutions doing research on Southern California earthquake hazards. Founded in 1991, SCEC is headquartered in USC’s College of Letters, Arts and Sciences.
Additional funding for this study was provided by the CDMG and the California Department of Transportation (Caltrans).
“SCEC has provided the leadership that has enabled different branches of science and engineering to work together to address earthquake hazards,” said James Whitcomb, deputy director of the NSF’s division of earth sciences, which funds SCEC. “It’s exciting that the techniques that recently have been developed can be applied to other parts of the U.S. and the world to reduce the destruction of earthquakes.”
USGS Director Chip Groat said, “These efforts are an outstanding example of the power of partnerships to protect lives and property. We look forward to continuing cooperation with SCEC, CDMG, the academic community and other public- and private-sector partners to reduce the rising human and economic toll of natural disasters.”