Strategy would temporarily place seismic sensors every couple hundred feet
Caltech's Rob Clayton is preparing to launch a project to blanket the Los Angeles Basin with 25 seismic sensors per square mile, gathering high-resolution data at an unprecedentedly wide scale over the 800-square-mile area.
The 3-D passive seismic survey will be a collaboration with seismic exploration company Sisprobe and geophysical consulting firm LA Seismic, and would serve the dual purposes of seismic hazard assessment and identifying areas for improving the resiliency of critical infrastructure.
"The sensors that we use for these surveys were developed for natural resources exploration, but also reveal a great deal about low-level seismicity and seismic hazard," says Clayton, professor of geophysics. "This is an example of how academia can partner with industry to launch large-scale projects."
The Southern California Seismic Network (SCSN), a partnership between Caltech and the USGS, covers the same region with about 20 permanent seismometers in the Los Angeles Basin. Clayton's project would increase that density on a temporary basis by three orders of magnitude, by creating an array of 5,000 portable nodes that will move across the Basin in approximately four months.
Ambient seismic noise is recorded in the Long Beach/Seal Beach area.
While the SCSN offers continuous monitoring of SoCal and focuses on magnitude 2.0 or greater earthquakes, Clayton's dense mobile array would detect earthquakes much less than magnitude 2.0 that can be used to study the structure of the earth beneath the LA Basin.
"Tracking how seismic waves move through the ground gives us clues to where fault lines exist. A denser array creates a higher-resolution image, letting us see structures that we might have previously missed," Clayton says.
Clayton anticipates that the survey would produce 60 terabytes of raw data-comparable to 90,000 CD-ROMs worth of information-and will take about four months to complete (not including data processing). It would cost about $11 million, a sum that the team is currently working on raising.
The end result would be a uniform and detailed ground-motion map revealing the faults beneath the LA Basin, and would be kept in an open-source database.
An earthquake in Carson is recorded using a seismic array deployed in Long Beach.
Clayton has used similar surveys on a smaller scale, blanketing Long Beach, Santa Fe Springs, West Orange County, and South West Los Angeles with thousands of sensors in the past. Those highly localized efforts, conducted over the past eight years, revealed previously undiscovered faults, folds in the earth, and creeping zones, Clayton says.
These surveys have shown that the potential level of shaking varies significantly over distances as short as a mile. This means that the hazard not only depends on the size and location of the earthquake but also on the material that you are standing on.
"This information can help cities evaluate earthquake hazards to people and critical infrastructure like highways, pipelines, and the Ports of Los Angeles and Long Beach," Clayton says. "Everyone in the LA area understands that we face the risk of an earthquake at any time, but being able to describe the level of risk on a block-by-block resolution can help us better plan for the future."
