Pasadena sits on much shallower sediment than downtown Los Angeles, according to a new Caltech study that produced a detailed underground map of the LA Basin.

The finding matters for building safety. Deeper soft sediments amplify seismic waves and produce longer-period shaking that hits tall buildings hardest. Beneath downtown LA, those sediments run up to 10 kilometers deep. Beneath the San Gabriel Valley, where Pasadena lies, the ground is far shallower, according to a Caltech news release published Aug. 31.

The peer-reviewed paper appeared Aug. 4 in the journal JGR Solid Earth.

Caltech graduate student Valeria Villa led the study alongside Robert Clayton, professor of geophysics, emeritus. Their team installed 273 small seismic sensors across LA County at 1- to 2-kilometer intervals, stretching from Pasadena to Long Beach and Santa Monica to Orange County. Each sensor stood about five inches above ground and recorded data for a month.

Villa developed a method combining gravity and seismic-wave measurements to see through the basin's scrambled signals. The result: a 3D map of the region.

"When LA is hit by a large earthquake, there are going to be some city blocks that are damaged and some that are relatively unscathed," Clayton said. "The underground basin variations control how the damage will be distributed."

In computer simulations of a magnitude 7.8 earthquake originating near the Salton Sea, seismic waves ripple into the LA Basin and shake downtown for minutes because of those deep sediments. Clayton noted that a quake within the basin itself could be just as destructive, pointing to the 1933 Long Beach earthquake. That magnitude 6.4 event on the Newport-Inglewood fault caused heavy damage, worsened by soft ground and weak construction.

Pasadena faces its own fault risks. The Raymond fault runs through the city and neighboring communities including South Pasadena, San Marino and Arcadia. It stretches 26 kilometers and could produce earthquakes between magnitude 6.0 and 7.0, according to the Southern California Earthquake Data Center at Caltech. The Sierra Madre fault zone carries the same magnitude range and sits near Altadena.

The city has already ordered soft-story apartment buildings to be retrofitted for earthquake safety, according to the Pasadena Star-News.

On Aug. 4, the LA County Board of Supervisors voted to require the Office of Emergency Management to deliver a comprehensive earthquake-readiness assessment by Sept. 29. Fifth District Supervisor Kathryn Barger introduced the motion. That review will engage seismologist Lucy Jones and researchers from Caltech and UCLA.

The Caltech team's next step is mapping how sediments meet bedrock below the basin. A hard boundary there can bounce seismic waves back upward, intensifying shaking at the surface. The researchers also want to study variations in the topmost kilometer of soil, where differences are most consequential for homes and buildings.

The National Science Foundation (NSF) funded the study. Villa is supported by an NSF Graduate Research Fellowship. The study's geographic data is publicly available on GitHub.

The Great California ShakeOut earthquake drill is scheduled for Oct. 15 at 10:15 a.m. Residents can register at shakeout.org.