8th-grade students in Torrance, California, take cover under their school desks during an earthquake drill

Children shelter under their desks during an earthquake drill in Torrance, California. The region’s number of active faults is higher than previously realized. Credit: Scott Varley/Digital First Media/Torrance Daily Breeze /Getty

Geophysics

Danger lurks in Californian fault long presumed dead

A Los Angeles fault could unleash significant destruction — but isn’t factored into the region’s earthquake planning.

A buried geological fault that has long been considered inactive is, in fact, capable of unleashing large earthquakes directly beneath the Los Angeles area.

Geologists thought that the Wilmington fault, which lies south of downtown Los Angeles and stretches over about 20 kilometres, had been inactive for several million years. As a result, the fault is not included in earthquake hazard assessments for Southern California.

Franklin Wolfe at Harvard University in Cambridge, Massachusetts, and his colleagues analysed seismic surveys from the region and information from hundreds of oil and water wells. The data allowed the scientists to visualize the underground geology in more detail than before.

The team found that rock layers along and above the Wilmington fault have moved in the recent geological past. The scientists calculated that the fault causes earthquakes of magnitude 6.3 to 6.4 roughly every 3,200 to 4,700 years, on average. If nearby faults were to move at the same time as the Wilmington, the resulting earthquakes could exceed magnitude 7.