This earthquake fault in northeast L.A. and Pasadena isn't well known, but experts say it poses dangers

California officials have mapped a new stretch of an earthquake fault through northeast Los Angeles — a fault that could cause major damage in the heart of the metro area.

The Raymond fault has long been known as a potentially dangerous fault for Los Angeles and the San Gabriel Valley, and caused the magnitude-4.9 Pasadena earthquake in 1988, said Tim Dawson, senior engineering geologist with the California Geological Survey.

But it’s a fault that can pack a punch. That 1988 earthquake literally threw seismologist Lucy Jones out of bed. “The wave was coming up at me,” Jones recently recalled. “It was the most scared I have been in earthquake shaking.”

And it’s capable of a much worse earthquake. It could cause an earthquake as large as a magnitude 7. The Raymond fault runs from northeast L.A. through South Pasadena, Pasadena, San Marino, Arcadia, Monrovia and the unincorporated area of East Pasadena. For stretches, the fault runs alongside parts of Eagle Rock, York and Huntington boulevards, and under a stretch of the 110 Freeway in South Pasadena.


A known hazard

Earthquake faults are giant cracks in Earth's surface; a boundary line of sorts between shifting tectonic plates. When a major earthquake hits a fault like the Raymond, the plates suddenly move in opposite directions along the path of the fault.

The Raymond fault has long been known to experts, and much of the fault was mapped in the 1970s by the state. On June 15, the California Geological Survey issued a revised map of the Raymond fault.

The most significant changes involved extending the fault to the west, adding areas of Glassell Park, Eagle Rock and Highland Park to the Raymond fault zone. The fault zone was also moved farther south in Highland Park.

In the San Gabriel Valley, changes to the so-called Alquist-Priolo earthquake fault zone were minor.

Anyone seeking to build a new structure for human occupancy in an earthquake fault zone is required to conduct testing to determine whether they would be building directly on top of a fault.

State law generally bans construction of new buildings directly on top of an earthquake fault.

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