Earthquake lifted county shoreline, researchers say

California’s first recorded earthquake, felt by the Spanish in 1769

and measured by the length of Hail Marys they could utter, may have been

powerful enough to raise the Orange County shoreline more than 11 feet in

some places, UC Irvine researchers say.

The “severe” earthquake, described in Spanish explorer Gaspar de

Portola’s diary, may have had a 7.3 magnitude, significantly larger than

the 6.7-magnitude Northridge earthquake of 1994, said Lisa Grant,

professor of environmental analysis and design at UCI’s School of Social

Ecology.

Grant and her colleagues have traced geological and historical records

to determine that a major earthquake occurred sometime between 1635 and

1855 in the San Joaquin Hills, most likely in 1769.

“The San Joaquin Hills cover an area previously thought to have low

earthquake potential,” Grant said. “In findings published in 1999, we

discovered that a large-magnitude earthquake could occur here; now we’ve

discovered it has occurred. Our research may be the first documented

evidence of an early historic or prehistoric blind thrust earthquake.”

Grant and her colleagues used radiocarbon dating to determine the age

of plants and shells from the elevated marsh bench, or ancient shoreline,

in Upper Newport Bay and along the coastal San Joaquin Hills, finding

that they must have been deposited no earlier than 1635. They measured

the elevation of the marsh bench and the current shoreline and calculated

the earthquake’s magnitude from the amount of displacement it caused.

They also studied data from the freshwater San Joaquin Marsh, which

lies inland between UCI and Upper Newport Bay. Pollen from the marsh

suggest the earthquake occurred just before or about the same time the

Spaniards arrived in Southern California.