Themagnitude 4.5 earthquakerattled Northern California Monday night but caused no major damage. It was followed Tuesday afternoon by a4.8-magnitude quakenear Hollister.
It was the latest reminder that seismic forces put the East Bay at high risk of a major earthquake, including from the dangerousHayward Fault,which runs along heavily populated areas.
The epicenter of Monday’s quake was the Walnut Creek and Pleasant Hill areas. The earthquake had a preliminary depth of about nine miles underneath the surface, fairly deep for this part of the world, Keith Knudsen, USGS geologist and deputy director of the agency’s Earthquake Science Center, said in an interview. The depth of the quake caused it to be felt over a broad area, but the shaking felt at the surface was less intense than if the quake had been more shallow, scientists said.
The East Bay is at high risk of a major earthquake, scientists say.
(Randi Lynn Beach / Los Angeles Times)
Q: What do we know about the area of the epicenter?
The earthquake was not directly on top of any of the main Bay Area earthquake faults. The epicenter was about three miles west of the Concord fault, and farther than that off the northern end of the northern Calaveras fault, Knudsen said.
The epicenter is just northwest of Mt. Diablo, one of the Bay Area’s tallest peaks. The Mt. Diablo area is also a seismically active zone, “a region of uplift, folding and thrusting,” said David Schwartz, USGS scientist emeritus.
There were a series of earthquakes in the magnitude 5 range on the southeast side of Mt. Diablo in 1980 in the Greenville fault area. The Greenville fault area, one of the Bay Area’s seven most significant faults, ruptured with quakes of magnitudes(5.8and5.1on Jan. 24 , 1980, that were, respectively, 12 and nine miles north of Livermore.
On Jan. 26, 1980, amagnitude 5.4 quakehit with an epicenter only six miles from downtown Livermore. Damage in 1980 was reported at Lawrence Livermore National Laboratory and a mobile home park.
On July 16,a magnitude 4.3 earthquakehit near the northern side of the Greenville fault, about 11 miles northeast of downtown Livermore. It hit on the western edge of the Los Vaqueros Reservoir, which holds water from the Sacramento-San Joaquin Delta and is an important water source in the late summer and early fall when high levels of salt creep into the delta; no damage was reported.
Q: What do we know about seismic risk on the Concord-Green Valley fault?
The Concord-Green Valley and Calaveras faults are among the Bay Area’s most significant.
Ahypothetical magnitude 6.8 earthquakeon the Concord-Green Valley fault is capable of causing strong shaking in Contra Costa, Solano and Napa counties, and cause damage to the Kinder Morgan Concord pumping station, responsible for pumping fuel across the northern half of California, the Assn. of Bay Area Governments said in areportpublished in 2014. All five of the Bay Area’s refineries export their refined fuel through that Concord pumping station.
There are separate refined fuel pipelines that reach San Jose and Brisbane from the Chevron refinery in Richmond, but they represent a small share of the region’s fuel supply, the report said.
Such a plausible quake would be strong enough to cause liquefaction in all Bay Area counties. Strong shaking would also be felt in the Carquinez Strait, the narrow waterway connecting San Francisco and San Pablo bays to California’s two longest rivers, the Sacramento and San Joaquin; the shaking could cause the edges of dredged water channels to fall into the waterways, according to the association.
The last large quake on the Concord-Green Valley fault occurred in 1610. A more Averagemagnitude 5.4earthquake occurred on the fault in 1955.
Q: What about the Calaveras fault?
The Calaveras fault can produce a quake in the magnitude 7 range, and it’spossible that it could rupture jointlywith the Hayward fault, one of the nation’s most dangerous because the Hayward fault runs directly underneath densely populated cities in the East Bay, like Oakland, Berkeley, Hayward and Fremont.
The largest historical quake on the Calaveras fault was amagnitude 6.6 temblor in 1911, according to Ross Stein, chief executive ofTemblor.netand a former USGS research geophysicist.
One swarm in 2015 that occurred close to theCalaverasfault generated 4, 000 quakes over five months, according to theBerkeley Seismology Lab. The cluster of quakes occurred in theSan Ramon Valley, which has had many swarmsover the last several decades that haven’t resulted in large earthquake.
Map of Hayward fault
(Joe Fox / Los Angeles Times)
Q: What about the Hayward fault?
Alandmark reportpublished in 2018 by the US Geological Survey estimates that at least 800 people could be killed and 18, 000 more injured in a hypothetical magnitude 7 earthquake on the Hayward fault centered below Oakland.
Hundreds more could die from fire following an earthquake along the 52 – mile fault. More than 400 fires could ignite, burning the equivalent of 52, 000 single-family homes, and a lack of water for firefighters caused by old pipes shattering underground could make Matters Worse.
The Hayward fault is so dangerous because it runs through some of the most heavily populated parts of the Bay Area, spanning the length of the East Bay from the San Pablo Bay through Berkeley, Oakland, Hayward, Fremont and into Milpitas.
This curb was once flush but became offset because the Hayward fault is pulling the curb apart.
(Rong-Gong Lin II / Los Angeles Times)
Out of the region’s population of 7 million, 2 million people live on top of the fault, Schwartz has said, and that proximity brings potential peril. The 1989 Loma Prieta earthquake was centered in the sparsely populated Santa Cruz Mountains. For all the devastation of the 1906 San Francisco earthquake, it was centered off the coast in the Pacific Ocean.
The Hayward fault’s most memorable earthquake in recorded history was in 1868, and is estimated to have been a magnitude 6.8 earthquake – rupturing 20 miles of the fault’s length between San Leandro to what is now the Warm Springs neighborhood of Fremont,according to the USGS.
It killed about 30 people and caused immense property damage, i ncluding the collapse of the Alameda County Courthouse’s second floor and heavy damage at the historic Mission San Jose adobe church in southern Fremont.
What are the Bay Area’s other major earthquake faults?
Theseven significant earthquake fault zonesin the Bay Area all roughly parallel to each other . Besides the Hayward, Calaveras, Concord-Green Valley, and Greenville faults, here are the three others:
A map of the Bay Area’s major earthquake faults. Generally speaking, there are seven major fault zones in the region: the Calaveras, Concord-Green Valley, Greenville, Hayward, Rodgers Creek, San Andreas and San Gregorio.
(US Geological Survey ) ******
•San Andreas fault:The world’s most famous fault, the San Andreas triggered the great 1906 earthquake , estimated to be a magnitude 7.8 event that ruptured an astonishing 296 miles of fault,betweenSan Juan Bautista and Cape Mendocino, and destroyed most of San Francisco in shaking and fire and is believed to have killed at least 3, 000 people and destroyed 28, 000 buildings.
The quake produced 22 times more energy than the magnitude 6.9 Loma Prieta earthquake in 1989, which killed 63 people; The 30 th anniversary of that event is on Thursday. (The Loma Prieta earthquake, centered in the Santa Cruz Mountains, occurred on aInvolved sub-parallel strandof the San Andreas, and not on the main fault itself, according to USGS research geologist Kate Scharer.)
•Rodgers Creek fault:The Rodgers Creek fault runs through Santa Rosa. The USGS said in 2016 that a large earthquake on theRodgers Creek faulthas the potential to intensely shake the sedimentary basin beneath Santa Rosa.
On Oct. 1, 1969, there weretwo damaging quakesnear Santa Rosa of magnitudes 5.6 and 5.7, which at that time were the largest temblors in the Bay Area since 1906.
The Rodgers Creek fault is capable of aquake in the magnitude 7 range.
•San Gregorio fault:The San Gregorio fault runs west of the San Andreas, roughly off the coast of the Bay Area and along seashore areas of Marin and San Mateo counties, continuing south toward Monterey Bay. The most recent large event on this fault – like between magnitudes 7 and 7. 25 – probably occurred between 620 and 1400, according to astudyin the Bulletin of the Seismological Society of America published in 1997.
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