A 4.4-magnitude earthquake hits the Bay Area and the Salinas Valley

Residents of Hollister and the Bay Area had their afternoon interrupted by a 4.4-magnitude earthquake on April 4. The event, which occurred just after 3 p.m. and was centered near Tres Pinos, was widely felt throughout the Salinas Valley and Bay Area.

Written by Alba M. Rodriguez Padilla, Ph.D. Candidate, University of California Davis (_absrp)

Citation: Rodriguez Padilla, AM, 2023 Magnitude 4.4 Earthquake Hits Bay Area and Salinas Valley, Templor, http://doi.org/10.32858/temblor.306

A 4.4-magnitude earthquake struck the Bay Area and nearby residents on Tuesday, April 4, at 3:23 PM local time. The epicenter was located near the town of Hollister, south of the densely populated Bay Area, and east of the cities of Santa Cruz and Monterrey. Residents from San Francisco to Santa Cruz reported mild shaking. In the towns of Hollister and Salinas, closest to the epicenter, local residents reported moderate shaking, which reached an intensity of five on the modified Mercalli intensity scale.

The 4.4-magnitude earthquake struck on April 4 near Tres Pinos, California, in the Calaveras Fault region. Tentatively, it appears that the earthquake may have struck the Tres Pinos fault, just east of the Calaveras fault.

The US Geological Survey’s ShakeAlert system issued an alert associated with the event, which smartphone users received at locations where the shaking was expected to exceed a certain intensity. By 5 p.m., less than two hours after the event, more than 7,000 people had submitted a “Did you feel it?” Report via the USGS page. Although the event was widely felt throughout the region, even north of the city of Santa Rosa more than 100 miles (160 kilometers) from the epicenter, the shaking was not enough to cause damage, according to the USGS PAGER report on the quake.

Central California Creeping Flaws

The event was the result of lateral motion on a slip fault, most likely the Calaveras fault, which meets its western neighbor, the San Andreas fault, south of Hollister. (Pending transfer, a process that seismologists perform after an earthquake to more accurately determine location and depth, it appears that this earthquake may have struck the Tres Pinos fault, a smaller secondary fault, east of Calaveras and part of the Calaveras fault zone.) Calaveras is a sliding fault. Right lateral, which means that whichever side of the fault you’re on, the other side moves to the right. The fault extends for about 75 miles (about 120 kilometers), from its junction with the San Andreas Fault to north San Francisco Bay. Calaveras are very active and are believed to be capable of producing a magnitude 7+ earthquake.

In this map, the epicenter, identified by the USGS, appears to be located on a strand of the Tres Pinos fault, just east of the Calaveras fault zone.

Close to the epicenter of yesterday’s earthquake, the Calaveras fault is creeping at a rate of a few millimeters per year (Lee et al., 2023). Instead of staying “closed” until it experiences large earthquakes, the fault moves slowly, or creeps almost continuously, accompanied by small events like yesterday’s earthquake, the fifth magnitude-3 event near Tres Pinos since January 2020. According to Heather Crum (Shadox) ), NSF postdoctoral fellow at the University of California, Berkeley, Events of less than 4.5 degrees are common at the junction of the San Andreas and Calaveras faults.

The interaction between creep and earthquakes continues to be a topic of active research, especially south of the Bay Area, where San Andreas transitions from shutdown to crawl. The Calaveras fault hosted a 5.1-magnitude earthquake, with its epicenter just north of yesterday’s event, in October 2022. Scientists noticed “stimulus creep” — a small motion in response to the passage of seismic waves from the Calaveras event — in the San Andreas fault after that quake, Cromm (Shadox) says. . Whether yesterday’s event was accompanied by the Calaveras Crawl or the San Andreas Fault awaits documentation.

The Calaveras Fault is in constant motion, slowly creeping up, causing roads, homes, pipes, sidewalks, and other infrastructure to slowly separate. Here at Hollister the fender is shifted to the right, which is how you tell the Calaveras fault is a right side sliding fault. Credit: Kelly Michaels, CC BY-NC 2.0

Location, location, location

Not only does the Calaveras break away from the San Andreas, but it also connects to the Hayward Fault, which has been called “America’s deadliest fault” because it runs directly through densely populated communities like Oakland and Berkeley, and its frequency interval suggests a major earthquake is possible in our lifetime. Like its sister Calaveras, the Hayward Fault snakes through the Bay Area. But creep does not remove earthquake risk from the equation. Crawling faults in the Gulf region can host devastating earthquakes. In 1868, the Hayward Fault ruptured in a devastating earthquake of magnitude 6.8. In 1984, the Calaveras Fault triggered a magnitude 6.2 earthquake near Morgan Hill. This event produced $7.5 million in damages.

The California III rupture predictions indicate that Calaveras will have the second-highest potential to explode in a large-magnitude earthquake—greater than or equal to magnitude 6.7—by 2043 (second only to Hayward). Both defects can affect each other. The joint rupture of the Calaveras with neighboring faults such as the Hayward increases the potential for larger events, and large events are expected at the Calaveras and Hayward because they have not had a major event (7+) in over a century.

In this image of a dock in Hollister, a shift to the right again indicates that the Calaveras Fault is a right-side slip fault. Credit: Kelly Michaels, CC BY-NC 2.0

California is home to the plate limit

Meanwhile, about 400 miles (640 kilometers) away, Southern California was hit by a large-scale earthquake with a magnitude of 4.2 last Friday. Yesterday’s event in Hollister is too small and remote to be connected to Friday’s event, but it nonetheless highlights that California is home to an active plate frontier. Small and large earthquakes are expected. If you feel the vibration, drop it, cover it and hold on.

References

Lee Y, Borgmann R, and Tyra TA (2023). Spatiotemporal differences of surface deformation, shallow creep rate, and slip drift between the San Andreas fault and the southern Calaveras. Journal of Geophysical Research: Solid Earth, 128(1), e2022JB025363. https://doi.org/10.1029/2022JB025363

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