Hollister and Bay Area residents saw their afternoon interrupted by a magnitude-4.4 temblor on April 4. The event, which occurred after 3 p.m. with an epicenter near Tres Pinos, was widely felt throughout the Salinas Valley and the Bay Area.
By Alba M. Rodriguez Padilla, Ph.D. candidate, UC Davis (@_absrp)
Citation: Rodriguez Padilla, A.M., 2023, Magnitude-4.4 earthquake rattles the Bay Area and Salinas Valley, Temblor, http://doi.org/10.32858/temblor.306
Editor’s note: This article was updated on April 6 with a link to automatic relocation of the earthquake.
A magnitude-4.4 earthquake rattled the Bay Area and neighboring populations on Tuesday, April 4, at 3:23 p.m. 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 Monterey. Residents from San Francisco to Santa Cruz reported light shaking. In the towns of Hollister and Salinas, closer to the epicenter, locals reported moderate shaking, reaching an intensity of five on the Modified Mercalli Intensity scale.
The U.S. Geological Survey ShakeAlert system issued an alert associated with the event, received by smartphone users in 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 through the USGS page. Though the event was widely felt throughout the region, as far north as the city of Santa Rosa more than 100 miles (160 kilometers) from the epicenter, the shaking was insufficient to cause damage, according to the USGS PAGER report for the earthquake.
The creeping faults of Central California
The event was the result of lateral motion on a strike-slip fault, most likely the Calaveras Fault, which meets its westerly neighbor, the San Andreas Fault, south of Hollister. (Pending relocation, like this automatic relocation created by scientists at Lamont-Doherty Earth Observatory that more accurately pinpoints the location and depth, this earthquake appears to have struck on the Tres Pinos Fault, which is a secondary, smaller fault, east of the Calaveras and part of the Calaveras Fault Zone.) The Calaveras is a right-lateral strike-slip fault, meaning 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 of the San Francisco Bay.
Near the epicenter of yesterday’s quake, the Calaveras Fault creeps at a rate of a few millimeters per year (Li et al., 2023). Instead of remaining “locked” until it experiences large earthquakes, the fault moves slowly, or creeps, almost continuously, accompanied by small events like yesterday’s temblor, which is the fifth event over magnitude 3 to occur near Tres Pinos since January 2020. According to Heather Crume (Shaddox), an NSF postdoctoral fellow at the University of California, Berkeley, events below magnitude 4.5 are common at the junction of the San Andreas and the Calaveras faults.
The interplay between creep and earthquakes remains a subject of active research, especially south of the Bay Area, where the San Andreas transitions from being locked to creeping. The Calaveras Fault hosted a magnitude-5.1 earthquake, with its epicenter north of yesterday’s event, in October 2022. Scientists observed “triggered creep” — small motion in response to the passage of the seismic waves from the Calaveras event — on the San Andreas Fault following that earthquake, Crume (Shaddox) says. Whether yesterday’s event is accompanied by creep on either the Calaveras or the San Andreas Fault awaits documentation.
Location, location, location
The Calaveras not only splays off the San Andreas, but it also connects to the Hayward Fault, which has been called “the most dangerous fault in America” because it runs right through densely populated communities like Oakland and Berkeley and its recurrence interval suggests a large earthquake is possible in our lifetimes. Like its sister the Calaveras, the Hayward Fault creeps through the Bay Area. But creep doesn’t remove earthquake hazard from the equation. Creeping faults in the Bay Area can host destructive earthquakes. In 1868, the Hayward Fault ruptured in a devastating magnitude-6.8 earthquake. In 1984, the Calaveras Fault produced a magnitude-6.2 temblor near Morgan Hill; that event produced $7.5 million in damage.
The Third California Earthquake Rupture Forecast suggests the Calaveras has the second-highest probability of rupturing in a large magnitude earthquake — greater than or equal to magnitude 6.7 — by 2043 (second behind the Hayward). Both faults could also affect one another. Co-rupture of the Calaveras with neighboring faults like the Hayward increases the likelihood of larger events (potentially more than magnitude 7.0), and large events are expected on the Calaveras and the Hayward because neither has had a major event in over a century.
California is home to a plate boundary
Meanwhile, about 400 miles (640 kilometers) away, Southern California experienced a widely felt magnitude-4.2 earthquake last Friday. Yesterday’s event in Hollister is too small and too far to be related to Friday’s event but nonetheless highlights that California is home to an active plate boundary. Earthquakes, small and big, are expected. If you feel shaking, drop, cover, and hold on.
Chaussard, E., Bürgmann, R., Fattahi, H., Nadeau, R.M., Taira, T., Johnson, C.W., and Johanson, I. (2015). Potential for larger earthquakes in the East San Francisco Bay Area due to the direct connection between the Hayward and Calaveras Faults. Geophysical Research Letters, 42, 2734– 2741. doi: 10.1002/2015GL063575.
Li, Y., Bürgmann, R., & Taira, T. A. (2023). Spatiotemporal variations of surface deformation, shallow creep rate, and slip partitioning between the San Andreas and southern Calaveras Fault. Journal of Geophysical Research: Solid Earth, 128(1), e2022JB025363. https://doi.org/10.1029/2022JB025363