On Sunday, a magnitude-5.1 earthquake struck about 7 kilometers (4.3 miles) southeast of Ojai, Calif., likely due to a small, as-yet unidentified fault near the San Cayetano Fault. The confluence of earthquake and Tropical Storm Hilary is an unfortunate coincidence.
By Fionna M. D. Samuels, Ph.D., Optimum Seismic Fellow (@Fairy__Hedgehog)
Citation: Samuels, F.M.D., 2023, Southern California shaken by earthquake during unrelated hurricane, Temblor, http://doi.org/10.32858/temblor.319
Rains from Tropical Storm Hilary began to drench Southern California on Sunday morning. At 2:41 p.m. local time, residents were surprised by a magnitude-5.1 earthquake. Quickly dubbed a #Hurriquake on X (formerly Twitter), the temblor struck just over 4 miles (about 7 kilometers) southeast of Ojai, Calif., at a depth of about 3 miles (4.8 kilometers) according to preliminary data from the U.S. Geological Survey (USGS). The epicenter is located near the San Cayetano Fault, with one of its smaller neighbors the likely culprit.
Because the quake surpassed the magnitude-5 threshold, the USGS issued a ShakeAlert, which is designed to detect earthquakes fractions of a second after they begin and compute where shaking is likely to be felt. That alert goes to partner organizations who deliver the warning by sending SMS or in-app messages to people who may be affected. This acts as an early warning system, giving some people time to brace for shaking in the seconds before a large quake reaches them.
After shaking from a quake subsides, the USGS can determine how strongly the earthquake was perceived by people from self-reported data in addition to the event’s measured magnitude and ground motions. As of this writing, nearly 14,000 people had filled out Felt Reports.
People in Ojai, near the epicenter, reported feeling strong shaking and captured videos showing parked cars rocking and bottles falling from shelves in at least one restaurant. Southwest of the epicenter, in Ventura, people reported moderate shaking. The earthquake was felt, albeit very weakly, as far south as Los Angeles, about 80 miles (about 130 kilometers) away from the epicenter. No injuries or structural damages have been reported at this time.
Good afternoon Southern CA. Did you feel the magnitude 5.1
earthquake about 4 miles southeast of Ojai at 2:41 pm. #ShakeAlert system was activated. See: https://t.co/2JRvHxmuYN @Cal_OES @CalConservation @CAGeoSurvey pic.twitter.com/DusXYbwr81— USGS ShakeAlert (@USGS_ShakeAlert) August 20, 2023
The mechanism
The magnitude-5.1 quake was large enough to be recorded by numerous seismometers in the region. From these data, researchers calculate a focal mechanism. Seismologists use these beachball-like diagrams to represent the type of fault slip that occurs during an earthquake. Strike-slip, normal and thrust movements each have distinct representations determined by the recorded compression and dilation of the quake’s initial P-wave. In this case, the focal mechanism looks like a combination of a thrust and strike-slip earthquake, which implies it’s complicated.
Usually, seismologists compare an earthquake’s proximity to different faults to determine its source fault, says U.C. San Diego structural geologist and paleoseismologist Thomas Rockwell. Although this earthquake occurred near the San Cayetano Fault, he says it’s unlikely to be the source of Sunday’s quake. “The surface trace of the San Cayetano Fault is along the northern margin of Upper Ojai Valley,” says Rockwell, “whereas this earthquake was to the south, under Sulphur Mountain.”
In a blog post detailing this earthquake, geologist Judith Hubbard wrote that the epicenter of this earthquake seems to sit between two faults – the Lion Fault and the Mission Ridge-Arroyo Parida Fault – based on the Community Fault Model designed by the Southern California Earthquake Center. The opaque focal mechanism further complicates matters because it does not match the typical movements of either of these faults, Hubbard wrote.
The Lion and Mission Ridge-Arroyo Parida faults reside between the San Cayetano Fault and the Ventura Fault. “The system is capable of much larger earthquakes,” says Rockwell, though this is the largest recorded in the last 100 years. Paleoseismic data suggest that the most recent slip on the San Cayetano Fault occurred between 1660 and 1850 and may have been responsible for a large earthquake in December 1812. The magnitude was not recorded by modern instruments, of course, but based on geologic evidence, the earthquake may have been as large as magnitude 7.5.
The last rupture of the Ventura Fault was likely even earlier, about 800 years ago. Careful analysis of rocks around the fault suggest that this event was likely between magnitude 7.7 and 8.4.
There have been a plethora of aftershocks recorded by the USGS since the initial quake, including more than a dozen aftershocks with a magnitude of 3 or higher — large enough to be felt. A magnitude-3.9 aftershock was felt early Tuesday. Though the cumulative number of aftershocks will increase over time, the number of aftershocks occurring per day will likely decrease.
There was also a sequence of 19 foreshocks, all under a magnitude 2.5, the first of which was detected on Saturday morning about 26 hours before the mainshock, according to Egill Hauksson, an emeritus research professor at Caltech’s Seismological Laboratory who has been diving into the preliminary data.
Ultimately, geologists may be able to use data from the foreshocks and aftershocks to illuminate which fault slipped and paint a more detailed picture of this complicated fault system. This earthquake may eventually be ruled background seismicity, Hauksson says. “This earthquake occurred on a minor fault that is about 5 kilometers long,” he explains. “It did not rupture the surface and is simply a reflection of ongoing tectonic deformation as the wider region moves around the big bend in the San Andreas Fault.”
#Hurriquake
Although the two natural hazards were immediately tied together by the public on social media, researchers have clearly stated that Tropical Storm Hilary did not cause this earthquake.
Hurricanes don’t cause earthquakes, but they can be detected by seismometers!
Hurricanes cause huge waves that pound on the coast and the seafloor. This can be detected by seismometers for 100s of miles. https://t.co/73969zLPU4
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) August 20, 2023
The foreshocks began on Saturday morning, while Hurricane Hilary was still churning the waters south of Mexico’s Baja Peninsula. According to the National Weather Service, no precipitation fell in Ojai that day.
Earthquakes are most often caused by built-up tectonic stress, which is relieved when the two sides of a fault slip past each other. Water can change stress on faults either through deforming the earth beneath a large lake or other waterway, or through water percolating (or being injected) into the ground and finding its way to the fault. But neither of these scenarios occurred on Sunday.
“It was a coincidence,” says Hauksson. “Earthquakes are not triggered by rainfall because water only penetrates a few inches into the ground in a day.” Rainfall infiltrating the ground couldn’t reach several kilometers down to influence the fault in such a short time.
However, this confluence of calamities is an important reminder that earthquake preparedness should include preparing for multiple disasters like flooding and landslides, or even fires. “The Ventura Basin region is undergoing rapid contraction and a very large earthquake will occur at some time in the future,” says Rockwell. “Be prepared!”
To learn more about how to prepare for an earthquake, the USGS explains what to do here.
For more about whether Tropical Storm Hilary could trigger a major San Andreas earthquake, check out this story.
Fionna M. D. Samuels is Temblor’s Optimum Seismic Fellow. She is a science writer hailing from the Front Range of Colorado where she got her Ph.D. in Chemistry from Colorado State University. Her work has appeared in Eos, Scientific American and Symmetry. Optimum Seismic is sponsoring their first Temblor science writing fellow to cover important news about seismic resilience of the built environment.
References
Dolan, J. F., & Rockwell, T. K. (2001). Paleoseismologic evidence for a very large (M w> 7), post-AD 1660 surface rupture on the eastern San Cayetano fault, Ventura County, California: Was this the elusive source of the damaging 21 December 1812 earthquake?. Bulletin of the Seismological Society of America, 91(6), 1417-1432.
Hubbard, J., Shaw, J. H., Dolan, J., Pratt, T. L., McAuliffe, L., & Rockwell, T. K. (2014). Structure and seismic hazard of the Ventura Avenue anticline and Ventura fault, California: Prospect for large, multisegment ruptures in the western Transverse Ranges. Bulletin of the Seismological Society of America, 104(3), 1070-1087.
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