Three unrelated magnitude-6-plus earthquakes shook Baja California in three weeks, on three different faults.
By Hector Gonzalez-Huizar, Ph.D., Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE)
Citation: Gonzalez-Huizar, H., 2022, Three earthquakes strike near the western US-Mexico border, Temblor, http://doi.org/10.32858/temblor.283
When a magnitude-6.2 earthquake struck Baja California (Mexico) on Nov. 22, it marked the third quake larger than magnitude 6 to be felt in the region in less than three weeks. Within a period of only 20 days, three large earthquakes were felt by people in Baja California (Mexico) and Southern California (United States). The first quake, a magnitude-6.0 event, occurred on Nov. 2, with an epicenter located in the Pacific Ocean, some 950 miles (about 1,500 kilometers) west of San Diego, California. The second quake, a magnitude-6.1, occurred in the Gulf of California, in Mexico, on Nov. 4. The epicenter of the third one was near the town of San Quintin, in Baja California. Fortunately, there are no reports of significant damage caused by these three events. However, they are a reminder that this region along the western U.S.-Mexico border is surrounded by faults capable of generating large, damaging earthquakes.
The three earthquakes were felt in many cities near the U.S.-Mexico border, including San Diego and Tijuana, according to the reports of the USGS Did You Feel It? website. Yesterday’s quake was felt as far as Los Angeles, California, and Phoenix, Arizona, around 250 and 350 miles (400 and 560 kilometers) from the epicenter, respectively.
The three quakes’ epicenters
The first quake, the magnitude 6.0, occurred in a region where large earthquakes are rare. The epicenter is far from any tectonic boundary (that is, the limit between two tectonic plates, where most earthquakes worldwide are generated).
The second event, the magnitude 6.1, occurred along the tectonic boundary between the Pacific Plate and the North American Plate in the Gulf of California. The system of faults that defines the plate boundary in this region connects to the north with the San Andreas Fault system in California (Castro et al., 2021). The epicenter of this earthquake is located close to 70 miles (about 110 kilometers) from the epicenter of a magnitude-7.0 earthquake that occurred in 2012. The USGS earthquake catalog reports that 15 earthquakes of magnitude 6 or larger have occurred along the Gulf of California since 2007, meaning on average, one of these events has occurred per year. However, it is important to mention that these numbers are slightly higher than what is reported in the catalog of the CICESE Seismic Network, which monitors the seismicity in this part of Mexico—one of the most seismically active regions in the country.
Yesterday’s quake, the magnitude-6.2 event, was located near the town of San Quintin, in Baja California, around 125 miles (200 kilometers) south of San Diego. The earthquake can be attributed to motion along the large San Clemente Fault system. This large strike-slip fault system can be considered as part of a broader North American-Pacific Plate boundary (Walton et al., 2020), which includes the San Andreas Fault Zone that gave us the 1857 magnitude-7.9 Ft.Tejon quake and the 1906 magnitude-7.8 San Francisco quake. The San Clemente Fault can be thought of as the westernmost strand, or sliver, of the San Andreas Fault Zone.
The San Clemente Fault extends for 200 to 300 miles (400 to 500 kilometers) offshore, where it could generate earthquakes up to a magnitude 8.0. Within 24 hours of the magnitude-6.2 quake, more than 100 aftershocks have been detected, the largest with a magnitude of 4.5.
Even though the three earthquakes occurred in the seafloor, no tsunamis were produced. The three earthquakes were generated by faults in which the blocks of rocks have moved mostly horizontally, known as strike-slip faulting. In these kind of quakes, the vertical displacement of the seafloor during the movement is expected to be small. Thus, no big sea waves are generated, and tsunamis are less likely to occur.
Castro, R., Carciumaru, D., Collin, M., Vetel, W., Gonzalez-Huizar, H., Mendoza, A., et al. (2021), Seismicity in the Gulf of California, Mexico, in the period 1901–2018, J. South Am. Earth Sci. 106, 103087, doi: 10.1016/j.jsames.2020.103087.
Gonzalez-Huizar, H., Fletcher, J. M., (2020), Baja quakes highlight seismic risk in northern Mexico, Temblor, http://doi.org/10.32858/temblor.116
Walton, M.A.L., Brothers, D.S., Conrad, J.E., Maier, K.L., Roland, E.C., Kluesner, J.W., and Dartnell, P., (2020), Morphology, structure, and kinematics of the San Clemente and Catalina faults based on high-resolution marine geophysical data, southern California Inner Continental Borderland (USA): Geo- sphere, v. 16, no. 5, p. 1312–1335, https://doi.org/10 .1130/GES02187.1.