A magnitude-5.1 earthquake that recently occurred in Baja California, Mexico was felt as far as Los Angeles, California. What is the seismic risk imposed by the faults in Baja California?
By Hector Gonzalez-Huizar, Ph.D. and John M. Fletcher, Ph.D., Centro de Investigación Científica y Educación Superior de Ensenada, Baja California (CICESE).
Citation: Gonzalez-Huizar, H., Fletcher, J. M., 2020, Baja quakes highlight seismic risk in northern Mexico, Temblor, http://doi.org/10.32858/temblor.116
On the morning of Monday August 17, 2020, a magnitude-5.1 earthquake struck Baja California, Mexico. The mainshock originated approximately 78 miles (126 kilometers) south of the USA-Mexico border and 62 miles (100 kilometers) southeast from Ensenada, Mexico. USGS “Did you feel it?” reports indicate that shaking occurred throughout northern Baja and southern California. Like almost any other geologic event, earthquakes do not recognize international borders.
Faults in Baja
Most of the main faults in southern California can be traced into Baja California. These faults are primarily the result of the relative plate motion along the boundary between the North American and the Pacific tectonic plates. The main fault separating these plates — the San Andreas Fault — stretches through southern California, where it becomes the Imperial Fault south of the Salton Sea. In northern Mexico it continues as the Cerro Prieto Fault before it dives into the Gulf of California. Motion between the Pacific and North American plates is not limited to this series of faults. Throughout southern California and Baja, a network of faults accommodates a broad zone of movement known as the Southern California Shear Zone.
A history of seismicity
The faults in Baja California are seismically active and have the potential to generate large magnitude earthquakes that could affect communities on both sides of the international border. The magnitide-7.1 El Mayor-Cucapah earthquake struck northeastern Baja California on April 4, 2010, activating a series of previously unmapped faults in the Sierra Cucapah and southern portion of the Colorado River Delta.
The earthquake was widely felt throughout northwest Mexico and southern California. Cities close to the epicenter, like Calexico and Mexicali, experienced violent shaking. Four people died and approximately 100 were injured. It is estimated that around 25,000 people were affected. Many other faults in Baja California are capable of generating large magnitude earthquakes. For example, in 1931 a magnitide-7.1 earthquake occurred on the Cerro Prieto Fault. In 1892 a magnitide-7-7.5 earthquake occurred on the Laguna Salada Fault. In 1956 a magnitide-6.8 earthquake originated on the San Miguel-Vallecitos fault. It is estimated that the Agua Blanca Fault has the capacity to generate earthquakes as large as magnitide-7.3.
Recent earthquakes activated multiple faults
The August 17 magnitude-5.1 earthquake and its aftershock sequence occurred near the projected region of intersections of four major fault systems including the Agua Blanca, Tres Hermanas, San Miguel-Vallecitos and San Pedro Martir faults. All of these faults extend over 100 km in length and lose displacement near the region of intersection. The complexity in the region of intersection, characterized by a series of NE and NW trending faults, appears to be the result of the variability in the directions and rate of displacement of the four major faults.
A magnitude-4.7 foreshock occurred 21 minutes before the magnitude-5.1. Two weeks after the mainshock, over 200 aftershocks were identified and located by the seismic network operated by the Ensenada Center for Scientific Research and Higher Education (CICESE), the institution responsible for monitoring and investigating the seismicity along the Baja California Peninsula and the Gulf of California. At least three of these aftershocks were greater than magnitude-4.0.
Most of the aftershock sequence occurred along a southwest-northeast trending line that extends 4 miles (7 kilometers). The surface exposure of a fault lies parallel to this line 1-2 miles (2-3 kilometers) to the west. These aftershocks suggest this fault surface is tilted moderately to the east at depth. We call this fault the Cardenas Fault.
Interestingly, at least 10 aftershocks define a second, roughly perpendicular line that extends approximately 2.5 miles (4 kilometers) and coincides with several smaller faults in the same orientation.
This bimodal aftershock distribution suggests that the magnitude-5.1 quake activated at least two perpendicular faults with a total rupture length of 7 miles (11 kilometers). It is unclear on which of these faults the magnitude-5.1 originated. The focal mechanisms (or beach ball) of an earthquake is obtained from seismograms in order to define the orientation of two orthogonal planes, one of which corresponds to the orientation of the fault where the earthquake originates. In the case of the magnitude 5.1 quake one of these planes is consistent with the San Miguel Fault and the other plane with the Cardenas Fault. Thereafter, the earthquake might have initiated on any of these two faults.
The northeast oriented Cardenas Fault, which was activated in this event, links the San Miguel and Tres Hermanas faults to the north with the Agua Blanca Fault to the south, each of which are major faults capable of generating much larger earthquakes. If the magnitude-5.1 quake occurred on the left-lateral Cardenas Fault, it should unclamp the southeasternmost sections of the San Miguel and Tres Hermanas Fault making them more susceptible to failure in the future. The shift toward failure should be greater on the Tres Hermanas Fault because it is oriented almost perpendicular to the Cardenas Fault.
International collaboration in Baja
Earthquake researchers at CICESE collaborate with scientists at institutions in California and worldwide. However, given that earthquakes do not respect international boundaries, we are convinced that stronger collaborations and data sharing between CICESE and research institutions in the USA will result in a better understanding of the earthquakes and their potential impact on the communities on both sides of the border.
Concerned about your earthquake risk? Check it at Temblor.
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