The 2010 Sierra El Mayor Earthquake: Tectonic Setting and Regional History

The magnitude 7.2 Sierra El Mayor earthquake struck Baja California, Mexico, on 4 April 2010 at 22:40 UTC. The event originated at a depth of 9.9 km and was centered near the Sierra El Mayor range, approximately 50 km south of the United States border. This earthquake remains the strongest to affect the region in more than a century and provides key insights into the ongoing deformation along the Pacific–North American plate boundary. The epicentral area lies within the southernmost extension of the San Andreas fault system. Here, right-lateral strike-slip motion is accommodated primarily along the Cerro Prieto and Laguna Salada faults. These structures form part of a broader network of northwest-trending faults that transfer slip between the Gulf of California rift and the San Andreas fault proper. The 2010 rupture initiated on a previously unmapped strand of the Laguna Salada fault system and propagated bilaterally for roughly 120 km, producing surface offsets up to 3 m. Geologically, the region is characterized by crystalline basement rocks of the Peninsular Ranges batholith, overlain by Neogene sedimentary basins. The Sierra El Mayor itself is an uplifted block bounded by active normal and strike-slip faults. Long-term slip rates on the Laguna Salada fault are estimated at 2–4 mm per year, consistent with geodetic measurements of contemporary strain accumulation. Paleoseismic studies reveal that large earthquakes have occurred on this and adjacent faults every few hundred years, although the historical record prior to the twentieth century is sparse. Since 1 January 2000, the only earthquake of magnitude 7.0 or greater within the immediate vicinity was the 2010 Sierra El Mayor event itself. The mainshock was followed by an energetic aftershock sequence that included several events above magnitude 5.0, distributed along both the main rupture trace and secondary faults to the northeast. Post-seismic deformation detected by GPS and InSAR indicated afterslip and viscoelastic relaxation continuing for years afterward. The 2010 earthquake produced widespread strong ground motion across northern Baja California and southern California. Damage was concentrated in the cities of Mexicali and Calexico, where unreinforced masonry structures suffered the greatest impact. No fatalities were reported, largely due to the rural location of the epicenter and relatively modern building codes on the U.S. side of the border. Liquefaction occurred along the New River and in the Colorado River delta, highlighting the seismic vulnerability of the thick alluvial sediments in the Imperial Valley. Tectonic models indicate that the 2010 event released a fraction of the accumulated strain along the plate boundary. Continued monitoring by regional seismic networks remains essential for assessing the likelihood of future large earthquakes on neighboring structures such as the Imperial and Cerro Prieto faults.

References

United States Geological Survey Earthquake Catalog (event page for 2010-04-04 M7.2 Sierra El Mayor) Southern California Earthquake Data Center Global Centroid Moment Tensor Project Geological Society of America Special Paper on the 2010 Baja California earthquake