M 7.2 - The 2010 Sierra El Mayor, B.C., Mexico Earthquake

Tectonic Summary

The April 4, 2010, M 7.2 Baja California Sierra El Mayor earthquake occurred as the result of shallow strike-slip faulting in northern Baja California, approximately 65 km south of the Mexico-USA border along the principal plate boundary between the North America and Pacific plates. Focal mechanism solutions indicate that rupture occurred on either a left-lateral northeast-southwest-striking fault or a right-lateral southeast-northwest-striking fault. At the location of the earthquake, the Pacific plate moves northwest relative to the North America plate at a velocity of about 46 mm/yr.

The principal plate boundary in northern Baja California consists of a series of northwest-trending strike-slip (transform) faults that are separated by pull-apart basins. The faults are distinct from, but parallel to, strands of the San Andreas Fault system. The April 4th main-shock occurred along a strike-slip segment of the plate boundary that coincides with the southeastern part of the Laguna Salada fault system. This complex event may have begun with east-down motion along faults on the eastern edge of the Sierra El Mayor, then progressed to the northwest with oblique slip, that is, a combination of lateral shift to the right and also east-down motion. Overall, the location and focal mechanism solutions of the earthquake are consistent with the shock having occurred on this fault system. Field geologists from the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE, written commun., 2010) observed surface rupture associated with the 2010 event at latitude 32.578621°, longitude –115.725814°. Highway 2 was offset at this location by a total of about 1.2 m across a zone of fractures that offset the road towards the right, with the east side also dropping downward. Aftershocks appear to extend in both directions along this fault system from the epicenter of the April 4, 2010, event. The aftershock zone extends from near the northern tip of the Gulf of California to 10 km northwest of the Mexico-USA border.

Historically, this is an area with a high level of seismicity, though the April 4th event is the largest earthquake to strike the area since 1892. The April 4th earthquake was larger than the M 6.9 earthquake in 1940 or any of the early 20th-century events (for example, 1915 and 1934) in this region of northern Baja California. The 1892 earthquake occurred along the Laguna Salada fault system, but surface offsets associated with the 1892 event lie farther northwest than the April 4th mainshock’s epicenter. The 2010 event’s aftershock zone extends to the northwest, overlapping with the portion of the fault system that is thought to have ruptured in 1892. The 1940 Imperial Valley earthquake approached M 7, though it occurred farther to the north and on the Imperial fault. Both the 1892 and 1940 earthquakes were associated with extensive surface faulting. An event of M 7.0 or 7.1 occurred in this region in 1915, and then a M 7.0 to 7.2 event in 1934 ruptured the Cerro Prieto fault with up to several meters of surface slip.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)