Earthquake Swarm S20101211.1 in Baja California: Geological Context and Event Analysis

The region encompassing Alberto Oviedo Mota in Baja California, Mexico, lies within the tectonically active boundary between the Pacific and North American plates. This zone features a network of strike-slip faults associated with the Gulf of California rift system, where right-lateral shear accommodates relative plate motion at rates of approximately 4–5 cm per year. The area experiences frequent seismic activity due to its position along the southern extension of the San Andreas fault system, with shallow crustal earthquakes common in the upper 15–20 km of the lithosphere.

Swarm S20101211.1 was recorded 14 km west-southwest of Alberto Oviedo Mota. It began at 04:38 on 11 December 2010 and concluded at 22:55 on 13 December 2010, spanning 66 hours and 16 minutes. During this interval, 50 earthquakes were detected, with magnitudes ranging from 1.5 to 4.7 and focal depths between 1 and 16 km. The largest event, magnitude 4.7, occurred at 12:52 on 11 December at a depth of 13 km. Other notable shocks included magnitudes 3.2, 3.1, and 3.0, clustered primarily in the first 36 hours.

The swarm exhibited typical characteristics of seismic swarms in transform-margin settings: a rapid onset followed by a gradual decline in event rate, without a single dominant mainshock-aftershock sequence. Depths remained consistently shallow, consistent with brittle failure in the seismogenic crust of the region. Historical records since 2000 indicate 19 prior swarms in the vicinity, with notable increases in frequency during 2010. This pattern underscores persistent strain accumulation along local fault segments.

The swarm occurred eight months after the magnitude 7.2 Sierra El Mayor earthquake of 4 April 2010, whose epicenter lay approximately 10 km away. That mainshock ruptured a northwest-striking fault and triggered widespread aftershock activity throughout the area. Post-mainshock stress redistribution likely contributed to the elevated swarm rates observed later in 2010, as fluids or aseismic slip may have facilitated the clustered events.

Seismic swarms in this part of Baja California often reflect interactions between major transform faults and subsidiary structures. The 2010 activity aligns with regional tectonics dominated by northwest-trending right-lateral faults that accommodate Pacific plate motion. Such episodes provide valuable data for understanding short-term strain release in a zone capable of producing infrequent but significant earthquakes.

References

USGS Earthquake Catalog (historical events and magnitudes)
Servicio Sismológico Nacional, Mexico (regional monitoring data)
SeismoSight internal swarm classification records