Seismic Swarm VS20130404.1: Activity Southwest of Niland, California

Seismic swarm VS20130404.1 occurred 11 km southwest of Niland, California, in the Imperial Valley region. The sequence began at 10:46 on 3 April 2013 and concluded at 23:01 on 5 April 2013, registering 59 earthquakes over 60 hours and 14 minutes. All events remained small, with magnitudes between 0.2 and 1.5 and focal depths ranging from 0 to 10 km, consistent with shallow crustal processes typical of the area.

The Imperial Valley lies at the southern termination of the San Andreas Fault system, where right-lateral strike-slip motion transitions into the Brawley Seismic Zone. This zone accommodates oblique extension linked to the northward propagation of the Gulf of California rift. The local geology features thick sedimentary fill overlying metamorphic basement, intersected by numerous northwest-trending faults and enhanced by high heat flow from the underlying Salton Sea Geothermal Field. Magmatic intrusions and hydrothermal fluid circulation contribute to elevated seismicity rates, often manifesting as earthquake swarms rather than mainshock-aftershock sequences.

Within the recorded swarm, activity clustered in two phases. The first phase on 3 April included events up to magnitude 1.3 at depths of 1–7 km. The second phase on 5 April produced the largest event of the swarm (magnitude 1.5) at a depth of 0 km, accompanied by multiple magnitude 1.2–1.3 shocks at similarly shallow levels. Depths remained predominantly between 1 and 5 km, suggesting involvement of fluid-filled fractures within the geothermal reservoir.

Historical records indicate frequent swarm activity in this locale. Since 1 January 2000, 56 swarms have been documented, with notable increases in 2009 (11 swarms), 2010 (13 swarms), and 2012 (11 swarms). Earlier years showed lower but persistent occurrences, reflecting the steady tectonic and hydrothermal drivers of the region.

Such swarms typically arise from aseismic slip or pore-pressure changes rather than magmatic intrusion. The shallow depths and low magnitudes observed align with patterns seen in other geothermal settings worldwide, where fluid migration along pre-existing faults generates episodic seismic energy release without producing large damaging events.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
SCEC Community Fault Model documentation