Seismic Swarm VS20210110.1: Analysis of Activity Near Niland, California
On 10 January 2021, a seismic swarm designated VS20210110.1 was recorded 10 km southwest of Niland, California. The sequence began at 07:01 and concluded at 13:26, encompassing 26 earthquakes over a span of 6 hours and 24 minutes. Magnitudes ranged from 1.2 to 3.3, with the largest event occurring at 08:21. Depths were predominantly shallow, clustered between 2 and 3 km, consistent with activity in the upper crust of this tectonically active zone.
The temporal distribution showed an initial cluster of smaller events followed by the peak magnitude at 3.3, after which activity gradually declined. Most events remained below magnitude 2.0, illustrating the typical low-to-moderate energy release pattern of swarms in the region. Such sequences often lack a single dominant mainshock and instead feature numerous events of comparable size distributed over hours.
Regional Geological Context
The swarm location lies within the Imperial Valley portion of the Salton Trough, a tectonically active rift basin formed by oblique extension along the Pacific-North American plate boundary. This area experiences distributed deformation through a network of northwest-striking strike-slip faults and northeast-striking normal faults. High heat flow associated with the nearby Salton Sea geothermal field and underlying magmatic intrusions contributes to shallow seismicity.
Earthquake swarms in this setting frequently arise from fluid migration and aseismic slip within the Brawley Seismic Zone, rather than from sudden stress release on a locked fault segment. Depths of 2–3 km align with the brittle-ductile transition influenced by elevated geothermal gradients.
Historical Swarm Activity
Since 2000, 83 swarms have been documented in the immediate vicinity. Annual counts vary considerably, with notable increases during 2009–2013 and renewed activity in 2020. This long-term record underscores the persistent, swarm-dominated character of seismicity southwest of Niland, reflecting ongoing transtensional tectonics and hydrothermal processes.
Implications and Monitoring
Shallow swarm events such as VS20210110.1 are routinely monitored by regional seismic networks because they can precede larger triggered earthquakes or signal changes in subsurface fluid pressure. Continued observation remains essential for understanding strain accumulation along the southern San Andreas and Imperial fault systems.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
SCEC Community Fault Model documentation