Seismic Swarm S20170313.1 Near Imperial, California

On March 13, 2017, a seismic swarm designated S20170313.1 was recorded 6 km northeast of Imperial, California. The sequence began at 08:55 and concluded at 21:16, encompassing 24 earthquakes over 12 hours and 21 minutes. Magnitudes ranged from 1.3 to 2.4, with focal depths between 5 km and 14 km. The largest events reached magnitude 2.4 at 17:13:42 and again at 18:38:45, both at 11 km depth. Smaller events clustered primarily between 10 km and 12 km depth during the afternoon peak.

This swarm fits within a broader pattern of seismic activity in the Imperial Valley. The region occupies a tectonically active zone at the boundary between the Pacific and North American plates, where right-lateral strike-slip motion occurs along the San Andreas Fault system. The Imperial Fault and Brawley Seismic Zone dominate local deformation, producing frequent small-magnitude earthquakes and occasional swarms linked to fluid migration and geothermal processes beneath the Salton Sea.

Historical records since 2000 document 18 swarms in the same area. Yearly counts include one swarm in 2000, two in 2003, one in 2005, two in 2008, four in 2009, four in 2010, one in 2011, one in 2012, and two in 2016. These episodes typically feature low-magnitude events distributed over hours to days, consistent with the March 2017 sequence.

The March 2017 swarm exhibited a rapid onset followed by a concentrated burst of activity between 17:00 and 18:00, after which rates declined toward the evening termination. Depths remained shallow to moderate throughout, reflecting the brittle upper crust of the Imperial Valley. Such swarms contribute to ongoing strain release without producing significant surface rupture or damage.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey regional fault and seismicity reports
• SeismoSight internal swarm classification records