Seismic Swarm S20110319.1: A Detailed Examination of the March 2011 Event near La Quinta, California

The seismic swarm designated S20110319.1 occurred approximately 23 km south-southwest of La Quinta in Riverside County, California. Registered by SeismoSight monitoring systems, the sequence began at 06:37 on 18 March 2011 and concluded at 06:06 on 20 March 2011. Over 47 hours and 29 minutes, a total of 34 earthquakes were recorded, providing a clear snapshot of clustered seismic activity in the Coachella Valley region.

All events remained below magnitude 3.0, with the largest reaching 2.4. Depths ranged primarily between 3 km and 15 km, indicating shallow crustal origins typical of the area. The majority clustered between 19 March 03:17 and 03:41, when 18 events occurred within a 24-minute window. Magnitudes during this peak period varied from 0.5 to 2.4, demonstrating rapid energy release without progression to a mainshock-aftershock pattern.

The Coachella Valley lies at the southern terminus of the San Andreas Fault, where the Pacific Plate slides northwest relative to the North American Plate. This transform boundary generates frequent small-magnitude seismicity. The valley floor consists of alluvial sediments overlying crystalline basement rocks of the Peninsular Ranges, which can amplify ground motion during events. Historical records show the region experiences both isolated earthquakes and episodic swarms driven by fluid migration or aseismic slip along subsidiary faults.

SeismoSight internal classification identifies 13 swarms in the same locale since 1 January 2000. These occurred in the following years and counts: 2001 (1), 2002 (2), 2003 (1), 2005 (2), 2009 (2), and 2010 (5). Such recurrence underscores the persistent low-level strain accommodation characteristic of the southern San Andreas system.

Analysis of S20110319.1 reveals a compact spatial footprint and brief duration. Event depths shallowed slightly during the most active phase before returning to deeper levels near the swarm’s end. No damage or felt reports were associated with these micro-earthquakes, consistent with their limited magnitudes.

Ongoing monitoring of swarm patterns in this sector supports improved understanding of fault interactions along the San Andreas Fault zone. Continued data collection aids regional hazard assessments for the densely populated Coachella Valley and adjacent desert communities.

References SeismoSight internal swarm catalog (S20110319.1 parameters and historical counts) USGS Earthquake Hazards Program regional tectonic summaries for Southern California