Seismic Swarm S20190318.1: Analysis of Activity Near Little Lake, California

The seismic swarm designated S20190318.1 was recorded in the region 13 km northeast of Little Lake, California. Activity began at 21:46 on 17 March 2019 and concluded at 04:20 on 22 March 2019, spanning 102 hours and 33 minutes during which 102 earthquakes were detected.

This sequence occurred within the Eastern California Shear Zone, a tectonically active belt that accommodates distributed right-lateral shear between the Pacific and North American plates. The local geology features a network of northwest-trending strike-slip faults that interact with the nearby Garlock Fault system. Shallow crustal seismicity is common here due to the brittle nature of the upper crust, often manifesting as earthquake swarms rather than isolated mainshock-aftershock sequences. Depths recorded in the swarm were consistently shallow, ranging from 0 to 4 km, consistent with the regional pattern of activity in fractured volcanic and sedimentary rocks.

Examination of the first 100 events reveals a predominance of microearthquakes. Magnitudes ranged from -0.2 to 3.2, with the majority below 1.5. Notable events included a magnitude 2.7 quake on 18 March at 04:53:54 and a magnitude 3.2 event on 20 March at 12:06:02. Depths clustered around 1–2 km for most events, though several occurred at 3–4 km, suggesting minor variations in rupture planes within the shallow crust. Temporal distribution showed peak activity during the first two days, followed by a gradual decline, typical of swarm behavior driven by fluid migration or aseismic slip rather than a single dominant fault rupture.

The Little Lake area has experienced repeated swarm episodes. Since 1 January 2000, 56 swarms have been documented in the vicinity. Yearly totals include 4 swarms in 2000, 4 in 2001, 2 in 2002, 7 in 2004, 2 in 2005, 6 in 2006, 1 in 2009, 7 in 2010, 1 in 2011, 4 in 2012, 6 in 2013, 2 in 2014, 3 in 2015, 4 in 2016, 2 in 2017, and 1 in 2018. This long-term recurrence underscores the persistent strain accumulation and release along secondary faults in the shear zone.

Such swarms contribute valuable data for understanding regional tectonics. The 2019 sequence aligns with historical patterns of low-magnitude, shallow activity that rarely produces surface rupture but can indicate evolving stress conditions near major structures like the Garlock Fault. Continued monitoring supports improved hazard assessment in this portion of California.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight Internal Swarm Classification Records