Seismic Swarm S20191116.2: Analysis of Activity Near Olancha, California

Seismic swarm S20191116.2 was recorded 17 km southeast of Olancha, California, beginning at 03:16 on 16 November 2019 and concluding at 07:39 on 19 November 2019. Over 76 hours and 22 minutes, the sequence comprised 90 earthquakes. Depths ranged primarily between 0 and 8 km, with most events clustered at 1–3 km. Magnitudes remained modest, peaking at 3.6, consistent with typical swarm behavior where numerous small events occur without a dominant mainshock.

The sequence initiated with a 1.2-magnitude event at 3 km depth. Activity intensified through 16 November, including events of 2.7, 3.5, and 3.4 magnitude, all at shallow depths of 1–2 km. Further notable shocks on 17 November reached 3.6 magnitude at 2 km. Later stages on 18–19 November featured lower magnitudes, generally below 2.0, with depths remaining shallow. This temporal pattern reflects classic swarm dynamics: an initial burst followed by gradual decay without clear aftershock scaling.

The Olancha region lies within the tectonically active Eastern California Shear Zone and the southern Walker Lane belt. These areas accommodate right-lateral shear between the Pacific and North American plates at rates of several millimeters per year. Local geology features Quaternary alluvial fans, basin-fill sediments, and proximity to the Owens Valley fault system. Historical large events, such as the 1872 Owens Valley earthquake of estimated magnitude 7.4–7.9, underscore the area's capacity for significant seismicity, though swarms like S20191116.2 represent a distinct, lower-energy mode of strain release.

Regional fault networks include normal and strike-slip structures associated with Basin and Range extension. Shallow focal depths observed in the swarm align with brittle failure in the upper crust, typical of this extensional regime. Fluid migration or aseismic slip on nearby faults may trigger such swarms, though precise mechanisms require further geophysical investigation.

Historical records since 2000 document 75 swarms in the vicinity. Annual counts vary, with elevated activity in 2004 (8 swarms), 2010 (7), 2013 (6), and notably 2019 (18). Earlier years such as 2000, 2001, and 2016 each recorded 4 swarms. This distribution indicates persistent swarm-prone conditions, possibly linked to the intersection of multiple fault strands and hydrothermal influences near Owens Lake.

In summary, swarm S20191116.2 exemplifies the frequent, low-magnitude seismic episodes characteristic of the Olancha area. Continued monitoring supports improved understanding of strain accumulation along the Eastern California Shear Zone.

References:
SeismoSight internal swarm classification data
USGS Earthquake Catalog (regional context)
California Geological Survey fault database (geological history)