Seismic Swarm S20190815.1: Analysis of Activity Near Little Lake, California
The seismic swarm designated S20190815.1 was recorded in the region 22 km east-southeast of Little Lake, California, within the tectonically active Eastern California Shear Zone. This zone accommodates a significant portion of the dextral shear between the Pacific and North American plates through a network of northwest-trending strike-slip faults and distributed deformation. The swarm initiated at 15:56 UTC on 14 August 2019 and concluded at 11:18 UTC on 17 August 2019, spanning 67 hours and 21 minutes during which 120 earthquakes were detected. Analysis of the first 100 events reveals predominantly low-magnitude seismicity, with magnitudes ranging from 0.1 to 2.5. The largest event reached magnitude 2.5 at a depth of 7 km. Depths were generally shallow, concentrated between 0 and 14 km, consistent with brittle failure in the upper crust. Activity showed temporal clustering, with multiple events occurring within minutes during peak periods on 15 and 16 August, reflecting typical swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence. The Little Lake area lies near the transition between the Sierra Nevada block and the Basin and Range province, where Quaternary volcanism and geothermal systems, including the nearby Coso Volcanic Field, contribute to elevated crustal permeability. Historical records indicate recurrent swarms in this setting, with 62 episodes documented since 1 January 2000. Annual counts include peaks of 11 swarms in 2019 and 7 each in 2004 and 2010, underscoring the region's persistent low-level seismic productivity. These swarms typically involve events below magnitude 3.0 and rarely produce surface rupture or significant ground shaking. Monitoring by regional seismic networks continues to track such activity, aiding in the characterization of fault interactions and potential links to geothermal fluid dynamics in the Coso region. Continued observation supports improved understanding of strain accommodation in this portion of the Pacific-North American plate boundary.