Seismic Swarm S20210708.2: Analysis of Activity Southeast of Markleeville, California

The seismic swarm designated S20210708.2 was recorded in the region 31 km southeast of Markleeville, California. It began at 22:49 on 8 July 2021 and concluded at 10:04 on 20 September 2021. Over a duration of 1763 hours and 15 minutes, a total of 4214 earthquakes were registered. This event aligns with the area's established pattern of episodic seismic swarms, which have occurred nine times since 1 January 2000, specifically in the years 2000, 2003, 2005, 2007, 2010 (two instances), 2013, and 2014 (two instances).

Examination of the initial 100 events reveals a rapid onset of activity dominated by shallow depths between 0 and 8 km. The sequence opened with events of magnitude 2.2 and 2.1 at 8 km and 7 km depth, respectively, followed shortly by a magnitude 6.0 event at 7 km depth at 22:49:48. Subsequent shocks included a magnitude 5.0 at the same depth four minutes later. Magnitudes in this early phase ranged from 1.2 to 6.0, with the majority falling between 2.0 and 3.5. Depths remained consistently shallow, clustering around 1–7 km, indicative of brittle failure in the upper crust. Notable later events within the first 100 included magnitudes 4.6, 4.4, 4.5, and 4.2, interspersed with numerous smaller tremors.

The Markleeville region lies within the tectonically active transition zone between the Sierra Nevada and the Basin and Range Province. This setting features normal and strike-slip faulting driven by northwest-directed extension and right-lateral shear. Historical seismicity in Alpine County reflects ongoing strain accumulation along regional fault systems, with swarms often occurring without a single dominant mainshock. Depths recorded in S20210708.2 are consistent with the shallow seismogenic zone typical of this portion of eastern California.

References:
SeismoSight internal swarm classification records
USGS Earthquake Catalog (historical swarm verification)
California Geological Survey regional fault maps