Seismic Swarm S20211029.2: Analysis of Activity Near Olancha, California

Seismic swarm S20211029.2 was recorded 13 km east-southeast of Olancha, California, beginning at 01:27 UTC on 29 October 2021 and concluding at 11:04 UTC on 1 November 2021. Over 81 hours and 37 minutes, the sequence produced 48 earthquakes. All events were of low magnitude, with the largest reaching 3.0, and hypocentral depths remained shallow, ranging from 0 to 6 km. The swarm exhibited a typical pattern of clustered, low-energy releases without a dominant mainshock-aftershock sequence.

The temporal distribution showed peak activity on 29 October, including multiple events between 15:00 and 17:00 UTC and again in the evening hours. Magnitudes generally stayed below 2.0 after the initial 3.0 event, with only isolated shocks exceeding 2.0 on subsequent days. Depths clustered near 1–2 km for the majority of events, indicating shallow crustal processes consistent with fluid migration or minor fault slip in the upper brittle zone.

This swarm fits within a broader historical context of frequent seismic swarms in the region. Since 1 January 2000, 104 swarms have been documented near Olancha. Annual counts varied considerably, with notable increases in 2019 (23 swarms) and 2020 (20 swarms). Earlier years recorded fewer episodes, such as 7 in both 2004 and 2010. The 2021 total reached 6 swarms by early November, underscoring ongoing episodic unrest.

Geologically, the Olancha area lies within the eastern Sierra Nevada–Owens Valley transition zone, part of the Walker Lane belt and Eastern California Shear Zone. This region accommodates dextral shear between the Pacific and North American plates at rates of approximately 10–12 mm per year. Active normal and strike-slip faults, including strands of the Owens Valley Fault system, cut through Quaternary alluvium and bedrock. The nearby Coso Volcanic Field, located roughly 30 km southeast, contributes geothermal fluids and elevated heat flow that can promote swarm-type seismicity through pore-pressure changes. Shallow depths observed in S20211029.2 align with fluid-driven mechanisms documented in similar volcanic-adjacent settings.

No surface rupture or significant damage was associated with this swarm. Such episodes typically reflect distributed strain release rather than large-scale tectonic loading. Continued monitoring remains essential given the region's history of both swarms and occasional larger events on regional faults.

References

• SeismoSight internal swarm catalog (S20211029.2 parameters and statistics).
• USGS Earthquake Catalog (regional fault mapping and historical context).
• California Geological Survey (Owens Valley tectonics and shear-zone rates).