Seismic Swarm S20130804.1 Near Olancha, California: Geological Context and Event Analysis

Seismic swarm S20130804.1 occurred 16 km south-southwest of Olancha in Inyo County, California, within the tectonically active Owens Valley. This region lies at the western margin of the Basin and Range province, where east-west extension drives normal faulting along the Sierra Nevada frontal fault system and related structures. The area experiences frequent low-magnitude seismicity due to its position in the Eastern California Shear Zone, which accommodates a portion of the Pacific-North America plate boundary motion.

The swarm initiated at 05:30 UTC on 4 August 2013 and concluded at 02:02 UTC on 6 August 2013, spanning 44 hours and 31 minutes. During this period, 50 earthquakes were recorded. Event depths ranged from -1 km to 5 km, with the majority clustered near the surface. The largest event reached magnitude 3.9 at 05:30:55 on 4 August, followed by numerous smaller shocks that exhibited a typical swarm pattern of gradually declining frequency and magnitude without a dominant mainshock-aftershock sequence.

Subsequent events included magnitudes of 2.9, 2.4, 2.2, and 2.1, interspersed with dozens of events below magnitude 2.0. Shallow focal depths suggest activity along near-surface faults or fracture networks influenced by regional extension. No damage or felt reports beyond minor shaking were associated with the sequence.

Owens Valley has a well-documented history of seismic activity, including the 1872 Owens Valley earthquake of estimated magnitude 7.4–7.9, which produced extensive surface rupture. Since 2000, 34 swarms have been identified in the broader region, with notable clusters in 2000 (6 events), 2002 (2), 2004 (9), and 2013 (3). These swarms reflect episodic fluid migration or stress triggering along distributed faults rather than single large ruptures.

The 2013 swarm aligns with patterns observed in prior years, where activity concentrates in short temporal windows and remains confined to small magnitudes. Such sequences contribute to the long-term strain release in this portion of the Walker Lane belt without posing immediate risk of a major event.

References

• USGS Earthquake Catalog (historical swarm data 2000–2013)
• California Geological Survey, Owens Valley Fault Zone reports
• SeismoSight internal swarm classification S20130804.1