Seismic Swarm S20090524.1: Analysis of Activity Near Olancha, California

The seismic swarm designated S20090524.1 occurred 21 km northeast of Olancha, California, in Inyo County. It began at 10:53 on 23 May 2009 and concluded at 20:16 on 29 May 2009, spanning 153 hours and 23 minutes. During this interval, 88 earthquakes were recorded. The events clustered in the Owens Valley region, a tectonically active zone within the eastern Sierra Nevada.

The swarm featured a range of magnitudes, with the largest reaching 4.7. Multiple events exceeded magnitude 3.0, including several at 3.3 and 3.0. Depths were predominantly shallow, with many registering at or near 0 km and others between 1 km and 9 km. Notable early activity included the 4.7 event on 23 May at 22:58, followed by aftershocks such as a 2.9 at 23:54. Later peaks occurred on 25 and 26 May, with events of 3.3, 3.0, and additional magnitudes around 2.8–2.9. The sequence tapered by 29 May, ending with a 1.5 event.

This pattern exemplifies an earthquake swarm, characterized by numerous events of similar magnitude without a dominant mainshock-aftershock sequence. Such swarms often reflect fluid migration or stress redistribution along faults. In this case, the concentration of shallow events suggests activity within the upper crust.

The Olancha area lies in the Owens Valley, part of the Basin and Range province. Tectonic extension between the Sierra Nevada and the Great Basin drives normal and strike-slip faulting. Key structures include the Owens Valley Fault Zone and segments of the Sierra Nevada Fault system. The region experiences ongoing deformation linked to Pacific-North American plate boundary interactions via the Eastern California Shear Zone.

Historically, the area has produced significant earthquakes, notably the 1872 Owens Valley event estimated at magnitude 7.4–7.9, which caused surface rupture along the Owens Valley Fault. Seismic swarms have recurred since 2000, with this being the third such episode; the initial swarm took place in 2000. These clusters highlight persistent low-level activity amid the broader fault network.

Monitoring by regional networks aids in distinguishing swarms from foreshock sequences, supporting hazard assessment in this populated corridor near major transportation routes.

References USGS Earthquake Hazards Program California Geological Survey Fault Activity Map Southern California Earthquake Data Center