Seismic Swarm S20091001.2: Analysis of Activity Near Olancha, California

An earthquake swarm designated S20091001.2 was recorded 18 km northeast of Olancha, California, beginning at 14:15 on 30 September 2009 and concluding at 15:53 on 19 October 2009. Over 457 hours and 38 minutes, the sequence produced 636 events. This swarm occurred within the tectonically active Owens Valley region of eastern California, part of the broader Eastern California Shear Zone where right-lateral strike-slip faulting accommodates a portion of the Pacific-North American plate boundary motion.

The local geology features Quaternary alluvial deposits overlying metamorphic and granitic basement rocks of the Sierra Nevada block, with nearby structures including the Owens Valley Fault and proximity to the Coso Volcanic Field. Seismicity in this area often reflects both tectonic strain and possible magmatic or hydrothermal influences from the adjacent volcanic province. Historical records indicate recurrent swarm activity, consistent with episodic fluid migration or fault creep along minor structures.

Analysis of the first 100 events reveals a rapid onset following the initial M1.1 event on 30 September. The sequence escalated on 1 October with a magnitude 5.0 earthquake at 3 km depth, followed within minutes by an M3.5 at 4 km. Subsequent events clustered at shallow depths, predominantly between 0 and 5 km, with isolated deeper occurrences reaching 10 km. Magnitudes ranged from 1.0 to 5.0, with the largest event serving as the primary energy release. Aftershocks displayed a typical decay pattern, including multiple M2.7–M3.5 events distributed over the following hours. Depths remained consistently crustal, aligning with regional brittle failure zones.

The swarm's temporal distribution showed peak productivity in the first 24 hours after the mainshock, with events occurring at intervals of seconds to minutes initially, then lengthening. Spatial concentration around the reported epicentral area suggests activation of a small fault network rather than a single through-going structure. No significant migration of hypocenters was evident in the early sequence.

Since 2000, six swarms have occurred in the vicinity, with prior episodes in 2000 (three swarms) and 2002 (one swarm), plus one additional swarm in 2009. This pattern underscores the region's propensity for clustered seismicity over isolated mainshock-aftershock sequences, likely driven by the interplay of regional extension and local volcanic processes.

Overall, S20091001.2 exemplifies the swarm behavior characteristic of the Olancha area, providing data on shallow crustal dynamics in a zone of distributed deformation.

References SeismoSight internal classification records for swarm S20091001.2 USGS Earthquake Catalog for regional context and historical swarm counts