Seismic Swarm S20010714.1: Analysis of Activity near Coso Junction, California
Seismic swarm S20010714.1 was recorded beginning at 04:08 on 14 July 2001 and concluding at 17:48 on 19 September 2001. The events were centered 6 km east-southeast of Coso Junction, California. Over 1,621 hours and 40 minutes, a total of 5,060 earthquakes were detected.
The Coso region lies within the eastern California shear zone, part of the broader Basin and Range province. This tectonic setting features active normal and strike-slip faulting driven by northwest-directed extension. The area hosts the Coso Volcanic Field, characterized by Pleistocene rhyolite domes and basaltic flows overlying Mesozoic granitic basement. Geothermal heat flow is elevated due to shallow magmatic sources, supporting long-term geothermal energy production that can influence local seismicity patterns through fluid injection and extraction.
The swarm initiated with a magnitude 2.0 event at a depth of 0 km. Within the first day, activity intensified markedly, including several events above magnitude 3. The largest shocks reached magnitudes 3.9, 3.8, and 3.6, all occurring within a narrow 12-minute window on 14 July at depths of 1–2 km. Subsequent events maintained a consistent shallow depth distribution, predominantly between 1 km and 3 km, with occasional deeper occurrences up to 5 km.
Examination of the first 100 recorded events reveals a typical swarm signature: rapid onset followed by a high rate of smaller-magnitude aftershocks. Magnitudes clustered between 0.5 and 2.7 for the majority of events, with only four exceeding magnitude 3. Depths remained shallow throughout, averaging near 2 km. This pattern indicates brittle failure within the upper crust, likely facilitated by hydrothermal fluid migration along pre-existing faults.
Historical records since 1 January 2000 document seven swarms in the vicinity. Four occurred in 2000 and three in 2001, underscoring the recurrent nature of clustered seismicity in this geothermal setting. Such swarms are common in areas of active fluid circulation and do not necessarily precede larger mainshock-aftershock sequences.
The 2001 swarm contributed valuable data on the mechanical behavior of the Coso geothermal reservoir. Shallow focal depths and temporal clustering align with observations from other fluid-influenced volcanic and geothermal provinces worldwide. Continued monitoring remains essential for understanding long-term hazard and reservoir dynamics in the region.
References
USGS Earthquake Catalog California Geological Survey regional reports Coso Operating Company geothermal field documentation