Seismic Swarm S20040712.1: Analysis of Activity Near Coso Junction, California

Seismic swarm S20040712.1 was recorded 15 km east of Coso Junction in Inyo County, California. The sequence began at 15:43 on 11 July 2004 and concluded at 03:36 on 29 July 2004, spanning 419 hours and 52 minutes. A total of 158 earthquakes were detected during this interval.

The Coso region lies within the western Basin and Range province, where extensional tectonics interact with the youthful Coso Volcanic Field. This field features rhyolitic domes, basaltic flows, and an active geothermal system that drives fluid migration along faults. Seismicity here frequently manifests as swarms linked to magmatic or hydrothermal processes rather than large tectonic events. Depths typically remain shallow, often between 1 and 6 km, consistent with the brittle-ductile transition influenced by elevated heat flow.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.1 to 1.9, with the majority falling between 0.4 and 1.0. The largest event in this subset reached 1.9 on 19 July. Depths clustered between 0 and 6 km, with several events registering at or near 0 km and occasional negative values likely reflecting location uncertainties. Timing showed initial clustering on 11–12 July, followed by episodic bursts through 21 July. Notable peaks included multiple events above magnitude 1.0 on 12, 15, 16, and 19 July. This pattern of repeating small events without a clear mainshock-aftershock decay is characteristic of fluid-driven swarms in geothermal settings.

Regional history underscores recurring swarm behavior. Since 1 January 2000, twelve swarms have occurred in the area. Yearly counts include four in 2000, four in 2001, two in 2002, and two in 2004. These episodes align with ongoing monitoring of the Coso Geothermal Field, where microseismicity aids in mapping fracture networks and fluid pathways.

The 2004 swarm fits established patterns of low-magnitude, shallow seismicity that rarely produces felt shaking or structural damage. Such activity contributes to refined models of volcanic and hydrothermal hazard assessment in the region.

References

• USGS Earthquake Catalog (historical swarm data and regional seismicity)
• California Geological Survey reports on Coso Volcanic Field
• SeismoSight internal swarm classification records