Seismic Swarm S20040930.1 Near Bodfish, California: A 2004 Earthquake Sequence Analysis

Seismic swarm S20040930.1 was recorded 25 km south-southwest of Bodfish in Kern County, California. Activity began at 07:33 on 29 September 2004 and concluded at 00:47 on 5 October 2004. Over 137 hours and 14 minutes, the swarm produced 119 earthquakes. This event represents the fourth swarm documented in the region since 1 January 2000, following one swarm in 2001 and three in 2003.

The Bodfish area lies within the southern Sierra Nevada, near the Kern River Valley and Lake Isabella. Regional tectonics are shaped by the intersection of the northwest-trending Kern Canyon Fault and the east-west Garlock Fault system. These structures accommodate right-lateral and left-lateral shear, respectively, within a zone of distributed deformation between the San Andreas Fault to the west and the Basin and Range province to the east. Crustal thickness in the southern Sierra averages 30–35 km, with shallow seismicity typically occurring above 10 km depth due to the brittle-ductile transition.

Analysis of the first 100 events reveals a classic swarm pattern dominated by low-magnitude earthquakes. Depths ranged primarily between 2 km and 7 km, with the majority clustered at 4 km. Magnitudes stayed below 3.0 except for a single M5.0 event at 22:54 on 29 September at 2 km depth. Subsequent events included multiple shocks of M2.0–M2.9, such as an M2.9 at 2 km depth on 1 October and several M2.3–M2.7 events distributed over the following days. The sequence exhibited no clear mainshock-aftershock decay; instead, events occurred in episodic bursts, consistent with fluid migration or aseismic slip triggering mechanisms common in intraplate swarm environments.

Geological records indicate that the Kern Canyon Fault has hosted recurrent moderate seismicity throughout the Holocene. Paleoseismic studies document surface-rupturing events with recurrence intervals of several thousand years. The 2004 swarm occurred in a region of elevated heat flow associated with Miocene volcanism and ongoing extension, which may facilitate swarm behavior through elevated pore-fluid pressures at shallow depths.

The 2004 sequence underscores the persistent seismic hazard in the southern Sierra Nevada. Although individual events remained small, the M5.0 shock was widely felt across Kern County and produced minor ground shaking near Lake Isabella. Continued monitoring of similar swarms provides critical data for refining fault models and assessing the potential for larger triggered events along the Garlock Fault.

References:
USGS Earthquake Catalog (2004 events)
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center swarm records