Earthquake Swarm S20070402.1 Near Bishop, California

An earthquake swarm designated S20070402.1 occurred 2 km northeast of Bishop, California, from 00:11 UTC on 2 April 2007 to 06:41 UTC on 3 April 2007. Over this 30-hour period, 40 events were recorded, with magnitudes ranging from 0.5 to 3.9 and focal depths predominantly between 9 and 13 km.

The sequence began with a magnitude 1.1 event at 12 km depth, followed rapidly by a magnitude 2.4 shock at 13 km. Activity intensified over the next hour, culminating in the largest event of the swarm—a magnitude 3.9 earthquake at 12 km depth at 01:04 UTC. Subsequent events showed a gradual decline in both frequency and magnitude, with later shocks clustering at shallower depths of 6 km.

Magnitudes remained modest throughout, with only one event exceeding magnitude 3. Depths stayed consistent near 11 km for most of the swarm before shallowing slightly toward the end. This pattern is characteristic of swarm activity in the region, where fluid migration or aseismic slip along fault segments can trigger numerous small events without a dominant mainshock-aftershock sequence.

The Bishop area lies within the Owens Valley, part of the Eastern California Shear Zone. This tectonically active region accommodates right-lateral shear between the Pacific and North American plates. The valley is bounded by the Sierra Nevada frontal fault system to the west and the White Mountains fault zone to the east. Historical seismicity includes the great 1872 Owens Valley earthquake, estimated at magnitude 7.4–7.9, which produced extensive surface rupture.

Since 2000, only one prior swarm has been documented in the immediate vicinity, occurring in 2003. The 2007 swarm therefore represents the second such episode recorded in the area during the early twenty-first century. Depths of 10–13 km align with the brittle-ductile transition zone in this portion of the shear zone, where strain is released through distributed small-magnitude events.

No damage or injuries were reported from the 2007 swarm, consistent with its low magnitudes and rural setting. Continued monitoring of the Owens Valley remains important given the region’s capacity for larger earthquakes along major range-front faults.

References

SeismoSight internal swarm catalog (S20070402.1 parameters).
USGS Earthquake Catalog (regional historical events and tectonic setting).
California Geological Survey (Owens Valley fault information).