Seismic Swarm S20000426.1 Near Olancha, California

A seismic swarm designated S20000426.1 occurred 8 km northwest of Olancha in Inyo County, California. The sequence began at 08:14 on 25 April 2000 and concluded at 18:25 on 27 April 2000, spanning 58 hours and 11 minutes. During this interval, 53 earthquakes were recorded.

The events clustered tightly in both space and time, with magnitudes ranging from 0.4 to 3.2. The largest shock reached magnitude 3.2 at a depth of 2 km shortly after midnight on 26 April. Most events occurred at depths of approximately 4 km, though several registered at or near the surface and a few yielded negative depth values consistent with location uncertainties. No single dominant mainshock was evident; instead, the sequence featured repeated events of comparable size distributed throughout the period, characteristic of swarm behavior rather than a classic mainshock-aftershock pattern.

The Olancha area lies within the southern Owens Valley, part of the Eastern California Shear Zone. This region accommodates right-lateral shear between the Pacific and North American plates at rates of several millimeters per year. Active normal and strike-slip faults bound the valley, including strands of the Sierra Nevada frontal fault system to the west and the Owens Valley fault to the east. The 1872 Owens Valley earthquake, with an estimated magnitude of 7.4–7.6, produced extensive surface rupture along the latter fault roughly 50 km north of the swarm location and remains the largest historical event in the area.

Southwest of Olancha, the Coso Volcanic Field exhibits ongoing geothermal activity and elevated heat flow. Seismicity in this setting often reflects fluid migration along fractured volcanic and metamorphic rocks. The 2000 swarm’s shallow focal depths align with this tectonic environment, where brittle failure occurs within the upper few kilometers of crust. Contemporary geodetic studies indicate continued strain accumulation across the shear zone, consistent with the recurrence of moderate earthquake sequences.

Analysis of the temporal distribution shows peak activity on 25 and 26 April, with a gradual decline by 27 April. Magnitudes remained below 3.0 after the initial 3.2 event, and no damage or felt reports of significance were associated with the swarm. Such sequences are common in the region and typically do not herald larger earthquakes, although they provide valuable data on local fault structures and stress conditions.

References

• USGS Earthquake Catalog (historical and instrumental data for Owens Valley)
• California Geological Survey, Fault Activity Map of California
• Dixon et al., 2000, Present-day motion of the Sierra Nevada block (Geophysical Research Letters)