Seismic Swarm S20000517.1: Analysis of Activity Near Olancha, California

Seismic swarm S20000517.1 was recorded 6 km northwest of Olancha, California, beginning at 09:28 on 17 May 2000 and concluding at 01:19 on 21 May 2000. Over 87 hours and 50 minutes, the swarm comprised 92 earthquakes. This sequence represents the sole swarm documented in the region since 1 January 2000.

The events exhibited a range of magnitudes, with the largest reaching 3.5 at 17:37 on 17 May 2000. Multiple events of magnitude 2.0 or greater occurred throughout the period, including a 3.4 event at 23:07 on 19 May 2000 and a 3.3 event at the swarm's onset. Depths were predominantly shallow, clustered between 0 and 5 km, though isolated deeper events extended to 23 km. The temporal distribution showed peak activity on 17 May, with sustained lower-level seismicity continuing through 20 May before tapering off.

Swarm sequences such as this are characterized by numerous events without a dominant mainshock-aftershock pattern. The data indicate clustered shallow foci consistent with fluid migration or localized stress adjustments along minor fault structures. Early events included a 2.5 magnitude shock at 10:12 on 17 May at 0 km depth, followed by repeated magnitude 1.6–2.2 events at similar depths. Later phases featured events such as the 2.8 magnitude shock at 23:15 on 19 May and several magnitude 1.7–2.0 occurrences on 20 May, all at depths of 4 km or less.

The Olancha area lies within Owens Valley in eastern California, part of the tectonically active boundary between the Sierra Nevada block and the Basin and Range Province. This region experiences deformation through a combination of normal faulting along the Sierra Nevada frontal fault system and right-lateral strike-slip motion associated with the Eastern California Shear Zone. Owens Valley has a well-documented history of seismic activity, including the 1872 Owens Valley earthquake of estimated magnitude 7.4–7.6, which produced extensive surface rupture. Ongoing tectonic extension and shear accommodate Pacific-North America plate motion, resulting in recurrent small-magnitude seismicity.

The 2000 swarm aligns with the area's established pattern of episodic, low-magnitude earthquake clusters rather than large single events. No surface rupture or significant damage was associated with this sequence, consistent with its maximum magnitude of 3.5 and shallow but contained depths.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Database
SeismoSight internal swarm classification records