Seismic Swarm S20000709.1 Near Olancha, California
The seismic swarm designated S20000709.1 occurred 6 km NNW of Olancha in Inyo County, California. It initiated at 23:58 on 8 July 2000 and concluded at 22:02 on 10 July 2000, encompassing a total duration of 46 hours and 3 minutes during which 44 earthquakes were recorded.
Magnitudes within the swarm ranged from 1.3 to 3.3. The largest event reached magnitude 3.3 at 04:02 on 9 July, followed closely by several magnitude 2.9 and 3.0 shocks. Depths remained predominantly shallow, between 0 and 6 km, consistent with activity along near-surface fault structures. The sequence exhibited typical swarm characteristics, with events clustered in time without a single dominant mainshock.
This episode represents the first of two swarms documented in the region since 1 January 2000. The concentration of activity over less than two days highlights the episodic nature of seismicity in this portion of eastern California.
Olancha lies within the Owens Valley, part of the broader Basin and Range province and the Eastern California Shear Zone. The area experiences transtensional deformation driven by the interaction between the Pacific and North American plates. Prominent faults include the Owens Valley Fault and segments of the Sierra Nevada frontal fault system. These structures accommodate right-lateral strike-slip motion and normal faulting.
Geological history includes the great Owens Valley earthquake of 26 March 1872, estimated at magnitude 7.4–7.9, which produced surface rupture extending tens of kilometers and caused significant landscape changes near Olancha and Lone Pine. Ongoing tectonic extension continues to generate both isolated events and episodic swarms.
Seismic monitoring indicates that shallow swarms such as S20000709.1 are common expressions of fluid migration or aseismic slip transients along favorably oriented faults. Although individual events remain small, their cumulative occurrence contributes to the long-term strain release in the shear zone.
References
- United States Geological Survey Earthquake Catalog
- California Geological Survey Fault Activity Map
- USGS Professional Paper on the 1872 Owens Valley Earthquake