Location:

Period:

21 Sep 2021 16:19:50 - 13 Oct 2021 08:18:19 (21 days 15 hours 58 minutes)

Volcanoes in 100km radius:

Mono Lake Volcanic Field(68km), Mono-Inyo Craters(86km)

Earthquakes:

354

12 swarms found nearby.

2000

S20000926.1(21.4km)

26 Sep

2 days 18 hours

103 earthquakes

2003

S20030713.1(27.6km)

12 Jul

1 day 14 hours

105 earthquakes

2005

S20051119.1(14.9km)

19 Nov

3 days 19 hours

62 earthquakes

2007

S20070309.1(14.1km)

8 Mar

11 days 2 hours

405 earthquakes

2010

S20101031.1(19.8km)

31 Oct

2 days 12 hours

39 earthquakes

S20101116.1(21.1km)

15 Nov

1 day 10 hours

29 earthquakes

2013

S20130124.1(12.2km)

24 Jan

1 day 21 hours

61 earthquakes

2014

S20140530.1(14.2km)

30 May

3 days 11 hours

47 earthquakes

S20140726.1(27.4km)

25 Jul

1 day 5 hours

25 earthquakes

2021

S20210708.2(1.4km)

8 Jul

73 days 11 hours

4214 earthquakes

S20211031.1(8.6km)

30 Oct

2 days 4 hours

33 earthquakes

2022

S20220808.1(12.1km)

8 Aug

10 days 23 hours

336 earthquakes

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Earthquake Swarm S20210922.1: Seismic Activity Near Walker, California

An earthquake swarm designated S20210922.1 was recorded 4 km west-southwest of Walker, California, in Mono County. The sequence began at 16:19 on 21 September 2021 and concluded at 08:18 on 13 October 2021, spanning 519 hours and 58 minutes. During this interval, 354 earthquakes were registered.

Analysis of the first 100 events reveals predominantly low-magnitude activity with shallow focal depths. Initial events on 21 September included magnitudes between 0.6 and 2.3 at depths of 0–7 km. Subsequent activity on 22 September featured a peak magnitude of 2.8. The largest event in the examined sequence reached magnitude 3.6 on 23 September at a depth of 7 km. Many events clustered between 3–7 km depth, with occasional negative or zero-depth readings likely reflecting measurement variability. Magnitudes remained mostly below 2.0 after the initial days, indicating a typical swarm pattern of numerous small events without a dominant mainshock.

The Walker area lies within the Walker Lane tectonic belt, a northwest-trending zone of distributed right-lateral shear accommodating approximately 20–25% of the Pacific–North American plate motion. This belt extends along the eastern margin of the Sierra Nevada and transitions into the Basin and Range province. Local faulting involves a combination of strike-slip and normal faults that contribute to the region’s moderate seismicity. The crust here is relatively thin and actively deforming, producing frequent small earthquakes and occasional swarms.

Historical records since 2000 show ten prior swarms in the immediate vicinity, occurring in 2000, 2003, 2005, 2007, 2010 (two events), 2013, 2014 (two events), and an earlier 2021 sequence. These recurring swarms reflect the ongoing tectonic strain accumulation and release along the Walker Lane system, consistent with the broader pattern of episodic seismic activity in the eastern Sierra Nevada region.

The September–October 2021 swarm fits within this established framework of low-to-moderate magnitude, shallow seismicity. No damage or significant ground deformation was associated with the sequence. Continued monitoring remains important given the area’s proximity to population centers and infrastructure along U.S. Route 395.

References

SeismoSight internal swarm classification S20210922.1 dataset
U.S. Geological Survey Earthquake Catalog (regional tectonics summary)
California Geological Survey, Fault Activity Map of California