Location:

Period:

17 Sep 2004 12:34:13 - 24 Nov 2004 06:16:06 (67 days 17 hours 41 minutes)

Volcanoes in 100km radius:

Mono Lake Volcanic Field(16km), Mono-Inyo Craters(32km), Mammoth Mountain(50km)

Earthquakes:

4783

20 swarms found nearby.

2001

S20010308.1(21.1km)

7 Mar

2 days 6 hours

47 earthquakes

S20010320.1(12.1km)

20 Mar

6 days 14 hours

96 earthquakes

2004

S20041214.1(19.1km)

13 Dec

15 days 7 hours

237 earthquakes

2006

S20060505.1(20.9km)

5 May

1 day 14 hours

30 earthquakes

2008

S20080115.1(17.6km)

15 Jan

2 days 10 hours

50 earthquakes

S20080201.1(18.2km)

1 Feb

2 days 6 hours

54 earthquakes

2009

S20090831.1(24.8km)

30 Aug

2 days 4 hours

34 earthquakes

2011

S20110704.1(19.8km)

3 Jul

3 days 20 hours

49 earthquakes

2015

S20150505.1(3.6km)

4 May

2 days 22 hours

111 earthquakes

S20150510.1(5.0km)

9 May

2 days 3 hours

35 earthquakes

2016

S20160805.1(24.1km)

4 Aug

5 days 7 hours

205 earthquakes

S20161228.1(18.5km)

28 Dec

58 days 12 hours

4521 earthquakes

2017

S20170415.1(25.2km)

15 Apr

12 hours

83 earthquakes

S20171009.1(24.7km)

8 Oct

3 days 3 hours

371 earthquakes

S20171015.2(20.0km)

15 Oct

16 hours

26 earthquakes

S20171020.1(13.1km)

19 Oct

2 days 13 hours

33 earthquakes

2018

S20180207.1(24.9km)

7 Feb

2 days 14 hours

56 earthquakes

S20181207.1(1.9km)

7 Dec

1 day 2 hours

33 earthquakes

S20181217.1(2.3km)

17 Dec

2 days 9 hours

44 earthquakes

2020

S20200411.1(7.9km)

11 Apr

33 days 8 hours

1019 earthquakes

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The 2004 Earthquake Swarm Northwest of Benton, California

The earthquake swarm designated S20040918.1 began at 12:34 UTC on 17 September 2004 and concluded at 06:16 UTC on 24 November 2004. Its epicentral area lies 26 km northwest of Benton in Mono County, California, within the tectonically active western Basin and Range Province. Over 1625 hours and 41 minutes the sequence produced 4783 events, illustrating a classic swarm pattern of numerous small earthquakes without a single dominant mainshock.

Examination of the first 100 recorded events reveals predominantly shallow foci between 0 and 11 km depth. Magnitudes ranged from 0.3 to 5.4, with the largest shock (M 5.4) occurring on 18 September at 23:02 UTC at 3 km depth. Early activity clustered tightly in time and space, featuring multiple events above magnitude 3.0 within the first 36 hours. Depths remained consistently crustal, consistent with brittle failure along distributed faults in the region.

This swarm constitutes one of only two such episodes recorded in the area since 1 January 2000; the preceding swarm occurred in 2001. Both sequences highlight episodic, swarm-type seismicity rather than typical mainshock-aftershock behavior.

Geologically, the site occupies the western margin of the Basin and Range extensional province, where northwest-trending dextral shear of the Walker Lane belt interacts with north-south normal faulting. The crust here is thinned and heated by Miocene-to-recent volcanism linked to the Long Valley Caldera system roughly 40 km to the south. Regional faults accommodate ongoing extension at rates of several millimeters per year, producing frequent small-magnitude earthquakes. Historical records document recurrent swarms in this corridor, often associated with fluid migration or magmatic unrest at depth.

The 2004 swarm’s shallow depth distribution and rapid onset align with these regional characteristics, underscoring the area’s persistent seismic hazard. Continued monitoring by regional networks remains essential for distinguishing purely tectonic swarms from those potentially influenced by deeper volcanic processes.

References

USGS Earthquake Hazards Program catalog (events 2000–present)
California Geological Survey, Quaternary Fault and Fold Database
USGS Professional Paper on Long Valley Caldera and Mono-Inyo Craters volcanic chain