Location:

Period:

28 Jul 2014 09:02:38 - 6 Aug 2014 06:38:50 (8 days 21 hours 36 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

346

13 swarms found nearby.

2001

S20010810.1(10.3km)

10 Aug

1 day 19 hours

59 earthquakes

2011

S20110914.2(26.5km)

14 Sep

1 day 10 hours

42 earthquakes

S20111002.1(26.9km)

1 Oct

14 days 16 hours

382 earthquakes

S20111019.1(26.2km)

19 Oct

4 days 5 hours

106 earthquakes

S20111027.1(19.7km)

27 Oct

5 days 18 hours

177 earthquakes

S20111108.1(24.9km)

7 Nov

2 days 7 hours

43 earthquakes

S20111111.1(27.9km)

10 Nov

5 days 3 hours

128 earthquakes

S20111229.1(24.9km)

29 Dec

3 days 3 hours

89 earthquakes

2012

S20120103.1(27.5km)

3 Jan

3 days 3 hours

69 earthquakes

S20120110.1(29.1km)

9 Jan

6 days 16 hours

139 earthquakes

S20120201.1(27.9km)

31 Jan

3 days 5 hours

62 earthquakes

2013

S20131109.1(6.5km)

9 Nov

21 hours

28 earthquakes

2015

S20150112.1(3.6km)

12 Jan

3 days 23 hours

157 earthquakes

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Seismic Swarm S20140728.1 Near Graeagle, California

A notable seismic swarm, designated S20140728.1, occurred 1 km south-southeast of Graeagle in Plumas County, California. The sequence initiated at 09:02 on 28 July 2014 and concluded at 06:38 on 6 August 2014, spanning 213 hours and 36 minutes. During this period, 346 earthquakes were recorded.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.0 to 2.0, with the majority falling between 0.1 and 1.2. Depths were generally shallow, concentrated between 0 and 8 km, though a few events registered negative depths or reached 15 km. Notable peaks included a magnitude 2.0 event at 16:00:50 on 28 July at 5 km depth and several magnitude 1.8 events. The swarm exhibited typical characteristics of clustered microseismicity, with events occurring in rapid succession during the initial hours before tapering.

Historical records indicate 12 swarms in the region since 1 January 2000. These occurred in 2001 (1 swarm), 2011 (7 swarms), 2012 (3 swarms), and 2013 (1 swarm). Such recurrent swarms underscore the area's persistent seismic behavior.

Geologically, Graeagle lies within the northern Sierra Nevada, a region shaped by Mesozoic subduction, Cenozoic uplift, and ongoing Basin and Range extension. The local tectonic framework involves the Walker Lane belt, a zone of distributed dextral shear accommodating Pacific-North American plate motion. Faults such as the Mohawk Valley Fault and nearby segments of the Sierra Nevada frontal fault system contribute to the seismicity. The crust here features granitic intrusions of the Sierra Nevada batholith overlain by volcanic and sedimentary units. Earthquake swarms in this setting often arise from fluid migration or minor slip on pre-existing fractures rather than large-scale rupture.

The 2014 swarm aligns with patterns observed in similar tectonic environments, where low-magnitude events dominate without producing significant surface rupture. Updated seismic monitoring by regional networks confirms the area's moderate hazard level, with historical events rarely exceeding magnitude 5 in the immediate vicinity.

References

USGS Earthquake Catalog (queried for regional context)
California Geological Survey, Geologic Map of California
USGS Professional Paper on Sierra Nevada tectonics (updated editions)