Location:

Period:

12 Mar 2019 02:16:49 - 12 Mar 2019 02:52:57 (36 minutes)

Volcanoes in 100km radius:

St. Helens(0km), West Crater(36km), Indian Heaven(40km), Adams(53km), Rainier(79km), Hood(99km)

Earthquakes:

11 swarms found nearby.

2000

VS20001105.1(1.5km)

5 Nov

2 days 9 hours

45 earthquakes

2004

VS20040923.1(0.6km)

22 Sep

24 days 15 hours

2267 earthquakes

VS20041024.1(0.4km)

23 Oct

258 days 6 hours

5484 earthquakes

2005

VS20050720.1(0.5km)

19 Jul

53 days 4 hours

548 earthquakes

2008

VS20080118.1(0.5km)

17 Jan

3 days 11 hours

63 earthquakes

2011

S20110130.1(1.4km)

29 Jan

2 days 1 hours

31 earthquakes

S20110214.1(9.7km)

14 Feb

3 days 21 hours

85 earthquakes

2013

S20130824.1(18.1km)

23 Aug

2 days 21 hours

56 earthquakes

2018

VS20180102.1(0.5km)

1 Jan

23 hours

24 earthquakes

S20180103.1(9.4km)

3 Jan

2 days 12 hours

81 earthquakes

VS20180507.1(1.5km)

6 May

7 days 23 hours

91 earthquakes

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Earthquake Swarm VS20190312.1: Analysis of Seismic Activity Near Amboy, Washington

On 12 March 2019, a brief earthquake swarm designated VS20190312.1 was recorded 38 km NNE of Amboy, Washington. The sequence began at 02:16 UTC and concluded at 02:52 UTC, encompassing 24 events within a 36-minute window. All events were of low magnitude, ranging from -0.3 to 0.9, and occurred at very shallow depths between 0 and 2 km.

The temporal distribution showed clustering in the initial 20 minutes, with events occurring at intervals of 20 to 90 seconds. Magnitudes remained consistently below 1.0 after the initial 0.9 event, indicating a rapid decay in energy release typical of swarm behavior rather than a mainshock-aftershock sequence. Depths predominantly at 1 km suggest activity within near-surface crustal layers.

This swarm aligns with patterns observed in the region since 2000, during which 11 swarms have been documented. Prior episodes occurred in 2000 (1 swarm), 2004 (2), 2005 (1), 2008 (1), 2011 (2), 2013 (1), and 2018 (3). Such recurrent swarms reflect ongoing tectonic and possibly magmatic influences in the Cascade Range.

The Amboy area lies within the Cascade volcanic arc, formed by subduction of the Juan de Fuca Plate beneath the North American Plate. This tectonic setting produces both volcanic edifices and shallow crustal seismicity. Nearby volcanic fields, including those associated with the Boring Lava Field and proximity to Mount St. Helens, contribute to hydrothermal and fracture-related earthquake activity. Historical records indicate that similar low-magnitude swarms often occur without surface rupture or significant ground deformation.

Seismic monitoring in the Pacific Northwest benefits from dense station coverage by regional networks, enabling detection of events at magnitudes below 1.0. The 2019 swarm’s characteristics—short duration, shallow focus, and small magnitudes—mirror those of fluid-driven or thermally induced sequences common in volcanic margins.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional tectonic context.
Pacific Northwest Seismic Network reports on Cascade Range seismicity.