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Earthquake Hazard – West Yellowstone, Montana: Yellowstone’s Seismic Swarms Resume

2017/07/21

Ongoing Yellowstone Earthquake Swarm North of West Yellowstone, MT.

July 19, 2017 UPDATE: The University of Utah Seismograph Stations (UUSS) is monitoring an earthquake swarm which is currently active on the western edge of Yellowstone National Park.  The swarm began on June 12th, 2017 and, as of 09:45 MDT on July 19th, 2017, is composed of 1,284 events with the largest magnitude of ML 4.4 (MW 4.4) (Figure 1).  The swarm consists of one earthquake in the magnitude 4 range, 7 earthquakes in the magnitude 3 range, 105 earthquakes in the magnitude 2 range, 407 earthquakes in the magnitude 1 range, 736 earthquakes in the magnitude 0 range, and 28 earthquakes with magnitudes of less than zero.  These events have depths from ~0.0 km to ~14.0 km, relative to sea level.  At the time of this report, there were 125 felt reports for the M4.4 event that occurred on June 16, 2017 at 00:48:46.94 UTC (June 15, 2017 at 18:48:46.94 MDT).  The M4.4 event has an oblique strike-slip moment tensor solution (Figures 1 & 2).  In addition, four other earthquakes in the swarm have been reported felt.

Figure 1. Location of the earthquakes that are part of the swarm as of July 19, 2017 at 09:45 MDT (red symbols).

Details at link

Figure 2. Moment Tensor solution for the M4.4 event showing the fit between data (black) and synthetics (red dashed).

Details at link

Figure 3. Animation of the June 2017 Yellowstone earthquake swarm.  Earthquakes appear as red circles as they happen, then transition to blue.  After they have occurred, they appear as black circles.  The size of the circles are proportional to the earthquakes magnitude.

Details at link

Earthquake swarms are common in Yellowstone and, on average, comprise about 50% of the total seismicity in the Yellowstone region.

UUSS will continue to monitor this swarm and will provide updates as necessary.

If you think you felt an earthquake, please fill out a felt report at: http://earthquake.usgs.gov/earthquakes/dyfi/.

[Byline Jamie Farrell]

19 July 2017
University of Utah Seismograph Stations


See Also

ANSS Yellowstone Earthquake map

UUSS and ANSS Live Seismograms

Recent Earthquake Near Yellowstone Nat. Park, Montana

Historical Events

Satellite Technologies Detect Uplift in the Yellowstone Caldera – 2006 article

1959Hebgen Lake, MT – M 7.3

1947 – Virginia City, MT – M 6¼

1928 – Helena, MT – M 5½ ±

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The July 6, 2017 M 5.8 earthquake southeast of Lincoln in western Montana occurred as the result of shallow strike slip faulting along either a right-lateral, near vertical fault trending east-southeast, or on a left-lateral vertical fault striking north-northeast. The location and focal mechanism solution of this earthquake are consistent with right-lateral faulting in association with faults of the Lewis and Clark line, a prominent zone of strike-slip, dip slip and oblique slip faulting trending east-southeast from northern Idaho to east of Helena, Montana, southeast of this earthquake. The Lewis and Clark line is a broad zone of faulting about 400 km in length, and up to 80 km wide (wider to the east). Faults within this zone, primarily of Middle Proterozoic to Holocene in age, can be traced as much as 250 km along strike, and typically change in strike from east (near Idaho) to southeast (near Helena). In the region of the July 6th earthquake, prominent faults include the St Mary’s-Helena Valley fault, and the Bald Butte fault, both right-lateral structures. More detailed field studies will be required to identify the causative fault responsible for this earthquake.

Western Montana, northwestern Wyoming, and eastern Idaho have experienced at least 16 other M 5.5+ earthquakes within 300 km of the July 6, 2017 event over the preceding century. The largest was the August 1959 M 7.2 Hebgen Lake earthquake – the largest historic event in the intermountain region – which occurred about 280 km to the south-southeast of the July 6th event. The Hebgen Lake earthquake triggered a large landslide that resulted in significant damage and more than 28 fatalities. A M 6.9 earthquake just over 100 km to the southeast of the July 6th earthquake in June, 1925, caused significant damage, but no fatalities. A M 5.6 earthquake occurred 170 km to the south of the July 6th event in July 2005, and caused minor damage in the surrounding region.

The most notable earthquake sequence in the immediate area of the July 6, 2017 event was centered near Helena in 1935. The first felt earthquake of the sequence occurred on October 3, 1935, and produced light damage. The M~6.2 earthquake of October 18, 1935, resulted in heavy damage and two deaths, and the M~6 earthquake of October 31, 1935, produced additional heavy damage and two more fatalities. Many hundreds of small earthquakes were felt at Helena over the two succeeding decades.


Previous Events

YELLOWSTONE VOLCANO OBSERVATORY

INFORMATION STATEMENT
Sunday, March 30, 2014 10:31 AM (Sunday, March 30, 2014 16:31 UTC)

M4.8 Seismic Event strikes Yellowstone Park, at site of rapid uplift

The University of Utah, a YVO member agency, sent out the following press release about a magnitude 4.8 earthquake that occurred this morning at 6:34 AM MDT.

PRESS RELEASE
University of Utah Seismograph Stations
Released: March 30, 2014 08:15 AM MDT

The University of Utah Seismograph Stations reports that a light earthquake occurred at 06:34 AM on March 30, 2014 (MDT). The epicenter of the magnitude 4.8 shock was located 4 miles north-northeast of Norris Geyser Basin in Yellowstone National Park, Wyoming. This earthquake is part of a series of earthquakes that began in this area on Thursday, March 27. As of 8:15 am today, this series has included at least 25 earthquakes in addition to the main shock, with the largest of magnitude 3.1. The magnitude 4.8 main shock was reported felt in Yellowstone National Park and in the towns of West Yellowstone and Gardiner, Montana.

Today’s event is the largest earthquake at Yellowstone since February 22, 1980, and occurred near the center of a region of recent ground uplift described in a YVO Information Statement on February 18, 2014. The Yellowstone Volcano Observatory has been tracking this uplift episode for about 7 months.

As discussed in the March 3, 2014 YVO Monthly Update, seismicity in the general region of the uplift has been elevated for several months. A previous period of uplift in this area occurred between 1996 and 2003, and it was also accompanied by elevated seismicity.

A USGS field team is in Yellowstone and will visit the area near the earthquake’s epicenter today. The team will look for any surface changes that the earthquake may have caused, and for possible effects to the hydrothermal system at Norris Geyser Basin.

Based on the style and location of today’s earthquake, at this time YVO sees no indication of additional geologic activity other than continuing seismicity.

The Yellowstone Volcano Observatory (YVO) provides long-term monitoring of volcanic and earthquake activity in the Yellowstone National Park region. Yellowstone is the site of the largest and most diverse collection of natural thermal features in the world and the first National Park. YVO is one of the five USGS Volcano Observatories that monitor volcanoes within the United States for science and public safety.

YVO Member agencies: USGS, Yellowstone National Park, University of Utah, University of Wyoming, UNAVCO, Inc., Wyoming State Geological Survey, Montana Bureau of Mines and Geology, Idaho Geological Survey

Source: U.S. Geological Survey

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More Information:

USGS/NEIC Denver, USA
Earthquake Information Center The University of Utah

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