Download Montana Bureau of Mines and Geology and Montana Disaster and

Montana Bureau of Mines and Geology
The northern Intermountain Seismic Belt remained seismically active between October 1, 2012 and
September 30, 2013. During this period, using data from the Montana Regional Seismograph Network,
the Earthquake Studies Office of the Montana Bureau of Mines and Geology (MBMG) determined
hypocenter locations and magnitudes for 2,043 earthquakes in Montana and adjacent parts of Idaho,
Wyoming, and Canada. This total included 41earthquakes with magnitudes ranging from 3.0 to 3.9, and
one earthquake of magnitude 3.7. Residents reported feeling 20 earthquakes but none caused any damage.
Information about recent Montana earthquakes is available on the Earthquake Studies Office website
http://mbmgquake.mtech.edu.
Montana’s largest earthquake during the past year occurred on June 6, 2013 in the southern Madison
valley and had a magnitude of 3.7. This persistently active area is sparsely populated and only two
residents reported feeling this earthquake on the US Geological Survey’s Did You Feel It website.
A swarm of 100 earthquakes with magnitudes ≥ 1.0 occurred in the southern Tendoy Mountains
southwest of Lima Montana. The swarm was active May 6-26, 2013 and included four earthquakes in the
magnitude 3.0 to 3.2 range between May 21and 23. None of these earthquakes were reported as felt.
A swarm of 27 earthquakes occurred May 4-23, 2013 just northwest of the town of Norris, Montana. The
largest event was a magnitude 2.7 on May 14th. A vigorous swarm that included a magnitude 4.1
earthquake and more than 700 smaller shocks occurred in this area during the summer of 1987.
A swarm of 40 earthquakes occurred in the Clarkston Valley, north of Three Forks, between June 18 and
27. The largest quake measured 2.5 on July 26th. The Clarkston Valley is the epicentral area of the June
27, 1925 magnitude 6.6 earthquake.
A brief swarm of 22 earthquakes occurred July 20-21, 2013 west of Marysville (northwest of Helena).
The largest event measured 2.4.
Between December 5, and 20, 2012, a sequence of nine seismic events occurred in the vicinity of the
Troy Mine in northwestern Montana. These unusual low-frequency events apparently correlate with rooffalls in an abandoned part of the mine. Event magnitudes ranged from 1.2 to 3.7.
Twelve seismic events with magnitudes ranging from 1.3 to 2.3 occurred throughout the year in the Coeur
d’Alene Mining District in north Idaho. These events were most likely induced by deep hard-rock mines
in the district.
Going into its eighth year, the aftershock sequence of the July 26, 2005 magnitude 5.6 Dillon earthquake
continues. During the past year, the Earthquake Studies Office located 28 aftershocks with magnitudes of
1.0 or larger. The number of magnitude ≥ 1.0 aftershocks located since the Dillon main shock is now
1,754.
The MBMG Earthquake Studies Office continues to receive funding from the U.S. Geological Survey for
partial support of the Montana Regional Seismograph Network. These funds are used for technical
assistance with repair and maintenance of seismographs and telemetry equipment, data archival, and
general network operations. This funding, together with generous support from the Confederated Salish
and Kootenai Tribes, allows the MBMG to fund a full-time assistant/seismic analyst in the Earthquake
Studies Office.
The MBMG has completed its first year of a cooperative agreement with FEMA’s Earthquake Hazards
Reduction State Assistance Program. The award has enabled the MBMG to make Montana’s 1982-2013
earthquake catalog available as a data layer on MBMG’s Online Mapping Application, providing visual,
easy, and versatile access to Montana’s extensive historical earthquake record. Improved access to these
data—including current seismicity—will enhance Montana’s earthquake/seismic hazard education and
outreach efforts.
As described in USGS Circular 1351, the MBMG is a Collaborating Organization in the Yellowstone
Volcano Observatory. The MBMG will coordinate with the seismic monitoring team in the event of
geologic unrest in Yellowstone.
Montana participated in its first ‘ShakeOut’ on October 23, 2013. Over 93,000 Montana residents
participated in this inaugural event coordinated by Katie Gallagher in the Governor’s Office of
Community Service.
Current Seismic Retrofits in progress:
Creative Arts Complex MSU Bozeman, MT; 2/3rds complete ($3 million total cost, FEMA PreDisaster Mitigation grant funding $2.2 million Federal Share).
Mathews Hall UM-Western, Dillon, MT; In Phase I Engineering-Environmental Historic Preservation
Award ($1.14 million total cost, FEMA Hazard Mitigation Grant Program funding $858,000 Federal
Share).
Montana Law Enforcement Academy Non-Structural Retrofit ready for HMGP award ($125,600 total
cost, FEMA Federal Share funding $94,200).
Map of 2,044 earthquake epicenters from October 1, 2012 through September 30, 2013 determined from
Montana Regional Seismograph Network data.
Submitted by Mike Stickney, Director, Earthquake Studies Office and Senior Research Geologist,
Montana Bureau of Mines and Geology with a contribution from Kent Atwood, Mitigation Officer,
Montana Department of Military Affairs, Disaster and Emergency Services.