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Mount St. Helens: “The Ides of March?”

Posted by feww on February 17, 2011

VolcanoWatch Weekly [17 Feb 2011]

Shinmoedake Update:

Some 2,500 people living near Shinmoedake volcano on Japan’s Kyushu island were advised earlier today to evacuate their homes after heavy rain threatened lahar avalanches, reports say.

VoW: Mount St. Helens


Mount St. Helens and Spirit Lake, as seen from Bear Cove. Image Source: U.S. Forest Service via USGS/CVO.


Phreatic eruption of Mount St. Helens, March 28, 1980, as seen from the north. Image by C.Dan Miller, USGS/CVO

Mount St. Helens, Washington  Ash Plume Path May 18, 1980


Click image to enlarge.

Mount St. Helens Volcano
Position: 46°12′ N 122°10’48″ W,
Summit Elevation: 2,549 m (8,363 ft )
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN
Source: USGS/CVO

Recent observations:

An M4.3 earthquake struck the Mount St. Helens region this morning, 14 February 2011, at 10:35 a.m. PST (18:35 UTC) and was felt widely through southwestern Washington and Northwestern Oregon (http://earthquake.usgs.gov/earthquakes/dyfi/events/uw/02141835/us/index.html). Its exact magnitude may change by a few tenths from this value as records are further analyzed. The earthquake was followed by several aftershocks up to M2.8 over the next few hours (http://www.pnsn.org/recenteqs/latest.htm), the three largest of which were also reported felt. All of the earthquakes are located in an area about 8 kilometers (5 miles) north of the crater of Mount St. Helens, near the Johnston Ridge Observatory, at a depth of about 4 to 6 kilometers (2.5 to 4 miles).

Today’s earthquakes are in the same place as a small swarm that took place about two weeks earlier, on 29 January. These earthquakes are reminiscent of a swarm that took place about 30 years ago, when a swarm of small earthquakes began in August 1980, a few miles northwest of today’s activity. The 1980-1981 sequence climaxed with an M5.5 earthquake on 14 February 1981. Analysis of the 1981 events suggested that they occurred along existing faults in the Mount St. Helens seismic zone, a northwest to southeast trending system of faults in which Mount St. Helens lies. The Mount St. Helens seismic zone exhibits strike-slip motion, with the southwestern rocks slipping horizontally northwest relative to the rocks northeast of the fault zone. The fault zone likely exerts control on the location of Mount St. Helens volcano. Studies following the 1980 eruption suggested that the magma removed during the May 1980 eruption and subsequent lava-dome building caused faults along the seismic zone to slip in response to the magma withdrawal. Similar interaction of volcanic activity and tectonic fault movement is possible in the case of today’s earthquakes, but at present there appears to be no signs of unrest in the volcanic system.  USGS/Cascades Volcano Observatory, Vancouver, Washington

Summary of Weekly Volcanic Activity Report – 9 February to 15 February 2011

[Source: SI/USGS]

New Activity/Unrest:

Map of Volcanoes


Map of Volcanoes.
Background Map: University of Michigan. Designed and enhanced by Fire Earth Blog. Click image to enlarge.

Ongoing Activity:

For additional information, see source.

Related Links:

FEWW Volcanic Activity Forecast

Other Related Links:

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