Weak earthquake right below Mount Rainier volcano, Washington, USA

On September 13, 2011 at 09:29:28 AM a weak tremor of 2.6 magnitude occurred below the crater of Mt Rainier, the Washington volcano.

This does not mean at all that the volcano will erupt, but this tremor below the crater is one of the strongest since many months. Activity is centered below sea level below the crater (+5,000 meter below the summit)

Update 14/09 : The many small shocks who started to occur a couple of hours after the 2.4 mainshock could be (only one scenario) any form of stress release from brittle material (click on this link to see the seismogram)

Update : the magnitude and depth has been changed by USGS to 2.4 @ a depth of 1 km (0.6 miles)

Todays tremor is by far the biggest since many months.

Typically, up to five earthquakes are recorded monthly near the summit. Swarms of five to ten shallow earthquakes over two or three days take place from time to time, predominantly in the region of 4 km below the summit (near sea level), and are thought to be caused by the circulation of hot fluids beneath Mount Rainier.

Seismic swarms (not initiated with a mainshock) are common features at volcanoes, and are rarely associated with eruptive activity.

We encourage people to take a look at this very interesting page describing the seismicity since 2001. Click here
Time-Depth, Map, and Cross-Sections of Mount Rainier Seismicity

List of previous quakes since June 19 (max. tremor was 1.4 vs 2.6 today)

Sate     Time      Location       Depth Mag
11/06/19 01:46:39  46.84N 121.76W   4.0 0.1  AB
11/06/20 21:08:36  46.84N 121.75W   3.4 0.2  AB
11/06/23 04:29:41  46.84N 121.75W   2.1 1.4  AB
11/06/24 18:40:11  46.84N 121.75W   2.6-0.1  AA
11/06/28 12:50:16  46.85N 121.74W   3.8 0.8  AA
11/07/04 18:31:13  46.85N 121.77W   6.2 0.0  AA
11/07/05 18:22:19  46.86N 121.72W   0.0 1.1  BC
11/08/03 19:00:27  46.85N 121.76W   4.6 0.9  AA
11/08/12 23:43:42  46.86N 121.73W   0.9 0.5  AA
11/08/16 15:41:46  46.84N 121.76W   3.3 0.5  AB
11/08/18 17:04:48  46.86N 121.73W   1.8-0.3  AC
11/08/21 17:24:45  46.87N 121.74W   3.2 0.0  AC
11/08/22 23:20:34  46.85N 121.75W   4.2 1.3  AA
11/08/29 15:25:58  46.86N 121.75W   3.7-0.5  AC
11/09/01 12:20:20  46.85N 121.78W   0.0 0.8  AC
11/09/04 02:56:33  46.85N 121.74W   3.8 0.3  AB
11/09/04 02:57:41  46.85N 121.74W   3.5 0.8  AA
11/09/04 10:50:09  46.85N 121.75W   3.9 0.6  AB
11/09/12 07:49:04  46.84N 121.75W   3.8 0.9  AA
11/09/12 07:51:05  46.85N 121.75W   3.7-0.1  AC

Seismogram (September 13) at Camp Muir (also embedded below the earthquake data)
September 14


Camp Muir is the location of the seismograph at the base of this page

We are searching for more data to explain the phenomena.

Most important Earthquake Data:
Magnitude : 2.6
UTC Time : Tuesday, September 13, 2011 at 16:29:28 UTC
Local time at epicenter : Tuesday, September 13, 2011 at 09:29:28 AM at epicenter
Depth (Hypocenter) : 800 1,000 meters (0.5 0.6 miles)
Geo-location(s) :
23 km (14 miles) ENE (62°) from Ashford, WA

Links to important maps
Satellite map of the greater epicenter area
I Have Felt It map

Volcano information
Mount Rainier, at 4392 m the highest peak in the Cascade Range, forms a dramatic backdrop to the Puget Sound region. Large Holocene mudflows from collapse of this massive, heavily glaciated andesitic volcano have reached as far as the Puget Sound lowlands.
The present summit was constructed within a large crater breached to the northeast formed by collapse of the volcano during a major explosive eruption about 5600 years that produced the widespread Osceola Mudflow. Rainier has produced eruptions throughout the Holocene, including about a dozen during the past 2600 years; the largest of these occurred about 2200 years ago. The present-day summit cone is capped by two overlapping craters. Extensive hydrothermal alteration of the upper portion of the volcano has contributed to its structural weakness; an active thermal system has caused periodic melting on flank glaciers and produced an elaborate system of steam caves in the summit icecap. Reported 19th-century eruptions have not left identifiable deposits, but a phreatic eruption may have taken place as recently as 1894.
Text : Courtesy Smithsonian Institute

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Not all the wiggles seen on the seismograms are due to earthquakes. Anything that produces ground vibrations could be recorded, for example a car that passes by the seismometer (this is why we try to locate most of our seismometers well away from roads). Since the electrical signals from the seismometers are typcally transmitted to the University of Washington over telephone wires any electrical noise on the telephone lines will also show up on the seismogram. Such noise is usually easy to distinguish from earthquake generated signals because the the noise is often "spikey" in appearance.

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