Probable snow avalanche signal at Mount Hood
The seismology lab got a phone call from a snow safety person at the Hood Meadows ski area on the morning of Mar. 10, 2011. He informed us about a "large avalanche that occurred overnight. His follow-up e-mail stated: "We think it occurred sometime between the previous afternoon after 4:00pm and before 6:30am(PST). It was a category D4.5-5.0 avalanche (see photos below) and it scoured the ground and took out trees along the way. The Ski Patrol had done avalanche control work (Howitzer cannon) at approximately 6-6:18am(PST). However, the avalanche debris the Ski Patrol encountered had about 1" of fresh snow on it, leading them to believe that this avalanche happened sometime during the night and was not a result of their control work. It was snowing at the mountain and so it was undetermined whether they had initiated this avalanche or not."
A quick review of our seismic records could not identify an obvious large avalanche signal such as was seen at Mount St. Helens during this same storm, but there were signals consistent with acoustic airwaves at one seismograph (howitzer fire?) and a possible low amplitude avalanche signal following those "booms". A detailed search of the records over the previous day (and even through the following day) found no other signals that had avalanche characteristics and recorded simultaneously on more than one station. Unfortunately the seismic monitoring system at Mount Hood is nowhere near as extensive nor sensitive as that at Mount St. Helens.
Below are the seismograms for a 9 minute period shortly after 6am PST on Mar 10, 2011 for 4 stations on or near Mount Hood. This is followed by a spectrogram figure of the same stations for the same time period. We think that the high-frequency, short blips seen on station VFP at 14:11:10, 14:13:20 and 14:16:55 GMT (6:11:10 am PST...) are the acoustic airwaves from the control charges. For some reason they only show up on this station even though it is 15 km away. Acoustic airwaves are funny things and can be strongly affected by local topography, wind conditions and the sensitivity of the seismic station to being shaken by something above ground. What we think are the avalanche signals are marked on the figures and are much lower frequency in a broad range from about 0.5 to 6 Hz but with extra energy around 2 Hz. By the way, The very strong and continuous signal on station "HOOD" is due to a mechanical pump in a nearby building that runs much of the time. The sensitivities of both station "HOOD" (located near the service area for the Hood Meadows ski facilities) and station "TIMB" (located near the Timberline Lodge) are fairly low due to the cultural "noise" generated by these establishments (machinery, vehicles, people, etc).
Based on these seismograms it is impossible to get a good location for the source of the signals. However, based on descriptions and photographs (see below) provided by the ski area, we know the rough area of the avalanche. The following map shows our seismic stations as blue triangles and the approximate avalanche path outlined in pink. The word from the ski area is: "This event most likely initiated at the 8000 to 10600 foot level in the Clark drainage on the southeast side of Mt. Hood, possibly as high as what is called the "Super Bowl". The toe of the avalanche was near the bottom of the Heather Chair Lift."
Here are a few of the photos of the avalanche path and debris field provided by the ski area. Clicking on any figure will provide it at higher resolution. The first is of the start zone looking up into the Super Bowl with the Mount Hood summit on the sky line. The avalanche fracture line is quite obvious even though it has been degraded by subsequent snow fall.
The following photo was taken from near the edge of a normally closed area looking down into the Clark Canyon where it narrows. Note the two skiers on the debris (lower center) and the scour marks with no debris on the lower left.
The following photo was taken within the avalanche channel in the lower Clark Canyon looking back up hill with a ski patrol member for scale.
See the exotic events web page for a list of other interesting non-earthquake seismic events including other avalanches on Cascade volcanoes. In particular, the large avalanche at Mount Hood of the previous year on March 29 had a much stronger seismic signal than the one this year, though these two avalanches seem to run over the same terain and be of comparable sizes.