June 15, 2016
by Steve Malone
While not recognized at the time the seismic waveforms at PNSN stations, MBW, SHUK and RPW and Canadian station VDB confirm the date and time of a debris avalanche off Sherman Peak at Mount Baker reported by John Scurlock, a well known mountain areal photographer, based on his flight of June 8, 2016. John sent out e-mail with the following comments: The upper east face of Sherman Peak slid again this year, as reported to us several days ago by Corey Vannoy, a climber on the Boulder Glacier route on Mount Baker. The date of the slide is unknown but looks to have been within the last couple of weeks.
Based on what appears to be a light dusting of snow on close ups of the debris surface stretching down the Boulder Glacier I searched back through PNSN seismograms for evidence of shaking that would be consistent with this avalanche and found the following records. Shown here are seismgrams from the four stations showing the waveforms best followed by a clip of the spectrograms from the standard PNSN spectrogram pages.
Date: May 25, 2016. Time is in Universal Standard Time (PDT+7)
Thus the avalanche started at about 21:00:30 Z (just after 2:00 pm PDT) and the strong shaking lasted a little over 2 minutes. This is similar to but maybe a bit larger or at least longer lasting that similar slides in 2013 and 2006. These Sherman Peak avalanches take place every few years and can represent a considerable hazard for climbers on the Boulder Glacier. More details on this avalanche and others can be found at the Mount Baker Volcano Research Center web site.
Also, of some interest is a note from Jackie Caplan-Auerbach from Western Washington University who reports: Of potential interest is that there are a handful of tiny seismic signals right before the slide (see attached figure). We've seen these precursory signals prior to other slides (largely at Iliamna volcano in Alaska), and they were observed by Steve and others in a Sherman Crater avalanche in 1977. But I've never seen them in association with these ones on Sherman Peak. Here is a longer duration seismogram from station SHUK illustrating these possible "pre-events".
Again John Scurlock has kindly provided some excelent photos to document this event (All copyrighted by him).
May 30, 2016
by Lauren Burch
All the mountains, oceans, and islands on Earth exist because of plate tectonics. Different plate boundaries produce different geologic features: divergent boundaries spread apart to form mid-ocean ridges and rift valleys, transform boundaries slide past one another to form strike-slip faults like the San Andreas, and convergent boundaries collide to form tall mountains, deep trenches, and volcanoes. This type of plate boundary is responsible for the numerous volcanic arcs around the Pacific Rim (often called the “Ring of Fire”), and formed our iconic Cascade Volcanoes. Here in the Pacific Northwest, the Juan de Fuca plate is subducting beneath the North American plate along a convergent plate boundary called the Cascadia Subduction Zone (CSZ). Subduction zones like this are the only fault systems capable of producing very large megathrust earthquakes, but they only do so occasionally - over the last 100 years, there have been 84 earthquakes of magnitude 8.0 or greater worldwide, and only 4 of them were greater than an M9.
The simplest answer to the question “Will there be another large earthquake on the CSZ?” is yes. However, the question of “when” is much more difficult to answer. Seismologists don’t know exactly when the next large earthquake will occur on the CSZ, but we do have a good picture of when they have happened over the past 10,000 years. If we divide 10,000 years by the number of ~M9 earthquakes found in that time period, the average recurrence rate for M9 earthquakes along the CSZ is roughly 550 years. We are 316 years past the last great CSZ earthquake in 1700, and we estimate that there is about a 15 % chance that an M9 will occur on this fault within the next 50 years. However, research on submarine landslide deposits shaken loose by big earthquakes indicate that M8+ earthquakes occasionally strike off the coast of Oregon in between “full rip” M9 events. This research suggests that there is a greater probability of reoccurence of a great earthquake in Southern Oregon than off the Washington coast, but there is not a consensus within the geophysical community as to specifically how much greater the hazard is.