Earthquake Monitoring at Mount Baker
The Pacific Northwest Seismic Network (PNSN) and CVO monitor seismicity at Mount Baker via a small network of two stations located within 20 km (12 miles) of the summit, as well as the broader regional PNSN network. With so few nearby stations, it is likely that very small (M < 1.0) earthquakes are not routinely detected and located at Baker. This is reflected by the fact that the smallest earthquake ever located within 3 miles of Baker was a M 0.8. Nevertheless, Mount Baker is still considered one of the most seismically quiet volcanoes in the Washington and Oregon Cascades.
From 1980 through 2012, a total of 45 seismic events were located by the PNSN within 5 km (3 miles) of the summit. Of these events, 34 are low-frequency events that scientists interpret to be glacier quakes (see below for more information on glacier quakes at Baker). Most of the remaining events are also low-frequency events, but with depths of 15-30 km (9-19 miles). These events are thought to be caused by the movement of fluids at depth (see below for more information on these so-called “DLP” events). Only three of the 45 Baker events are normal earthquakes, translating into a recent seismicity rate of one earthquake every 11 years. The largest of these was a M 1.4 recorded in 1993, with the most recent event being a M 1.3 in 2004. No earthquakes have ever been reported felt by people on or near Mount Baker.
Seismicity and unrest at Mount Baker
Typically, an increase in seismic activity is the most commonly observed precursor of magma intrusion. However, there was a notable lack of seismicity surrounding the period of heightened degassing activity in 1975-76. Part of the reason for unrecognized seismicity could be due to the sparse seismic network at the time – there was one station about 5 km from the summit and the remaining permanent stations were more than 30 km away. Additional temporary seismic instruments were installed on the flanks of Baker in 1975 in response to the increased degassing, but despite the enhanced network only one earthquake was located beneath the volcano (see Malone (1979) for more details).
Glacier quakes at Mount Baker
Between June-September 2009 over 30 low-frequency (LF) events occurred at very shallow depths beneath Mount Baker, some of which were large enough to be recorded on seismic stations on Mount Rainier, over 200 km (120 miles) away. These events had all the attributes of classic volcanic “LP” (long-period) events, but their occurrence near glaciers raised questions about whether they were volcanic or glacial in origin. Gas measurements were made in mid- September 2009 and showed no abnormal readings. Subsequent analysis of older Baker events found other examples of shallow LF events occurring in 2005, 2006, and 2007, and similar events have been recorded in 2010-2012, all during the months of May-October. Based on several lines of evidence, including their seasonal occurrence, scientists believe that these events are occurring along the base of a glacier at Mount Baker, most likely the Easton or Deming glaciers (where most of the events locate), and are not related to volcanic activity.
Deep long-period (DLP) events at Mount Baker
In contrast to the virtual absence of shallow earthquakes at Mount Baker, at depth it has the highest concentration of so-called “Deep Long-Period” (or DLP) events of any volcano in the Washington and Oregon Cascades. A 2011 study cataloged DLP events in the Washington and Oregon Cascades, and found that, since 1980, more DLP events have been located in the vicinity of Mount Baker (31) than at the rest of the Cascade volcanoes combined. These events are thought to be related to movement of fluids or magma, and at some other volcanoes have been found to correlate in time with eruptive activity at the surface. At Mount Baker they have shown no such relationship to eruptive activity or other forms of observable unrest at the surface (i.e., increased degassing), and are considered to be a part of the normal background seismicity within this active volcanic system.