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GPW seismic monitoring station at Glacier Peak, Washington. (Credit: Malone, Steve. Public domain.)

The Pacific Northwest Seismic Network (PNSN) and CVO monitor seismicity at Glacier Peak via a single station (GPW) located about 2 km (1 mi) from the summit plus the broader regional PNSN network. Only two earthquakes have been located by the PNSN within 10 km (6 mi) of the summit (a magnitude (M) 2.0 in 2001 and a M 1.3 in in 2011), including just one within 5 km. With just one nearby station, it is likely that very small (M < 1.0) earthquakes are not routinely detected and located at Glacier Peak. The ability to detect and locate very small earthquakes is critical to early recognition of reawakening volcanic systems. Despite the small network and likelihood that small earthquakes have been missed, Glacier Peak is still considered one of the most seismically quiet volcanoes in the Washington and Oregon Cascades.

At the first sign of unusual earthquake activity, scientists will deploy additional instruments on and around Glacier Peak to monitor earthquakes, deformation, and other symptoms of volcanic unrest. The monitoring information will be used to assess the state of unrest and to issue appropriate advisories and warnings to emergency-response officials and the public.

Glacier seismicity at Glacier Peak

Station GPW has recorded many seismic events since its installation by the PNSN in 2001. However, the vast majority (if not all) of these events are likely related to glacier movement. Reasons for this interpretation include: 1) They do not record on the next-nearest station (WRW, 33 km distant); 2) They have something of a seasonal cycle (more occur in the summer than winter); and 3) There is an active glacier (the Scimitar) less than 1 km (0.5 mi) from GPW. In addition, they have been a regular feature on GPW records since it was installed in 2001 without any associated volcanic activity at the summit.

Deep long-period (DLP) events near Glacier Peak

In contrast to the virtual absence of earthquakes near Glacier Peak, about 15 km (9 mi) to the west of the vent there is one of the higher concentrations of so-called 'Deep Long-Period' (or DLP) seismic events in the Cascade Range. DLP events are thought to be related to movement of magma or other fluids, and in some cases have been found to correlate in time with eruptive activity at the surface. At Glacier Peak they are do not occur near enough to the volcano to determine their relationship to the magmatic system.