Earthquake monitoring at Newberry volcano
Newberry Volcano, despite being a Very High Threat Volcano, was once thought to be one of the most seismically quiet of the monitored volcanoes in Washington and Oregon before a major seismic network upgrade in 2011. Prior to 2011, only one seismic station (NCO) had operated near Newberry, and from the time of its installation (1987) through 2011 the Pacific Northwest Seismic Network (PNSN) had located only 7 earthquakes within 20 km (12 mi) of the caldera - and none were located inside. In 2011 the Cascades Volcano Observatory installed 8 additional real-time seismic and deformation (GPS) stations around Newberry, significantly bolstering monitoring capabilities.
Since the new stations were installed in 2011, the number of earthquakes located near Newberry and inside the caldera has increased significantly, with an average of 10-15 earthquakes per year located by the PNSN within the caldera. This increase reflects the improved detection and location capabilities of the enhanced Newberry network - before 2011, it turns out that we were in essence unable to detect and locate earthquakes at Newberry much below M 2.0.
The types of seismicity observed at Newberry include a steady background of small (M < 2) so-called "volcano-tectonic" earthquakes that are a common feature of background seismicity at many volcanoes. In addition, we have observed occasional deep (6-8 miles) long-period earthquakes and shallow (2-3 miles deep) hybrid earthquakes that are commonly observed at other calderas with active magmatic and hydrothermal systems. The Newberry seismic network also routinely picks up signals from multiple non–volcanic sources, including rumbles from nearby thunderstorms (also electrical spikes if lightning is close by), pops from the fracturing of lake ice when Paulina and East Lakes begin to freeze in the late fall and early winter, and sonic booms from passing jets. Luckily, given the new monitoring network, these signals are relatively easy to distinguish for seismic signals emanating from beneath the caldera.
An additional factor that influences Newberry seismicity is the likely presence of magma at relatively shallow depths (2-3 miles) beneath the caldera floor. Evidence for this magma comes from studies performed by seismologists at the University of Oregon using data recorded by many temporary seismometers deployed during short-lived experiments in the 1980s and 2000s. This magma chamber and the hydrothermal system above it are likely responsible for most of the seismicity. Newberry's caldera collapsed about 75,000 years ago, and there is evidence for caldera-fill material that mutes the seismic energy from earthquakes that occur within the caldera. We infer that this fill/mush combination acts to absorb seismic energy from earthquakes occurring at Newberry. This absorption makes seismic observations difficult on seismometers outside of the caldera, and is one reason that only a few earthquakes were recorded in the decades before the monitoring boost in 2011.