2010-2011 Seismicity Summary

In Washington and Oregon for the period 7/1/2010 - 6/22/2011, the PNSN detected and located about 1045 onshore earthquakes. The two largest earthquake in this period had magnitudes of M4.3 and M4.2, and nine earthquakes had magnitudes of 3.0 or greater. One M=3.1 event in southwest Oregon, probably originating in the subducted Juan de Fuca/Gorda plate, was not catalogued because the data constraining it just missed our quality criteria. Of the 1045 earthquakes, 167 were Mount St. Helens events, and 19 were reported as felt events.

Mt. Rainier Seismicity

81 events occurred at Mt. Rainier. The two largest had M=2.5 and M=2.1, 18 Rainier were between M1.0 and M1.9, and the remaining 61 earthquakes had magnitudes less than M1.0.

Mt. Hood Seismicity

23 of the 1045 earthquakes catalogued occurred close to the Mt. Hood volcano, the largest of which was a M2.6 event. 17 were members of a modest earthquake swarm about 5 km WSW of Hood’s summit. This area was also active during the last reporting period (7/1/2009 – 6/30/2010). These earthquakes appear to be tectonic in origin. Swarm-like activity in the vicinity of Mt. Hood is a fairly common occurrence.

Wooded Island Swarm

The Wooded Island earthquake swarm on the southeastern edge of the Hanford Nuclear Reservation, some 6 km southeast of the only commercial nuclear power plant in Washington state, burst into activity in late January 2009. By July 2009 the swarm had released the bulk of its energy, but continued at greatly reduced levels until about March 2010. Since 7/1/2010, only 7 earthquakes were located in the swarm source zone, the largest having a magnitude of M3.3 and the others being M2.0 or smaller.

Maupin Swarm

25 of the 1045 earthquakes catalogued were part of the continuing swarm of earthquakes 12 km ESE of Maupin (in north-central Oregon). The largest Maupin earthquake in this period was a M3.6, only two were between M2.0 and M2.9, and the remainder were less than M2.0. The Maupin swarm has been ongoing since December 2006, but the rate of earthquake occurrence (i.e. earthquakes/day) is approximately 5 times less after about January 2010 than the period July 2008 through December 2010. In early 2011 the rate started to increase. The rates of earthquake occurrence and seismic energy release have fluctuated substantially since the swarm’s inception in late 2006.