A swarm of small earthquakes is taking place in the vicinity of
Wooded Island on the Columbia river about 8 miles north of Richland,
WA. As of Apr 6, 2009 the Pacific Northwest Seismograph Network
(PNSN) has recorded and located over 750 earthquakes since the swarm
began on Jan. 4, 2009.
(An earthquake "swarm" is distinguished from
an earthquake
"mainshock-aftershock" sequence in that the largest event of the sequence
does not come at or near the beginning, and there may be several
larger events of roughly the same size scattered throughout the swarm.)
These events occur within a small region no more than 2 km on a side and at shallow (< 3 km) depths. The largest event has a magnitude of only 2.9 and most events are less than magnitude 1. Earthquake swarms such as this one are not uncommon in eastern Washington. In fact there have been at least three previous swarms in this same area (1970, 1975 and 1988).
Because of the shallow nature of these swarms, their history of having no large (greater than magnitude 4) events within them and the mechanism of faulting we think that they occur within the Columbia River basalts due to the small but steady north-south compression taking place over most of the state. Comparatively weak, more easily deformed layers under the basalts cause stress concentrations within basalt layers causing them to under go brittle failure along small zones of weakness. Because of the limited size of the basalt flows we think that large earthquakes within them are very unlikely. Thus these swarms are not indicators of large earthquakes in the near future.
The PNSN continues to monitor this activity and are working with the geophysicists from PNNL Battelle to obtain the best data and understanding of this swarm. A set of routinely updated pages of technical information (catalog, time history, locations) can be found at: page for main event.