Offshore Oregon Sequence
Two interesting series of earthquakes are currently occurring off the central Oregon coast, approximately 180 to 250 miles west of Waldport. The earthquakes can be placed in two categories.
- Dr. Robert Dziak with Oregon State University has used a hydrophone network to detect and locate a “swarm” of hundreds of small earthquakes in an approximately 20 x 35 square mile area about 220 miles west of Waldport (for details about this earthquake swarm see http://www.pmel.noaa.gov/vents/oregon-quakes.html).
- A less numerous set of earthquakes, including three with magnitudes between 5.0 and 5.2, have occurred on the Blanco Fault Zone (BFZ). These earthquakes were located with land-based seismic networks. The 200 mile long BFZ forms part of the western boundary between the Juan de Fuca and Pacific tectonic plates. The three M5 earthquakes are spaced along an approximately 110 mile long section of the BFZ, centered about 220 miles west-southwest of Waldport. These three M5 earthquakes and other recent smaller events scattered along the entire length of the BFZ are too spread out to be considered an earthquake swarm.
Land-based seismic networks cannot locate these earthquakes because they are too small and too far from land.
According to Dr. Dziak, the current earthquake swarm is not typical of other swarms in the region, both with respect to its location (well away from the plate boundary), and its high rate of earthquake occurrence (averaging about 60 per day). The swarm’s center is about 40 miles from the nearest of the recent M5 earthquakes. The swarm began on 30 March, three days before the first of the M5’s. A question that naturally arises is whether or not there is a physical connection between the BFZ earthquakes and the earthquake swarm. Such a relation might be one of “cause and effect”, or perhaps both phenomena share an underlying cause. Perhaps it’s a matter of coincidence. It is not possible to make a definitive statement regarding this matter.
The Blanco Fault Zone is a very seismically active feature. Since 1980, it has produced about 70 earthquakes with magnitudes of 5 or greater. In some years there have been as many as eight such events; in other years there have been none. So far in 2008 there have been four (the three mentioned above plus a fourth, a M6.3 event that occurred on 10 January).
Earthquakes along the BFZ involve primarily horizontal (“strike-slip”) movement along the fault and this greatly limits their potential for generating a significant tsunami, even if an earthquake had a magnitude as high as 7. Rarely have swarm earthquakes been observed with magnitudes above 5 to 5.5. Such small earthquakes cannot produce a significant tsunami, regardless of the direction of motion on the fault that is producing the swarm.
All of the earthquakes discussed here are 70 miles or more from where the Cascadia subduction zone (CSZ) fault intersects the sea floor. Thus none of these earthquakes are occurring on the CSZ.
More on earthquake swarms
An earthquake “swarm” is an episode with many earthquakes in which the largest earthquake does not occur at the beginning of the episode and in which the largest earthquake is not substantially larger than other earthquakes of the episode. Earthquake swarms generally continue for weeks, months, or even longer, without the occurrence of a substantially larger event. Earthquake swarms sometimes (but not always) indicate volcanic activity. Earthquake swarms also occur on land, in a variety of geologic and tectonic settings. One such swarm is currently active in north-central Oregon, near the town of Maupin. Nearly 40 years ago there were two small earthquake swarms in the Seattle urban area. Swarms have also occurred along the San Andreas fault system in California, in the central United States, and in many other places around the world.