Magnitude 5.8 Off the Coast of Oregon

Monday, November 19, 2007 at 20:32:47 UTC

(12: 32:47 AM Pacific Standard Time)

Epicenter: Latitude 43.541N, Longitude 127.507W

Depth: approximately 10 kilometers.

Undersea plate tectonics west of Washington and Oregon

The Blanco Fracture Zone is one of several seismically active transform faults off the coast of Oregon. The Blanco FZ offsets the Juan de Fuca (JDF) Ridge from the Gorda Ridge (ridges are spreading centers where new ocean floor is created) and forms the boundary between the Pacific and Juan de Fuca plates. On transform fault boundaries, the plates on opposing sides of the boundary move horizontally (side-by-side) with respect to each other (yellow arrows on map).  Although earthquakes occur frequently along the Blanco Fracture Zone, they are rarely felt. Generally, the transform motion does not produce tsunamis because the horizontal motion of the plates does not raise or lower the seafloor as is required to produce a tsunami.

(Reference:  http://www.ess.washington.edu/SEIS/PNSN/INFO_GENERAL/blanco.html)

 

Vectors showing the motion of the Juan de Fuca and North American plates with respect to the Pacific plate.

Vectors showing the motion of the Pacific and Juan de Fuca plates with respect to the North American plate.

Two seismograms from AS-1 instruments located in Oregon are shown below.  Note that although the first P arrival is quite clear, it would be difficult to pick an S-phase arrival with any confidence.  It is not unusual for the S-phase, especially on a vertical instrument, to be difficult to pick.

Corvallis AS-1 Record (CHOR).  The P phase is clear, but the S phase is not.

 

 

UPOR seismogram of the Blanco fracture zone event.  A high-pass filter with the corner set to 5 seconds has been applied.

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