Map of Cascadia Seismicity

Map showing locations of earthquakes in the Cascadia region. Geotouch was used to separate the locations by tectonic regimes; green symbols indicate earthquakes in the North America plate, and blue symbols indicate offshore or Benioff zone earthquakes within the Explorer, Juan de Fuca, and Gorda plates. Open black triangles represent Cascade volcanoes.

Black lines offshore indicate plate boundaries including the deformation front at the margin of the North America and the Explorer, Juan de Fuca, and Gorda plates. The deformation front approximates the position of the Cascadia Subduction Zone.

Earthquakes shown were located by pnw (USGS special studies), PNSN , PGC, NCEDC, DNAG, UTAH (see web site for acronym).

Where duplicate locations existed, a preferred location was selected as follows: we used athoritave geographic regions for earthquakes that were well located instrumentately and locations from specials studies for older significant earthquakes.

The magnitude criteria varied with time. From 18??-1960 events magnitude 6 or greater were included; from 1960-1979, events magnitude 5 or larger; and from 1980-2006 well-located events magnitude 2 or greater were used. "Well-located" for the 1980-2006 period was defined as having phase readings on 6 or more stations, rms 0.35 sec. or less, and estimated depth error 5 km or less for earthquakes east of 125 degrees, or depth error 20 km or less for locations west of 125 degrees.