This small earthquake was well recorded by AS-1 stations located in Corvallis and Portland, OR. This IRIS site: http://www.iris.edu/quakes/make_event_search_map.pl?USER_DEF=Y makes it easy to locate an event using the S-P interval method. I've assumed that the distance to a station is approximated by multiplying the time between the P and S phase arrivals by 8 km/s. This is a rough rule of thumb for nearby earthquakes.
|UPOR (sac file)
University of Portland, Portland, Oregon
S minus P time = 7.6 seconds
|CHOR (sac file)
College Hill, Corvallis, Oregon.
S minus P time = 8.2 seconds
|Station||(S-P) Seconds||Distance, km, ~ (S-P)*8||Distances, corrected for 22.2 km depth|
Circles uncorrected for earthquake depth. The star is located at the catalog epicenter.
The circles above can be corrected for the depth provided by the catalog location, 22.2 km. Consider the diagram below. We estimate that the earthquake is located at a distance R from a station. This defines a hemisphere centered on the station. If the depth is Z, then the radius, R', of the circle on which the event is located is related to Z and R by:
R**2 = Z**2 + R'**2 Solving for R'
R' = sqrt(R**2 - Z**2)
Circles corrected for earthquake depth of 22.2 km. The triangle marked EQ is located at the catalog epicenter.
For this case, knowing the depth provides enough added constraint to get a rough location with just two stations.