Even after careful relocation, no obvious single fault plane can be discerned. Several features of the earthquake cluster can be noted. The earthquakes are grouped in a very narrow depth range, from 8 to 10 km. In map view, the cluster is elongated in the NW-SE direction, with the mainshock closer to the southeast end. In comparing the locations of the very be st-located earthquakes (AA quality) to a broader selection (BB and better quality), the AA quality solutions cluster in an even narrower depth range, but have a similar distribution in map view.
Step 1: A subset of 10 stations with good azimuthal coverage and reading quality was selected, for which 6 of the well-located events had "good" readings on all ten stations (i.e. residuals low enough that the reading was used in the solution). The six events were relocated using the CO (Cascade) velocity model, and P readings from the ten stations which were common to all six master events. The azimuthal gaps (after relocation) ranged from 68-78 degrees. and the nearest station was within 1 km for all six of the master events. Magnitudes of master events ranged form 2.5-3.2.
Stations used: CHC, HTW, BHW, TTW, RMW, JCW, BLN, FMW, SMW, CKA).
Events used: 96050314153p, 96050316021p, 96050316070p, 96050412163p, 96050419574p, 96050507253p. Six events were chosen
Step 2: The average residual determined for each of the 10 stations from Step 1 was set as the station correction, and the full set of 27 events with close-in readings was relocated using P waves only, and the station set of 10 stations with newly-determined station corrections. The locations were then fixed, and another location run performed to compute average station residuals for additional stations. An expanded station set (sta.CE) and an improved version of the Cascade velocity model (vel.CE) was formulated by adding station corrections for stations on which at least 10 P-wave readings were available from among the set of 27, and for which the standard deviation between the residuals was .1 seconds or less.
Corrections were determined for additional stations: GSM, RPW, HDW, XTL, QTZ, TWW, CMK, RER, MBW, LO2, ELL, WAT, CKA, GRL, TBM, MUP, WPW, CBS, MCW.
Step 3: The revised set of stations (sta.CE) and velocity model (vel.CE) were used to relocate all events in the map area shown. Events of location quality "BB" or better are shown in the map view and cross sections. In cross-sections, topography is displayed with vertically exaggerated by a factor of 3.
