THE 1993 KLAMATH-FALLS, OREGON, EARTHQUAKE SEQUENCE - SOURCE MECHANISMS FROM REGIONAL DATA

We use regional broadband seismograms to obtain seismic moment-tenser solutions of the two September 20, 1993, M(w)=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (M(D)>3.5). Several sub-groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment-tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east-dipping normal fault, possibly related to the Lake of the Woods fault system. Rotation of T-axes between the two main shocks is consistent with the two dominant trends of the aftershocks and mapped faults. We propose that a change in fault strike acted as temporary barrier separating the rupture of the main shocks. Empirical Green's function analysis shows that the first main event had a longer rupture duration (half-duration 1.7 s) than the second (1.2 s). In December, vigorous shallow activity commenced near Klamath Lake's western shore, 5-10 km east of the primary aftershock zone. It appears a M(w)=5.5 aftershock occurring the day before, though within the primary aftershock zone, triggered the activity.