SOURCE COMPLEXITY OF THE 1994 NORTHRIDGE EARTHQUAKE AND ITS RELATION TO AFTERSHOCK MECHANISMS

We determined the source process of the 1994 Northridge earthquake in relation to the aftershock mechanisms. To study the source complexity of the mainshock, we inverted the P and SH waveforms recorded by the I RIS and IDA/IRIS networks, using the method of Kikuchi and Kanamori (1 991) in which the rupture is represented by a series of discrete subev ents with varying mechanisms. The waveforms show that the rupture cons isted of several subevents with about 2 sec in between. Our solution c onsists of three subevents with essentially the same mechanism, viz., strike, dip, and slip of 130 degrees, 42 degrees, and 116 degrees, res pectively. The first subevent occurred at a depth of about 19 km, foll owed after 2 sec by the second and largest subevent at a depth of 17 k m and the third subevent again 2 sec after the second at a depth of ab out 13 km. The total moment from the body waves of this sequence is ab out 1.1 x 10(26) dyne . cm (M(W) = 6.6) with a source duration of 7 se c. The large depths of these subevents explain the lack of any surface rupture, Furthermore, the upward propagation of the subevents is cons istent with the depth of the hypocenter and the distribution of the af tershocks, which are shallower and more northerly than the mainshock h ypocenter. The aftershocks were analyzed using data from the TERRAscop e network. We inverted short-period surface waves to determine the mom ent tensor for 70 events with M(W) > 3.5. The aftershocks can be group ed into three regions based on the mechanisms: the eastern part of the aftershock zone, where we find thrust events with mechanisms very sim ilar to the main event; a central area with predominantly strike-slip events; and an area to the west, where we find oblique thrust events b ut with more northerly P axes than in the eastern region. This distrib ution suggests that the fault system on which the Northridge earthquak e occurred is segmented and that the extent of the Northridge rupture is controlled by a change in geometry of the fault. We find a high str ess drop (270 bar) for the mainshock; we propose that the changes in t he fault geometry prevented a slip pulse from propagating, thereby cau sing a high ratio of slip-to-rupture length.