THE 1994 NORTHRIDGE AND 1971 FAULT RUPTURES AND SURROUNDING SEISMOGENIC FAULTS AS ILLUMINATED BY SMALL EARTHQUAKES

Earthquakes can be used to characterize the faults that generate them. We produced a set of accurate locations and quality-selected focal mechanisms for southern California from phase data of the Caltech-USGS network in the last three decades. We interpreted these abundant data for seismogenic faults. Much of the data in the central Transverse Ranges is associated with the 1994 Northridge sequence but include also the 1971 San Fernando aftershocks. Aftershocks provide information on the geometries of mainshock ruptures, but also on many other faults that interact with these ruptures, both structurally and dynamically. Some of the aftershocks in 1971 originate on the fault that will rupture in 1994. The 1971 and 1994 mainshocks are thought to be mechanically coupled. This fault interaction is therefore manifested by a wide range of magnitudes over a substantial amount of time. Other small earthquakes illuminate faults which may be seismogenically insignificant, but structurally important. An example may be diffused seismicity which is interpreted to stem from flexural folding induced by the 1994 rupture in the hangingwall block of the Santa Susana thrust fault. This fault is the main regional structure illuminated by seismicity. It is characterized by a flat and a listric ramp. A lateral ramp connects the flat in the San Fernando area to a deeper flat in the Northridge area. Ongoing seismicity is steadily improving constraints on the westward extension of this master fault system into the Ventura basin area. The 1994 rupture as imaged by aftershocks matches well in position, shape, and slip-geometry results obtained from the mainshock. In addition, seismicity suggests that the rupture is twisted, increasing in dip and in strike (clockwise) with depth. The deepest aftershocks on the rupture are stable in rate and kinematics suggesting creep near the brittle-ductile transition. This northwest-striking southeast-dipping 1994 rupture is interpreted as a transfer fault connecting the east-west striking and south-dipping Pico fault with a parallel fault to the northwest