The 1994 Northridge and 1971 Fault
Ruptures and Surrounding Seismogenic Faults as Illuminated by Small Earthquakes
L. Seeber, J. G. Armbruster, and P. Geiser
Lamont-Doherty Earth Observatory of Columbia Univ, Palisades,
NY
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 southeastdipping
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.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California