Earthquake
Focal Mechanisms from the New
Johnson, Horton, Withers and Cox
We evaluate 346 well constrained focal mechanisms
(0 < md < 3.9) generated using P and SH first motion
polarity observations recorded in
the northern Mississippi Embayment during the period of 2000-2007. The
New
Madrid Seismic Zone (NMSZ) can be broken into three interconnecting
segments
based on the distribution of micro-earthquake locations. Earthquakes
located
within the central segment illuminate a fault plane that strikes ~N21W
with ~45°
dip to the southwest. Based on rake 31% of the earthquakes located in
the
central segment have reverse slip, 28% have strike slip, and 41% have
normal
slip. Only 17% of the total focal mechanisms in the central segment
show reverse
slip where a nodal plane has a similar strike and dip as that observed
in the
seismicity. Earthquakes located within the southern segment illuminate
a fault
plane that strikes ~N50E with nearly vertical dip. Based on rake 27% of
the
earthquakes located in the southern segment have reverse slip, 55% have
strike
slip, and 18% have normal slip. In this
segment 34% of the focal mechanisms show right-lateral strike-slip on a
nearly
vertical nodal plane striking about N50E. Earthquakes located within
the northern
segment illuminate a fault plane that strikes ~N30E with nearly
vertical dip. Based
on rake 38% of the earthquakes located in the northern segment have
reverse
slip, 57% have strike slip, and 5% have normal slip. Almost half (~48%)
of the
focal mechanisms identified with this segment show right-lateral
strike-slip on
a nearly vertical fault striking close to N30E. We will extend these
preliminary results to produce a seismo-tectonic model of the NMSZ.