*click on illustration for a larger view

This is a map view of the New Madrid Seismic Zone and surrounding areas. The fault
zone (yellow) is located in the upper middle of the diagram. The red arrows
show the annual motion of each site with respect to the ITRF96 North American
Plate motion.

*click on illustration for a larger view

Left panels: Profiles of site velocities, with 1sigma error bars, parallel to the
approximate strike direction of the major rightlateral strikeslip faults in the
NMSZ. Panels show the entire network and its far and near field subsets. For each,
the profile mean is removed. Also shown are bestfitting profiles computed
for a locked vertical strikeslip fault driven by farfield motions. Misfit as a function
of farfield velocity is shown on the right, with 2 sigma range colored yellow.
None of the best fitting velocities differ significantly from zero.
It is very interesting that motion across the fault is indistinguishable from zero,
within errors. This signifies that if the current motions are representative of
longterm dynamics, then the recurrence time for a magnitude 8 earthquake
(assumed magnitude of 18112 series, with 510 meters of slip), is greater than
3,000 years!

*click on illustration for a larger view

Locations of continuously recording GPS sites used to estimate an Euler vector
for the presumablystable portion of North America. For each, the misfit between
the observed velocity and that predicted for a single plate is shown. These misfits
have a mean of 1 mm/yr.
Inset: Euler poles for the eastern and western subsets of the sites (divided by
dashed line) compared to that for the entire set. Because the poles for the east
and west data overlap at 95% confidence, the platewide GPS data show no resolvable
motion across the NMSZ.

*click on illustration for a larger view
 Relation between interseismic motion and the recurrence time of large New Madrid
earthquakes. For an assumed horizontal slip in 18111812 of 510 m, the geodetically
observed interseismic motion of less than 2 mm/yr discussed here implies recurrence
times greater than 2500 yr. Also shown are recurrence estimates from paleoseismic
studies. The paleoseismic and geodetic data are jointly consistent with slip in
18111812 being about 1m, corresponding to a magnitude 7 earthquake.

*click on illustration for a larger view

Top: Earthquake frequencymagnitude data for the New Madrid zone. Both the recent
and historic (18161984) data have slopes close to one, and predict a recurrence
interval exceeding 1,000 yr for magnitude 7 earthquakes and 10,000 yr for magnitude
8 earthquakes.
Bottom: Earthquake history for the NMSZ. Since 1816, there have been 16 earthquakes
with magnitude greater than 5 (about a 10yr recurrence), and two with magnitude
greater than 6 (about an 100yr recurrence). We thus expect magnitude 7 and 8
earthquakes to have about 1,000 and 10,000 yr recurrence.

*click on illustration for a larger view

National Seismic Hazard map showing predicted peak ground acceleration expected
in 50 yrs at 2% probability [Frankel et al., U.S.G.S. OFR 96532]. Because these
maps incorporate assumptions of rapid deformation and frequent great earthquakes,
the predicted acceleration for the New Madrid zone exceeds that in San Francisco,
and the predicted very high acceleration (exceeding 1.2g) area for the NMSZ is
larger than for Los Angeles or San Francisco. Our results imply that the seismic
hazard for the New Madrid area is lower and that such maps should be revised.
