The New Madrid Seismic Zone (NMSZ), in the central United States,
was the site of large magnitude (M) earthquakes in 1811-12. Paleoseismic
studies find events of similar size recurring about once every 500 years.
Assessing the causes of deformation and estimating
the recurrence of these events are necessary for determining earthquake hazard
in the region.
Global Positioning System (GPS) measurements made in 1991, 1993
and 1997 at 23 sites within 300 km of the NMSZ show minimal
motion across the fault zone. Using a locked
fault model, I find -0.2 +/- 2.4 mm/yr (2 sigma) of right-lateral
fault parallel (NE-SW) motion across the network.
This result is consistent with
continuous GPS results away from the NMSZ which indicate
less than 1 mm/yr of differential plate motion.
Assuming horizontal slip accumulates at less than 2 mm/yr, the recurrence
interval (T_r) for ``New Madrid style'' M 8 earthquakes, with 5-10 m slip, would exceed 2,500-5,000 years,
significantly greater than the 500-1,000 years previously estimated.
However, these GPS results are consistent with my re-evaluation of the
paleoseismic recurrence estimates,
and lack of fault-associated topography if the 1811-12 earthquakes and the
paleoearthquakes if the events were smaller, low M 7, and recurring every 500-1000
years. Also, smaller M 7 events are consistent with a new earthquake
intensity study for the 1811-12 series.
Reducing the size of the maximum magnitude events (M_max) has considerable effect on the
predicted seismic hazard in New Madrid.
Currently, the U.S.G.S. seismic hazard maps show
large accelerations for the central U.S. due to the NMSZ.
These maps reflect crucial parameter assumptions,
having large uncertainties because little
data exists for large events.
Since my GPS and seismic recurrence studies, along with other recent results,
that the M_max may be 7 with T_r
of 500 years, rather than M_max 8 and T_r of 1000 years used by the U.S.G.S.,
I explore the effects that these parameters,
along with several predicted ground shaking models, have on predicted 1 Hz and peak ground
accelerations from the NMSZ.
I find that changing these parameters significantly affects the predicted
demonstrating the considerable uncertainties in seismic hazard from the NMSZ.
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