Following a conversation with Dr. Art Frankel, USGS, at last week's Seismological Society of America meeting in Seattle, Washington, the following runs show the result of using different Mmax values for the characteristic New Madrid earthquake. In the 1996 USGS computations, Mmax = 8.0. The 1000 year repeat time of these events is distributed over three possible fault locations, each with a 3000 year repeat time.
The computational results show here compare the 1996 USGS computations to those with Mmax = 7.5 and 7.0!
Note: Only the Mmax of the characteristic
earthquake is changed. The Mmax values used with the
Gutenberg-Richter recurrence relation is not changed. This value is 6.5 in the
craton, and 7.5 in failed rifts, including New Madrid.
Hopefully, this simple change serves to highlight the relative importance
of the characteristic earthquake in the scheme of things.
| Map | Probability |
| Peak Horizontal Acceleration | 10% 50 year |
|
|
05% 50 year |
|
|
02% 50 year |
| 1.0 sec Pseudo Acceleration | 10% 50 year |
|
|
05% 50 year |
|
|
02% 50 year |
| 0.2 sec Pseudo Acceleration | 10% 50 year |
|
|
05% 50 year |
|
|
02% 50 year |
| 0.3 sec Pseudo Acceleration | 10% 50 year |
|
|
05% 50 year |
|
|
02% 50 year |
10% 50 years - there is little difference as a function of Mmax
5% 50 years and 2% 50 years - there is little difference as a function of Mmax outside of the Embayment, reflecting the fact that nearby sources are very important to the hazard level, except when a large distant source dominates. Of course, New Madrid and Memphis are very sensitive to the size of this nearby event.
The communities adjacent to the New Madrid Seismic Zone seismicity continue to have high expected ground motions, in spite of the fact that the numerical values decrease with decreasing Mmax for the 5% and 2% in 50 year hazard level.