<br><font size=2 face="sans-serif">Zhenming,</font>
<br>
<br><font size=2 face="sans-serif"> I would like to amplify some
of the points that Buddy Schweig made in his excellent response to you.</font>
<br>
<br><font size=2 face="sans-serif"> When deciding on measures for
public safety, such as building codes, it is critical to consider the random
variability that occurs in natural processes, such as the timing of earthquakes
and the severity of ground motions produced by rupture on a fault.</font>
<br>
<br><font size=2 face="sans-serif"> The 7-10% probability of having
a 1811-12 type New Madrid earthquake in the next 50 years is derived
from considering a 500 year AVERAGE recurrence time, along with an assumption
on the probability distribution around that average recurrence time. Unfortunately,
there is randomness to earthquake occurrence and there is a significant
probability that we will have an 1811-12 type earthquake in the next 50
years, well before the 500 year average interval has elapsed. </font>
<br>
<br><font size=2 face="sans-serif">Take the example of great earthquakes
on the Cascadia subduction zone (CSZ) of the Pacific Northwest. There
is good paleoseismic evidence stretching back at least 5000 years that
great earthquakes occur along the entire CSZ with an average recurrence
time of about 500 years (see Atwater and Hemphill-Haley, 1997, USGS Professional
Paper 1576; Nelson et al., 2006 in Quaternary Research). Of course,
there is uncertainty in our dating of these past earthquakes. Furthermore,
there is natural variability in the occurrence of these earthquakes. For
example, two great earthquakes (the U and W events described in Atwater
and Hemphill-Haley) may have occurred as little as 100 years apart,
given the closeness in time of the central estimates of their dates
and the uncertainites in the dating. </font>
<br>
<br><font size=2 face="sans-serif">The variability in earthquake recurrence
times is understandable: we know that slip on a fault during an earthquake
is usually very heterogeneous, with some patches on a fault slipping a
great deal and other areas of the fault hardly slipping at all. Slip
on these other areas is sometimes observed to be taken up by the next earthquake
(see the 1940 and 1979 Imperial Valley earthquakes for example).
It is also likely that the rate of loading of faults is not constant
in time or space. As I said in my previous email, the occurrence
of a large earthquake can increase stress in some areas and affect the
timing of other earthquakes in a region, adding to the randomness of earthquake
occurrence.</font>
<br>
<br><font size=2 face="sans-serif"> Buddy is also correct in
noting that the seismic hazard maps also include the possibility of magnitude
6 earthquakes, which will be more frequent than the 1811-12 events.</font>
<br>
<br><font size=2 face="sans-serif"> When an 1811-12 type New
Madrid earthquake occurs again, it will produce a range of ground motions,
even for locations at similar distances from the earthquake fault. Of
course, there are differences in soil conditions that will affect the ground
shaking and we can predict that areas of soft soils will have stronger
shaking on average. Even considering sites with similar soils at
similar distances from the earthquake, there will be substantial variation
of ground shaking. For example, if a large earthquake on the Reelfoot
fault ruptures from northwest to southeast, it will produce higher ground
motions than average for some locations in northwest Tennessee because
of the strong pulse of velocity produced by rupture directivity. Areas
of the fault where there is larger slip during the earthquake will likely
produce larger ground velocities at nearby sites. These areas of
larger slip will vary from earthquake to earthquake, as could the direction
of rupture. Thus, we can't predict in advance (at least at our present
stage of knowledge) where the areas of higher (and lower) than average
ground motions will occur. But we can account for this variability
in our seismic hazard calculations and in our building codes. That
is the essence of probabilistic seismic hazard assessment.</font>
<br>
<br>
<br><font size=2 face="sans-serif">Art Frankel<br>
U.S. Geological Survey<br>
MS 966, Box 25046<br>
DFC<br>
Denver, CO 80225<br>
phone: 303-273-8556<br>
fax: 303-273-8600<br>
email: afrankel@usgs.gov</font>