[CEUS-earthquake-hazards] alternative hazard maps

[Arthur D Frankel]

Zhenming,

  I would like to amplify some of the points that Buddy Schweig made in 
his excellent response to you.

  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.

  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. 

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. 

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.

   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.

    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.


Art Frankel
U.S. Geological Survey
MS 966, Box 25046
DFC
Denver, CO 80225
phone: 303-273-8556
fax: 303-273-8600
email: afrankel at usgs.gov
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