[CEUS-earthquake-hazards] FW: alternative hazard maps

[Wang, Zhenming]

Art,

First, I would like to clarify the bases for all the discussions we have: the basic earthquake science and the assumptions that are used in the national seismic hazard mapping.

Second, as you describe bellow, there are temporal measurements (including uncertainty) (such as the timing of earthquakes), some are spatial measurements (such as the severity of ground motions produced by rupture on a fault), and others are instrumental measurements (such as carbon dating). All my comments and discussions are on temporal and spatial measurements of earthquakes.

The main point is that "The temporal and spatial measurements are two fundamental and critical elements for quantifying earthquake and seismic hazard. The temporal and spatial measurements are also two basic elements of our society. Mixing these two measurements one way or the other will cause problem."

Your probability estimates for a 1811-12 type New Madrid earthquake and the ground motions at a site from this 1811-12 type New Madrid earthquake demonstrates the problem. Under the assumption that earthquake occurrence follow Poisson distribution, you have calculated 10% probability of having a 1811-12 type New Madrid earthquake in the next 50 years, derived from considering a 500 year AVERAGE recurrence time.  As demonstrated in your paper (Frankel, 2004), you can provided a range of probability, 10, 5, 2, 1, ..., percents, of having the ground motions for this 1811-12 type New Madrid earthquake in the next 50 years (hazard curve), by mixing temporal measurement (500-year recurrence interval) and spatial measurement (ground motion uncertainty).  These probability estimates are contradictory.

"the seismic hazard maps also include the possibility of magnitude 6 earthquakes, which will be more frequent than the 1811-12 events."  This hazard calculation could only be done by mixing the temporal and spatial measurements.

Thanks.

Zhenming


________________________________
From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of Arthur D Frankel
Sent: Thursday, February 07, 2008 2:56 PM
To: ceus-earthquake-hazards at geohazards.usgs.gov; ceus-earthquake-hazards-bounces at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] alternative hazard maps


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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