[CEUS-earthquake-hazards] Comment on China quake and hazards map

[Wang, Zhenming]

Dear Julio,

Your explanation (for a sole characteristic M7 earthquake with recurrence interval = 250 years that lead to PGA ~ 0.4 g on average in a site) is a deterministic one and inconsistent with what is known.

"If we have a fault with sole characteristic M7 earthquake with recurrence interval = 250 years that lead to PGA ~ 0.4 g on average in a site, then
PGA ~ 0.4 g has a mean return period ~ 500 years."  A PGA ~ 0.4 g with a mean return period ~ 500 years means that this PGA will occur in 500 years (return period - the mean [average] time between occurrences of a seismic hazard). This contradicts to the basic probability principle.  The probability that one or two PGA of ~0.4g could be observed at the site is about 63% if the M7 earthquake occurs twice in 500 years (assuming Poison distribution).  An event with an occurrence probability of 63% MAY not occur. Everybody can test this by flipping a coin. The probability that you get one or two heads is about 63% if you flip the coin twice. You may get two tails from time to time.

"In 2000 years we expect 8 M7 earthquakes and in 4 of them PGA >= 0.4 g."  The only thing known here is the probability of 98.2% that one or more PGA of ~0.4g could be observed at the site if the M7 earthquake occurs 8 times in 2,000 years (assuming Poison distribution).  What is the probability that four PGA of ~0.4g could be observed at the site? 100%?

There are very limited observations for characteristic earthquakes and the ground motions generated by the characteristic earthquakes at a site. For example, there are only 3 to 4 characteristic earthquakes (~M7.5) were documented in the past couple thousands years in the New Madrid Seismic Zone, and there is no ground motion record. A recurrence interval of about 500 years was inferred for the characteristic earthquake (~M7.5). The aim of seismic hazard analysis is to predict the probability that a level of ground motion could be expected at a site, for example in Memphis or Paducah, if the characteristic earthquake (~M7.5) occurs.

Greg Hempen has made a very good statement: "with so much to learn about earthquakes, we all should take care to be consistent and not misstate what is understood."

Thanks.

Zhenming



________________________________
From: Julio J. Hernández [mailto:julher at cantv.net]
Sent: Wednesday, May 21, 2008 2:24 AM
To: Central and Eastern U.S. Earthquake Hazards Listserve
Cc: Wang, Zhenming
Subject: Re: [CEUS-earthquake-hazards] Comment on China quake and hazards map

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