29 August 2011

Guest Post: Kevin Vranes on the Virginia Earthquake

NOTE: This is a guest post by Kevin Vranes.

The overwrought reaction to the M5.8 Virgina earthquake on Tuesday had a lot of native Californians snickering (ok, me included, I grew up just south of San Francisco), but I had to concede that a M5.8 in the east is not the same thing in the well-prepared west.  The fact is, the eastern seaboard has plenty of shaking in its geologic history, but little in its human history.  The frequency of very large quakes on the eastern seaboard is much longer than 1-2 generations, a period which in California seems to keep the risk fresh in the mind of locals and in the policy considerations of local governments.

Building codes in the U.S. have traditionally been highly localized, and only recently are beginning to reflect the realization that almost every state in the U.S. has some seismic risk and therefore codes should incorporate seismic design principles. Combine slow code change with old building stock (apparently the building stock is older in D.C. than in any of the 50 states - link), and you have an area at much higher relative risk to moderate shaking than California.  

The question we should be asking is, what is the true risk?  Since this is an intraplate region with lots of old, hidden faults that do not move often enough to reveal shaking risk, this is a difficult question to answer (perhaps nearing impossible in the foreseeable future).  But what about from an economic damages perspective?  In the absence of seismic data, can some measure of risk be calculated?

In 2009 Roger and I published a paper in Natural Hazards Review to examine this question. The analysis was an attempt to put historical quakes into current-day context by asking essentially this question: “If the 1906 San Francisco happened again today, with today’s population, increase in wealth and increase in damage mitigation, what would we figure the economic losses would be?”  We did this calculation for every earthquake in the U.S. since 1900 for which we could find a credible estimate of economic losses -- 80 in all.  How does this apply to east coast earthquakes, and therefore how does it reflect on Tuesday’s quake?  By giving us a measure of the frequency of quakes that cause economic damages in a certain region.

So what does our analysis tell us about Tuesday’s Virginia earthquake?  Unfortunately, the answer is: just about nothing. Since 1900 only two quakes have occurred in the east that were given damage estimates: a 1944 quake in Massena, NY (on the Canadian border, as far from NYC as you can get in New York state) and a 1954 event in Wilkes-Barre, Pennsylvania.  The Massena quake was given a $1.5M - $2M price tag, and the Wilkes-Barre event was given a $1M estimate.  Although damaging events have occurred in the mid-Atlantic and southeast in human history, none have occurred since 1900, the period for which we have good economic comparison data.

And this is the problem.  We know the major quakes can occur in the intraplate east, but they happen so infrequently and the last big ones happened so long ago, they tell us very little about what would occur today with the same shaking conditions.  The two major historic quakes in the east occurred in 1811 (New Madrid, Missouri) and 1886 (Charleston, SC).  Damages from the Charleston event was given a price tag of about $5 million in 1886 dollars.  By comparison, the 1906 San Francisco earthquake was estimated at over $500 million in 1906 dollars.

Were these quakes to occur again today damage would undoubtedly be extreme, but how extreme?  What’s left is to model with HAZUS, which can give a peek into what kind of damages you might expect from certain shaking types, and hope that your estimate for shaking risk in certain intraplate regions is fairly accurate.  Whether it is?  We’ll find out when the next big eastern seaboard quake happens near a major city.

2 comments:

Mark B. said...

An earthquake shook Boston in 1755, taking down chimneys and church steeples. Estimates put the quake in the 6+ range. Since then, the city has increased in size drastically by filling in the harbor and wetlands. A similar quake today would leave little standing, including large residential districts of brick row houses and 20th Century skyscrapers.

This fact has been publicized in the newspapers occasionally, so it's fair to say that everyone knows that Boston sits on a fault line that is 'active' in a geological sense. Yet absolutely no one advocates for reinforcing every building in the inner city district up to earthquake standards.

Is Boston a city of quake-deniers who can't (or refuse to) understand risk?

EliRabett said...

You are making a category error here, comparing DC to states. A better comparison would be DC to Philadelphia or Wilmington (nothing compares to NYC)

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