Is Charleston due for an Earthquake?

Vulnerability

Most of the current residential housing stock is of woodframe construction with masonry veneer. “Manufactured housing,” which typically rests on an unreinforced foundation without anchorage, is the second most common residential building type.

Many public buildings, including schools, emergency services and fire stations, are built of unreinforced masonry – some with heavy precast concrete roofs. These structures are very vulnerable to seismic forces due to their high mass, non-ductile deformation and poor connections.

The primary driver of building vulnerability in the region is the lack of seismic design. Unreinforced construction was permitted in South Carolina until 1994, and many such structures are still being built in some areas.

Until very recently, construction jurisdictions in South Carolina did not review structural design calculation, and few buildings have been designed for earthquake resistance perhaps because the focus on requirements for wind. However, because the dynamic impacts of wind and earthquake on structures are completely different, a building well suited to withstand high winds may be very vulnerable to ground shaking. In addition, some of the most densely populated areas are at risk from liquefaction.

Were the 1886 event to recur today, losses would be significant. The population in the area that would be affected by such an earthquake has grown to more than 5 million. AIR Worldwide estimates that the current value of residential and commercial properties within the damage footprint of the quake is close to $800 billion.Using the AIR U.S.

Earthquakes model, AIR estimates that a recurrence of the 1886 Charleston earthquake would result in insured losses of almost $38 billion, based on current numbers and values of exposed properties.

What Some earthquake experts say:

Charleston Should expect at least a magnitude 5.0 here within the century

Caused by the Woodstock and Ashley River faults, this massive earthquake caused a staggering (for 1886) $5 million in damage within a 200-mile radius, roughing up buildings and twisting railroad tracks like licorice sticks.

Summation map showing areas of persistently upward tectonic motion (light gray), areas of persistently downward tectonic motion (darker gray), and known recently active tectonic features in the Charleston area. The meizoseismal area of the 1886 Charleston earthquake is delimited by dark gray dashed-and-dotted ellipse. The zone of river anomalies (ZRA) of Marple and Talwani (1993) is bounded by dashed lines. Locations of the Ashley River fault and Woodstock fault are from Talwani (2000), the location of the Summerville fault is from Weems et al. (1997a), and the location of the Charleston fault is estimated from Lennon (1986). The Adams Run fault is proposed here. Area of exceptionally dense and well-developed sand blows near Hollywood is indicated by asterisk. Large gray arrows show direction of principal stress

Map provided by: http://www.gsajournals.org/perlserv/?request=display-figures&name=i0016-7606-114-1-24-f14

“The coastal plain here is sand,” says Pradeep Talwani, a geophysics professor at the University of South Carolina in Columbia. “When an earthquake strikes, the saturated sand behaves like liquid. It will not withstand anything built on top of it. It will topple over.”

Talwani estimates that a quake like the 1886 one occurs, on average, every 500 years. Although such a quake cannot be ruled out within the next 100 years, a magnitude 5 is more likely.

The city, like many others, follows building standards set by the International Code Council. These guidelines are intended to ensure that Charleston’s buildings will not collapse during a serious earthquake like the 1886 disaster.

But Charleston’s geography is tricky. “We also have to protect our buildings from flooding and winds reaching 130 mph gusts or higher,” says Douglas M. Smits, director of inspections, who oversees construction for the city.

Charlestonians are well aware of the danger.

“I worry about it,” says Sue Kramer, a member of the Garden Club of Charleston. “I even bought earthquake insurance for my house. But I feel safer here than when I visit my son. He lives in San Francisco.”

Additional reporting by Jill Bock, Erik Sherman, Apryl Chapman Thomas and Mary Logan Wolf

article provided by:http://www.usaweekend.com/04_issues/040905/040905earthquake.html

Other Experts say:

Charleston won’t get another earthquake for at least 250 years

A recent study from Northwestern University seismologists suggests the worries regarding an imminent earthquake in Charleston may be a good 250 years too soon. It’s not that the threat is nonexistent — the region was home to one of the largest historical quakes on the East Coast in 1886, killing 60 people and causing about $6 million in damage.

While standard hazard predictions suggest that the next quake could happen at any time, the new study by seismologist Seth Stein and Northwestern senior James Bebden notes existing data does not account for the relatively short time that’s passed since the last quake.

 They argue that the threat for the next few decades is less than half what federal hazard maps currently predict. Under Stein’s model, it will take until about 2300 or later before we’re at the threat levels that Charleston is currently designing buildings to withstand.

The traditional way to predict earthquake hazards has been to determine where a quake had happened before and the previous magnitude, and then plot out just how bad the next one will be.

Under that model Charleston is just as likely to have an earthquake tomorrow as Reno or St. Louis, where quakes have shaken residents in the last few weeks. But Stein argues that earthquakes are not only dependent on where earthquakes have happened in the past, but also how long it’s been since that last one.

“In general, (seismologists) tell you that some places are overdue for an earthquake,” Stein says. “Like parts of the San Andreas fault — we’re amazed it hasn’t happened yet. That implicitly means that we believe the world is building up for an earthquake.”

If the stress has to build before the next quake, Charleston (which has an estimated cycle of about 550 years) is far less likely to see an earthquake today than it is hundreds of years from now.

 

article provided by:  http://www.charlestoncitypaper.com/gyrobase/Content?oid=oid%3A44868

What Richard N. Côté Says:

He is President of the South Carolina Earthquake Awareness Association, Author of City of Heroes: The Great Charleston Earthquake of 1886, and Webmaster of www.scearthquakes.com

Recent research by Dr. Pradeep Talwani and William T. Schaeffer determined that the massive 1886 earthquake was not a freak event. Instead, they found that it was at least the seventh magnitude 7.0+ (severe) earthquake generated by the Middleton Place-Summerville Seismic Zone, a twenty-five by fifteen-mile oval area that lies astride what Dr. Talwani named the Woodstock Fault (the red lines on the map to the right).

Dr. Talwani determined that the “earthquake cycle” in this zone—that is, the average time between catastrophic earthquakes—is approximately 500-550 years. However, this does not give any prediction of when the next catastrophic earthquake will strike.

That could happen tomorrow—or not for many years. South Carolina will certainly suffer the effects of a major earthquake again. The only questions are where it will strike, when will it happen, and how much damage it will cause.

 information provided by:http://scearthquakes.com/