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SECTION 2
THE NATURE OF
THE EARTHQUAKE THREAT

THE EARTHQUAKE THREAT OF THE NEW MADRID FAULT

Seismologists have provided us with an estimated probability of 40% to 60% for the occurrence of a 6.5R earthquake happening in the New Madrid Fault within the next 15 years. Their probability projections for the 6.5R earthquake is 93 % to 98 % within the next 50 years.

Estimates for 6.5R earthquakes based on the actual seismic event occurrence is one to occur ever 55 to 85 years.

Looking at the last event in that range (6.2R) in 1895 and adding 85 years to that date, 6.5R activity should have presented itself during 1980.

 

QUESTION: What is the likelihood that another damaging earthquake will occur in the New Madrid area in my lifetime?

ANSWER: The U.S. Geological Survey estimates a 40% to 60% chance of a large earthquake--magnitude 6.5 R or greater--in the New Madrid Fault area.

 

QUESTION: What are the most dangerous areas in Arkansas relating to the New Madrid Fault?

ANSWER: The 24 counties in Arkansas most likely to sustain damage from large earthquakes in the southern part of the New Madrid Fault are:

Mississippi Crittenden
Poinsett Cross
Saint Francis Lee
Clay Greene
Craighead Lawrence
Jackson Woodruff
Monroe Prairie
Arkansas Phillips
Randolph Sharp
Independence White
Lonoke Lincoln
Desha Chicot

 

Had the area above, which was affected by the past damaging quakes been heavily populated, as it is now, with today's many unreinforced masonry structures and non-seismically built roads, bridges and overpasses, the death and destruction would have been enormous.

Seismologists expect a 6.5R occurrence to happen once every 55 years to 85 years, a 7.0R every 200-300 years and an 8.0 to 8.8R every 600-1,000 years. *

*DATA FROM: Estimation of Earthquake Effects Associated with Large Earthquakes in the New Madrid Zone by Margaret G. Hopper, USGS and The Effects of Earthquakes In The Central United States, by Otto W. Nuttli

Because all of the counties listed are in an alluvial plane that consists of a water saturated sandy loam type soil to a 3 to 5 mile depth, strong ground motion can cause great damage to non-seismically built buildings.

Also because of unstable soils, in the event of an 8.6R or larger quake, serious damage can occur in parts of an additional 22 to 30 counties to the west and southwest of the above mentioned counties.

Most highways, bridges, utility systems and other lifeline networks in the New Madrid Fault Zone are constructed on or beneath these unstable soils as well. This fragile infrastructure could be seriously disrupted for an extended period of time following a significant earthquake.

 

QUESTION: Which lifelines are at greatest risk from strong ground shaking?

ANSWER: Movement along highways and streets built on soft soils will be severely impaired and will hamper emeraency response activities. Another reasonable assumption i s that all utilities will be out for part of the time in the first 72 hours after a major earthquake.

 

QUESTION: Which types of buildings are at greatest risk from strong ground shaking"

ANSWER: The most vulnerable building types are unreinforced masonry (UFAI). tilt-up concrete, and pre-1972 non-ductile concrete frame buildings. There are thousands of URMs in the cities and hundreds of tilt-ups and concrete frame structures were built in the 50s and 60s in the downtowns and industrial parks of communities.

 

QUESTION: I'm in a well-designed and engineered building; are there other potential hazards besides structural collapse I should be aware of?

ANSWER: Yes. Nonstructural hazards such as unbraced bookcases and file cabinets, light fixtures, glass, and electronic equipment may fall, shatter, and fly about. Contents can be just as hazardous as unsafe buildings.

 

Earthquakes cannot be prevented; however, damage, destruction, loss of life, and even disruption can be significantly reduced by preparing homes, neighborhoods, schools, workplaces, and communities for the next inevitable quake.

 

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School Earthquake Preparedness Guide - State of Arkansas
Arkansas Office of Emergency Services, 1993