We've seen the movies and the made-for-TV dramas, but in the real world, what damage can an earthquake cause?
Are we talking about broken dishes and crooked picture frames, or would a big tremor beg comparisons to the apocalypse?
Geohazards Specialist Scott Ausbrooks of the Arkansas Geological Survey said a magnitude-5 quake would cause a fair amount of damage to buildings.
"At a 5 we would see damage to chimneys and things like that. Brick can fall from walls at that point. If it gets a little larger, damage is more widespread. For example, the Mount Carmel, Ill., earthquake in 2008 was felt in 28 states. That was a 5.4. Damage showed in brick buildings and porches collapsed. At 5.2, porches that are up on two columns may be brought down.”
What may occur at a higher register, such as a 5.5?
“For every tenth in magnitude, 32 times more energy is released," Ausbrooks said. "A few tenths of a point does matter. By this time, shaking is more intense, and you would see more damage. Architectural damage and structural damage to poorly built structures. This number is the threshold for structural damage.”
A poorly built or weak structure, according to Ausbrooks, may be an older home, or an inexpensive building without rebar, or steel reinforcement.
What damage is expected at a 6-magnitude?
“At a magnitude 6, you have very strong shaking with localized, heavier damage as well as widespread damage. At this point, damage occurs in better-built structures. Structural damage and extensive damage will occur in poorly built structures. A 6 is a threshold for low-end buildings. We haven’t seen a 6 in Arkansas since 1843.”
Ausbrooks said bridges are affected at 6 magnitudes: “Poorly built or older bridges lose strength at 6 because of liquefaction.”
Ausbrooks explained that liquefaction is a loss of bearing strength in soil, especially sandy or water-filled soil.
“As those grains are pulled apart it becomes like quicksand. If something heavy is on top, it’s going to sink, and things that are buried will pop to the top. Depending on the duration and magnitude of an earthquake, you may get quicksand or liquefaction.”
Ausbrooks said that the recent 4.7 earthquake was at the “upper end of historical earthquakes.”
“It’s not out of range, historically speaking, but it’s near the max of what we’d expect.”
Ausbrooks said that a fault-length rupture, were it to occur, would produce a magnitude near 5.5.
He said he does not expect a larger event than a 5.5 in the area, but ...
“We can never say never.”
(Staff writer Courtney Spradlin can be reached by e-mail at firstname.lastname@example.org or by phone at 505-1236. To comment on this and other stories in the Log Cabin, log on to www.thecabin.net. Send us your news at www.thecabin.net/submit)