Oklahoma's quakes prove that you can't predict them
BY WAYNE GREENE World Senior Editor
Tuesday, November 08, 2011
5/30/12 at 6:09 AM
Correction: A Tuesday Tulsa World graphic didn't specify the year in which Oklahoma has had at least 1,200 earthquakes. The state has had at least that many earthquakes this year, according to the Oklahoma Geological Survey. This story has been corrected.
The third earthquake to rock Tulsa in three days proves one thing about geology: You can't predict it.
The U.S. Geological Survey said the 8:46 p.m. Monday earthquake that was felt in Tulsa had a magnitude of 4.7 and an epicenter five miles northwest of Prague, very close to the spot where the first quake in the spate struck early Saturday.
Oklahoma Geological Survey instruments put Monday night's quake at a 5.0 magnitude.
Monday night's shaking came just as geoscientists thought Oklahoma might be ready to settle down.
Before the latest temblor, State Geologist Randy Keller said history teaches that as aftershocks get further apart - as they were earlier Monday - they fade away.
Except, he added, earthquakes simply aren't predictable in human time.
Austin Holland, a research seismologist for the Oklahoma Geological Survey, says more shaking along the Wilzetta Fault - which stretches from near Tecumseh to Creek County - is possible anytime, and Oklahoma has many other faults.
The immediate cause of the earthquakes is one of their great uncertainties, said University of Tulsa Geosciences Chairman Bryan Tapp.
"We really don't understand the actual triggering mechanism for the quakes that occurred on the weekend," he said.
From a broad perspective, the cause is known but certainly not simple. The region has seen intense tectonic deformation - although the really dramatic earth-trembling days ended about 300 million years ago, Tapp said.
The North American tectonic plate is moving to the west, which creates tension throughout the continent. That tension will break free from time to time at weak spots - faults, Keller said.
In this region the Anadarko Basin and the Arkoma Basin are subsiding, while other areas are being pushed up, meaning tension builds along those fault planes, and at some critical moment, the two sides of the fault slip, Tapp said.
That's when things get rolling.
There is one thing the three geoscientists agree on: It doesn't make any sense to think hydraulic fracking by petroleum exploration has anything to do with the weekend's excitement - no matter what you may have read on the Internet.
Holland said that hypothetically, fracking can cause small, localized earthquakes but that there's a much simpler explanation for these events - a known fault line and a continent's worth of geopressure.
Keller agreed.
"It's just too deep. If somebody wants to believe something, they will, but it just isn't logical to believe anything going on 10,000 feet above you is coming down into the solid basement rock and causing things to move," Keller said.
Magnitude unclear
How big was Saturday night's earthquake? Depends on whom you ask.
The U.S. Geological Survey puts the quake at 5.6 in magnitude, making it the largest in contemporary state history.
The Oklahoma Geological Survey puts it at 5.3, slightly smaller than a 1952 earthquake near El Reno.
Austin Holland, a research seismologist for the Oklahoma Geological Survey, said the numbers are going to change and are imprecise by nature.
"There's as many different magnitude scales as there are seismologists. And so just because you do it in a different way and there's a different number that comes out of it, does that make the earthquake any different? It doesn't," he said. "Whether you want to call it a 5.3 or a 5.6, it really is a significant earthquake."
The number stuck on the El Reno event is no more specific, he added.
By California standards, Oklahoma's earthquake wasn't much, but by Oklahoma standards, it's about as big as you're likely to see - with one significant exception.
State Geologist Randy Keller said a well-studied fault near Meers, in far southwestern Oklahoma, could unleash a very large earthquake - something on the 7.0 scale. Such a quake could cause significant damage in Lawton and Wichita Falls and be felt throughout the eastern half of the United States, he said.
By the Numbers
At least 1,200: The number of earthquakes reported in the state so far in 2011.
2.5 or above: The magnitudes of a majority of earthquakes reported in Oklahoma. Earthquakes are usually only reported when people can feel them.
Four: The number of Oklahoma earthquakes so far in November that had a magnitude of 4.0 or higher.
Source: Amie Gibson, Oklahoma Geological Survey research scientist in Leonard and OGS data
'Size' and 'strength'
The Richter scale is based on a logarithmic equation that quantifies the earthquake's magnitude, according to the U.S. Geological Survey.
The scale compares amplitudes of waves on a seismogram, not the strength, or energy, of the quakes, according to the USGS. So, a magnitude 9.0 is 2,512 times bigger than a 5.6 quake as measured on seismograms, but the 9.0 quake is about 125,893 times stronger than the 5.6. The strength of a quake is what leads to damage. This means that it would take about 126,000 quakes of magnitude 5.6 to equal the energy released by one magnitude 9.0 event.
The approximate strength of a 5.6 earthquake is equal in seismic energy to 3.8 kilotons of TNT. A 9.0 would be equal in seismic energy to 480 megatons of TNT.
The earthquake in Japan this year measured 9.0 in magnitude.
This explains why big quakes are so much more devastating than small ones. The amplitude ("size") differences are big enough, but the energy ("strength") differences are huge. The amplitude numbers are neater and a little easier to explain, which is why those are used more often in publications. But it's the energy that does the damage.
Source: USGS
Active Earth: Understanding earthquakes
The rupture of stressed rock results in an earthquake. A massive release
of energy passes through the Earth’s interior and along the surface.
STRONGEST
RECORDED
EARTHQUAKES IN
OKLAHOMA:
Nov. 5, 2011: Prague,
Lincoln County, 5.6
(OGS estimate 5.3)
April 9, 1952: El Reno,
Canadian County, 5.5
Nov. 5, 2011: Prague,
Lincoln, County, 4.7
(OGS estimate 4.8)
Oct. 13, 2010: Noble,
Cleveland County, 4.7
June 1, 1939: Spalding,
Hughes County, 4.4
Original Print Headline: All shook up
Wayne Greene 918-581-8308
wayne.greene@tulsaworld.com
Associated Images:
An earthquake late Saturday night caused extensive damage to the two-story ranch-style home of Joseph and Mary Reneau near the community of Sparks in Lincoln County. Contents inside their home were damaged earlier Saturday when an earthquake struck the same area. The Reneaus have lived in their house for 25 years. JIM BECKEL / The Oklahoman
|
