Geologist discusses Japan earthquake with Texas Exes

By Laurel L. Scott
Posted February 29, 2012

SAN ANGELO, Texas — Nearly a year ago, water levels in wells on the Edwards Aquifer near San Antonio "sloshed" as seismic waves from the magnitude-9 earthquake that struck Japan traveled across the planet.

Geologist Mark Cloos, a professor at the University of Texas' Jackson School of Geosciences, shared that fact at a meeting of San Angelo Texas Exes — University of Texas alumni — on Tuesday while describing how one of the worst natural disasters of modern history came to happen.

"We didn't feel a thing, but the water table shifted about a foot," Cloos said.

He said the earthquake could be explained by the theory of plate tectonics, which says the surface of the planet is made up of giant plates that are moving about as fast "as fingernails grow." Some plates are "slipping" past each other, such as the San Andreas Fault in California.

In Japan, the Pacific Plate is pushing under the Eurasia Plate, lifting up what has become over the millenniums the islands that make up Japan.

"Japan is here because the plates are coming together," Cloos said.

The earthquake struck March 11, 2011.

"It was the largest earthquake in Japan since 869 A.D. We think that's the last time something like this happened in Japan," Cloos said. "What happened was about 500 years of plate convergence was released in about one minute."

He said the rupture was 250 miles wide from north to south and 100 miles wide from east to west.

"There were a couple of little precursor earthquakes that we didn't know were precursor at the time," he said.

The greatest damage, Cloos said, was not done by the earthquake, though it was the one of only five magnitude-9 earthquakes in modern history.

"It was very, very violent shaking for about a minute in Tokyo, but what got the attention was the shaking continued for about six minutes," he said. "The energy released in that one minute was the equivalent of 9,300 billion tons of TNT or 600 million Hiroshima bombs."

Cloos said about 3,000 people were killed in the earthquake, but the tsunami, which struck the east coast north of Tokyo about an hour later, left 20,000 dead or missing.

"The tsunami traveled about 800 kilometers an hour, as fast as jet airplanes," he said. "It was just unstoppable. It was 131 feet high at its highest."

The tsunami was responsible for the nation's next crisis, the meltdown of four nuclear reactors at the Fukushima Dai-ichi Nuclear Power Plant. The plant was built on a site picked in 1962, using a 1967 design meant to deal with a magnitude-8 earthquake, he said.

The backup generators were built at 10 to 13 meters above the ground. The tsunami height was 14 meters, Cloos said. As a result, the plant lost all its backup generators.

"Another nuclear power plant that was much closer had three reactors that came through just fine because it was a little higher," he said.

For scientists, this was the first earthquake to benefit from Global Positioning System technology.

"Japan had 1,600 GPS stations, and they all moved," Cloos said. "They provided the information on how the ground actually moved. The coastline subsided as much as 2½ feet. It had been pushed up slowly over time, and then a lot of land was lost in a minute."

What geologists learn has applications closer to home. Cloos said there is active faulting in southeast Texas, especially in the Houston area, which is slowly sliding into the Gulf of Mexico. He also said scientists have known since the 1960s that smaller earthquakes can be induced by human activity, such as injection wells at oil fields, but that out of dozens of injection wells in the Dallas or San Antonio areas, one will have some minor earthquake activity.

"We think we are changing the state of stress by changing the fluid pressure," he said.

Pierce Miller, a rancher in Ozona and a San Angelo resident, said "it was outstanding" to have Cloos explain tectonic plates and the cause of the events in Japan.

"Being from San Angelo, I don't think we're subject to anything so dramatic," he said. "We have great people in San Angelo, but to have great people come in and to hear about it firsthand is special."

Roy Harrell, a retired foreign service officer who ranches in Ozona, said the topic was "extremely timely because I'm concerned about water."

"This situation is something beyond our shores, and it's something we can do little to mitigate, but it is very important," he said.