‘Nightmare’ scenarios show what could happen if Dallas’ earthquakes return bigger than before

It’s been two months since Dallas last heard the boom and felt the rattle of an earthquake, but that doesn’t mean the region is done with them.

On Monday, the U.S. Geological Survey plans to release a report that concludes Dallas and Fort Worth’s risk of more quakes is higher now than it has ever been.

Meanwhile, the Federal Emergency Management Agency has drafted a study of worst-case scenarios that shows North Texas faces at least some risk of a quake that could lead to loss of life and billions of dollars in damage.

The Dallas Morning News obtained the study through a records request after local and federal agencies declined to release it.

To date, the strongest quake to rattle Dallas occurred Jan. 6 and registered a magnitude of 3.6. FEMA’s worst-case scenario imagines a major 5.6 magnitude quake.

Despite that, a U.S. Geological Survey researcher said there’s no reason to panic.

The scenarios, said Robert Williams, are considered “unlikely but plausible.” 

If a 5.6 magnitude quake were to happen, northwest Dallas, West Dallas and downtown would bear the brunt, according to the U.S. Geological Survey ShakeMap included in the FEMA report.

Levees and dams could collapse. About 80,000 buildings would be at least slightly damaged, causing $9.5 billion in “direct economic losses.” Some 290 area bridges — those with a “10 percent or greater chance of exceeding slight damage” — would need to be inspected to make sure they didn’t crack or buckle.

“Critical infrastructure” could be seriously affected.

Most important, there is the “possibility for injuries, including fatalities,” the report states.

There would be significantly less damage should there be a 4.8 magnitude quake, the FEMA report says. But the cost would still be enormous, estimated at $2.3 billion in Dallas County building losses alone.

And, it says, “there is a small possibility for minor injuries.”

Rocky Vaz, Dallas’ director of emergency management, said this week that either scenario would have the same result: “You’re talking about massive disruptions and evacuations,” he said. “A nightmare.”

And not just for Dallas: A 5.6 magnitude quake — the same size as the quake that hit Oklahoma and was felt in Texas, Kansas, Arkansas and elsewhere in 2011 — would probably inflict a billion dollars’ worth of damage in Tarrant County, and about half that in Collin and Denton.

Last July, FEMA delivered to local officials its study titled “Potential Impacts and Mitigation Strategies: Dallas-Fort Worth Earthquake Scenarios.”

It was prepared at the request of the Irving-Dallas Area Earthquake Working Group, which formed just days after quakes became a way of life in a part of the world not long ago unaccustomed to feeling the earth move under it.

The group wanted to begin preparing for a worst-case scenario. That’s precisely what FEMA delivered.

Here is how Vaz put it: “The probability of having a big one — even a 4.6 or 4.8 — is very, very low. Less than 1 percent.”

But, he cautioned, “that doesn’t mean anything.”

Why not? Because to put that in perspective, the Trinity River levees are built to withstand a 100-year flood — which is to say, a flood that statistically has a 1 percent chance of occurring in any given year.

Last spring’s torrential downpours came very close to reaching that benchmark.

Vaz said some places in the Trinity floodway hit the 70-year flood stage, which is why there was flooding south and west of the levees.

But at least Dallas was prepared with levees and pump stations put in place decades ago to safeguard against the rising tide.

The region is not prepared for that 1-percent quake. Not even close, according to officials. And that preparation is unlikely to happen anytime soon.

The city hasn’t even begun to address changing building codes, because, so far, there’s been no need in the midst of small-scale quakes that have done little more than cause a few cracks in homes near the epicenters of the Irving and northwest Dallas quakes.

And, city officials say, they have little data to go on outside of a full year’s worth of quakes and some light tremors dating back to 2008. More data is on the way: The USGS report Monday will, for the first time ever, incorporate “induced” earthquakes into federal seismic hazard maps.

And by induced, the USGS means quakes caused when wastewater from natural gas drilling is injected back into the ground.

“The new report estimates where, how often and how strongly earthquake ground shaking is expected to occur,” said USGS’ Williams.

“As USGS has reported at various times over the past few years, places that have experienced increases in the rate of earthquakes also face small, but increased chances for larger damaging earthquakes,” he said.

“We still believe the earthquake hazard in the Dallas-Fort Worth region is elevated and that people should continue to learn what to do before, during and after an earthquake.”

The USGS and FEMA aren’t the only ones still dealing with the quake what-ifs. The Texas Department of Transportation is in the process of putting together its own statewide seismic risk map focusing on bridges.

Last year, the agency said that to “ensure public safety and adequate response after a seismic event, TxDOT needs to understand its exposure to seismic risk and have, on hand, a clear plan of action for post-earthquake response.”

Vaz and other Dallas officials have known about the FEMA nightmare scenario for months and have insisted they couldn’t turn it over without the feds’ consent.

Vaz said he is relieved the study is finally seeing the light of day.

“It’s not a bad idea for people to be exposed to those scenarios,” he said. “They need to understand that when something like this happens, there’s not a whole lot any one entity can do. We all have to come together. We have to worry about how to respond and recover. Fear can’t get in the way of preparation.”

http://cityhallblog.dallasnews.com/2016/03/nightmare-scenarios-show-what-could-happen-if-dallas-earthquakes-return-bigger-than-before.html/