When a big earthquake hits, the world often sees horrific images of collapsed bridges.
In 1989, during a 6.9-magnitude quake in the San Francisco area, the double-deck Nimitz Freeway pancaked, killing 42 people. Fifty-foot sections of the Bay Bridge also collapsed, killing a woman.
North Texas is unlikely to experience an earthquake of that scope, researchers say. But in recent years, the region has experienced dozens of smaller quakes, with the strongest having a magnitude of 4.0 — enough to potentially damage buildings and bridges.
Those in geology and engineering circles are increasingly concerned that the wave of seismic activity in Dallas-Fort Worth could damage the area’s transportation infrastructure — not only bridges but also tunnels, roadways and rail lines.
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“We’re talking a lot about it,” said Brian Barth, the Fort Worth district engineer for the Texas Department of Transportation. “It is important for us to make sure we’re covered. We’ve been discussing it statewide. This isn’t the only area where we’re having these issues.”
The updated information about the region’s earthquake risks could lead to the publication of new guidelines for building bridges, roads and rail lines to withstand higher-magnitude quakes. Over time, that could add millions of dollars to construction costs.
Until a couple of years ago, North Texas had little recorded history of seismic activity. But since November 2013, at least 50 earthquakes have been recorded along a 2-mile strip from Irving to west Dallas. A magnitude-2.6 quake hit the area Saturday morning.
On May 7, a magnitude-4.0 earthquake struck in Venus, shaking homes and rattling nerves in the small community between southeast Fort Worth and Mansfield.
At least 27 earthquakes near Reno and Azle, recorded since late 2013, were likely caused by drilling operations, according to a study in the science journal Nature Communications. Scientists from the U.S. Geological Survey, Southern Methodist University and the University of Texas at Austin have partly linked the quakes to injection wells used in natural gas production in the Barnett Shale.
Some well operators have disputed those and similar findings, and Gov. Greg Abbott approved $4.5 million last month for a comprehensive earthquake study.
Regardless of the cause, the quakes are occurring with a strength that was once thought improbable in this part of the United States.
Larger than expected
These “induced seismicity” earthquakes — so labeled by the U.S. Geological Survey because they’re believed to be influenced by human activity rather than just by natural fault lines or tectonic plate movements — were initially considered less potentially harmful than, say, an earthquake along the San Andreas Fault in California.
Induced seismicity earthquakes were typically below a magnitude of 4.0 — capable of rattling dishes and knocking a picture off a wall but not much more.
“The general rule of thumb is that earthquakes below magnitude 4 do not cause infrastructure damage,” said SMU geophysics professor Heather DeShon, although she cautioned that the university’s seismologists aren’t earthquake engineers.
But the Venus earthquake raised new alarm bells. In the hours after the May 7 temblor, highway inspectors fanned out to check nine bridges for damage. The bridges were along U.S. 67, Farm Road 157, and Johnson County Roads 502, 615 and 616.
They searched for clues — perhaps a misaligned pavement stripe or a buckled metal beam — that the bridges had been jostled. Bridges built during the past three decades in the Fort Worth area were designed according to standards considered appropriate for a low earthquake risk.
“Inspectors searched for recent cracking or spalling on the deck and beams, as well as the substructure, distortion at expansion joints, shifting of the beams away from the supports,” said Michael Peters, a Transportation Department spokesman.
Inspectors found no damage related to the quake. But that temblor and several larger ones in Oklahoma are rewriting what researchers understand about the kind of transportation damage that human-caused earthquakes can produce, said Mark Petersen, a Colorado-based research geophysicist at the U.S. Geological Survey.
For example, he said, the largest quake in more than 50 years in Oklahoma was a 5.6-magnitude temblor in 2011 centered near Prague, along an old fault line that inexplicably reawakened in the middle of the state. It buckled streets and damaged homes and other buildings.
“The magnitudes are increasing to a level that concerns us, so now we’re starting to take it into account,” Petersen said. For decades, his agency published data-driven risk reports with Texas in the lowest category for earthquakes. Now the agency is rewriting those reports.
The updated document is known as the National Seismic Hazard Model.
“We found the hazard is much higher due to the earthquakes we have had in the recent past,” Petersen said. “We’re trying to finalize that model and come up with something that forecasts the types of ground shaking we can expect in the future.”
Updated computer models show several spots in DFW — Venus, Irving and Azle among them — that face a significantly higher risk of earthquakes that could damage infrastructure.
SMU, in conjunction with the U.S. Geological Survey, has installed 11 seismic stations to monitor the Venus area, DeShon said.
Retrofitting may be needed
The new information doesn’t necessarily mean that bridges or other infrastructure are in danger of collapsing, several officials said.
But it could mean that millions of dollars in state and federal highway funding will be needed to repair old bridges or build new ones that adhere to higher earthquake-proof standards, officials said.
California has a retrofit program in which roughly $14 billion is being spent to shore up about 4,700 bridges, according to a 2013 report by the Congressional Research Service.
Closer to home, around 13,000 vulnerable bridges in seven states in the New Madrid seismic zone, which roughly follows the Mississippi River in and around Memphis, likely need retrofitting to meet new design standards, according to the congressional report.
The study did not estimate the cost of the work.
Guidelines used by states to determine how much earthquake-proofing to incorporate in bridge and road design are drafted with input from the American Association of State Highway and Transportation Officials, the Federal Highway Administration and other agencies.
Highway engineers in states such as California and Alaska have used specific design standards to earthquake-proof their roads and bridges since the early 1990s. But officials in Texas haven’t done so until now because the area was considered low-risk.
Now officials from various scientific backgrounds acknowledge that the earthquake risk in North Texas is much higher than previously thought. How high? It could take months or even years of research to figure that out.
This report includes material from the Star-Telegram archives.
Gordon Dickson, 817-390-7796
Although relatively small earthquakes have become part of life in North Texas, the region has no recorded incidents of damage to bridges, roads or other infrastructure.
The largest recorded earthquake in Texas history was in 1931, when a magnitude-5.8 temblor shook the far West Texas town of Valentine, according to the U.S. Geological Survey. In the deep desert, there wasn’t much infrastructure to damage. More recently, on May 7, a magnitude-4.0 earthquake struck the Johnson County community of Venus.
But in other parts of the United States, bridge damage is a common result of earthquakes. Some examples:
▪ In 1989, 63 people were killed in a 6.9-magnitude earthquake in the San Francisco area, including 42 who died when the double-deck Nimitz Freeway pancaked in Oakland.
▪ Five years later in Los Angeles, 57 people died in a 6.7-magnitude quake that left large sections of the Santa Monica Freeway and other busy highways unusable for months.
▪ In 1962, a 9.2-magnitude temblor struck Alaska — the strongest earthquake in U.S. history. The damage included the collapse of the Miles Glacier Bridge, which would not be fully repaired until 2005.
Sources: U.S. Geological Survey, news archives