Twelve earthquakes shook North Texas Tuesday and Wednesday -- and seismologists are intensifying their focus on all of the rumbling and rattling near the old Texas Stadium site.
Meanwhile, Southern Methodist University says it will install 22 more seismographs in the Irving area over the next few days.
Fifteen of those monitors were being deployed Wednesday. Two more from the U.S. Geological Survey should be installed Thursday, with others to be placed on Friday.
SMU experts, who have been studying North Texas earthquakes in recent years, stress it’s going to take time to learn more about the quakes.
“In the near term, our first step is to put out seismographs to confirm and refine the location of the quakes and define the faults in the area,” Heather DeShon, associate professor of physics at SMU, said in a statement. “Only after we get that data will we be in a position to investigate the potential cause of the earthquakes.”
SMU seismologist Brian Stump, who plans to discuss the quakes with the Irving City Council Jan. 15, says it’s premature to speculate on the cause.
What happened when
The most recent earthquake swarm started at 7:30 a.m. Tuesday. The earthquakes kept coming ... and coming ... and coming. But as Tuesday evening progressed, they tended to get smaller.
By Wednesday morning, 12 quakes had rattled North Texas within 27 hours. The U.S. Geological Survey says the 12th quake struck at 9:57 a.m. Wednesday in Irving -- it had a 2.7 magnitude.
The 12 quakes ranged in magnitude from 1.6 to 3.6.
Most of them were recorded near the old Texas Stadium site.
Some of the quakes were felt in several cities across Dallas-Fort Worth. There are no reports of significant damage or injuries.
But as the week went on, more earthquakes were recorded. Thursday brought three more, including two in Irving (both were 1.9 in magnitude). Then, a 2.7-magnitude quake was recorded in Midlothian, about a half-hour south of Irving. As of Friday morning, 15 earthquakes had been recorded in North Texas this week.
USGS geophysicist Carrieann Bedwell told The Associated Press that the agency's senior scientists will be looking at the data from a seismic standpoint.
Tuesday's 3.6 quake is the strongest to hit Irving in recent months. Since early September, more than 25 quakes have rattled Irving around State Highways 114 and 183, near the old stadium land and a gas well site.
Geologists say earthquakes of magnitude 2.5 to 3.0 are generally the smallest felt by humans.
Emergency officials meet
Emergency management officials from Dallas, Irving, Dallas County and the state met Wednesday to discuss the Irving-area earthquakes.
Dallas County officials plan to meet with SMU and Dallas emergency planners to “develop additional guidance to better ensure our plans and procedures are up-to-date,” County Judge Clay Jenkins said in a statement Wednesday afternoon.
[Related: For the latest developments on the North Texas earthquakes, visit KERA's Earthquake Blog.]
'Remote possibility' for a larger earthquake, USGS says
Robert Williams, another USGS geophysicist, told KERA there is concern about the increase in North Texas earthquakes.
“There is some concern that has increased because of the activity increase over the last 36 hours or so,” Williams told KERA. “So we don’t know if there will be a larger earthquake that’s more damaging, but we can’t rule out that possibility. Studies of earthquakes around the world for decades have shown that the more small earthquakes that you have in an area, the more likely you’re to have a larger earthquake. But it’s too early to say how likely, but we do want to say that there is a remote possibility for a larger earthquake and people should, I think, take general precautions.”
Dozens of small earthquakes have hit parts of North Texas in recent years, including several in and near Azle, which is northwest of Fort Worth.
Some blame North Texas earthquakes on natural gas well drilling -- and the use of disposal wells to store wastewater from the drilling. There’s been a gas drilling boom in the Barnett Shale, a massive geological formation that covers about 20 North Texas counties.
Irving also sits on the Balcones Fault, which extends from Del Rio in southwest Texas to the Dallas area.
The Railroad Commission of Texas, the state’s oil and gas regulator, hired a seismologist last year. Commission members have adopted tighter disposal well rules.
The commission said this week it is not investigating the Irving quakes.
“The Railroad Commission is not investigating seismic activity around Irving,” Ramona Nye, a commission spokeswoman, told StateImpact Texas. “Specifically, there are no disposal wells in Dallas County, and there is only one natural gas well in the vicinity, and it is an inactive well.”
North Texas has seen earlier earthquakes with magnitudes of at least 3.0. In 2013, two 3.6-magnitude quakes rattled Azle and two 3.7-quakes hit Mineral Wells, Southern Methodist University earthquake researchers say.
The USGS says the largest earthquake recorded in Texas was near the town of Valentine in southwest Texas in 1931 -- it was 5.8 in magnitude.
In Irving, earthquake drills
Back in Irving, school district spokeswoman Lesley Weaver at first thought someone bumped into a wall in the office Tuesday afternoon.
“We could feel something in our administration building,” Weaver said. “I would describe it as a slight shaking and we all said ‘What is that?’ Someone who works here in our office and lives in Irving as well said: ‘That’s an earthquake.’”
On Wednesday, Irving students participated in earthquake drills.
"We have earthquake procedures in place and are continuing to educate our students and staff on the proper actions to take during and after earthquakes," Weaver said.
Officials inspect roads, bridges and trains
Crews inspected roads, bridges and trains -- no damage was reported.
Morgan Lyons with DART says the light-rail Orange Line through Irving was unfazed by the shaking.
“Our system was built to withstand certainly seismic activity of this magnitude,” Lyons says. “We do daily inspections. The folks who do the inspections are certainly aware of what’s going on and are paying close attention for anything that might be unusual as a result of the recent seismic activity.”
On the roads, Tony Hartzel with the Texas Department of Transportation says crews doing their regular daily inspection routes haven’t found any quake damage. He’s confident in the strength and stability of Texas bridges even though state construction standards don’t even mention earthquakes.
“There’s not a specific standard, but the bridges are definitely built to withstand stresses, such as these earthquakes,” Hartzel says.
A look at one of Tuesday's quakes
SMU studying Irving quakes
Seismologists from SMU installed seismic monitoring equipment in northeast Irving Monday after the recent rash of minor earthquakes.
Irving City Manager Chris Hillman told KERA last week it was time to call in the experts.
“The frequency seems to have increased over the past month, month-and-a-half," Hillman says. "So therefore we’re asking the experts from SMU to come in and help assist. They’re going to be bringing in some additional equipment to help monitor and get more information if another earthquake were to occur.”
Video: Watch the SMU team install the equipment
More on SMU's efforts
SMU seismologist Brian Stump wrote on the SMU website:
The seismology team also can retrieve data from a University-controlled portable seismometer previously installed at a site south of DFW airport, as well as the seismometer permanently installed in the basement of Heroy Hall on the SMU campus.
SMU experts have also monitored activity in the Azle area, where there was an earlier swarm of quakes. The quakes in and near Azle are considered relatively small, but they’ve been large enough to cause damage and raise alarm. Both the mayors of Reno and Azle believe there is a link to the disposal wells from oil and gas drilling in the area.
Video: Watch the Jan. 6 press conference featuring SMU experts
Here's what USGS had to say on Twitter:
— USGS (@USGS) January 6, 2015
USGS posted this on its website about earthquakes in central and eastern North America:
As is the case elsewhere in the world, there is evidence that some central and eastern North America earthquakes have been triggered or caused by human activities that have altered the stress conditions in earth's crust sufficiently to induce faulting. Activities that have induced felt earthquakes in some geologic environments have included impoundment of water behind dams, injection of fluid into the earth's crust, extraction of fluid or gas, and removal of rock in mining or quarrying operations. In much of eastern and central North America, the number of earthquakes suspected of having been induced is much smaller than the number of natural earthquakes, but in some regions, such as the south-central states of the U.S., a significant majority of recent earthquakes are thought by many seismologists to have been human-induced.
Learn more: KERA earthquake coverage
KERA has been exploring the North Texas earthquakes in recent months. Here's a look at some of our recent coverage.
- The Digger: Reno Mom Helps Neighbors Find Answers About Texas Earthquakes
- The Seismologist: SMU's Heather DeShon Studies The Shaking In Azle And Reno
- A Rocket Scientist And His Wife Blame Disposal Wells For North Texas Quakes
- Company Man: Oil and Gas Energy Rep Says Industry Understands Quake Concerns
- This Azle Couple Worries About Another Round Of Earthquakes
- Earthquake Swarm In North Texas 'Needs To Be Solved Right Now,' Azle Mayor Says
KERA's Bill Zeeble, StateImpact Texas and The Associated Press contributed to this report.
Video: Watch aerial footage of the old Texas Stadium site