The number of tremors recorded during an earthquake swarm in Irving climbed to eight Tuesday, according to the United States Geological Survey; three additional quakes were reported Wednesday morning and a fourth that afternoon.
The USGS said the two strongest of Tuesday's quakes, a 3.5 and a 3.6, each ranked as an MMI V on the Modified Mercalli Intensity Scale, which indicates they had the strength to be felt by nearly everyone, wake those sleeping, break windows and dishes and to overturn unstable objects.
No injuries or significant damage have been reported in connection with the earthquakes and police have requested people stop calling 911 to report only shaking. Atmos Energy said Wednesday the earthquakes have not caused any gas leaks in the area.
The recent earthquake swarm, where a number of quakes are concentrated in a single area over a few days, is centered in an area of Irving where more than 20 quakes have been recorded since October 2014, but where only one earthquake was ever recorded before 2008, according to a scientist at Dallas' Southern Methodist University.
Scientists at SMU said these are some of the largest quakes to hit North Texas in recent history. Before this most recent swarm, the largest magnitude quake recorded in the Irving area since October 2014 was a 3.3-magnitude quake on Nov. 22. The most recent temblor before this swarm was a 2.4 MMI III recorded at 8:29 p.m. on New Year's Day.
Since 2008, more than 100 quakes have hit North Texas; 45 were in the Irving area alone and more than 20 happened in just the last three months.
Geologists and seismologists at SMU are trying to figure out what is causing the increase in earthquakes, whether they are induced or if they are simply the result of natural, believed to be dormant, fault lines.
SMU researchers are taking a hard look at disposed waste water as a possible cause, but experts add that earthquakes of this size can happen virtually anywhere in the world at any time.
“There’s been some indication that disposable fluids can trigger small earthquakes,” said Dr. Brian Stump, a professor of earth sciences at SMU. “That's still an open question in this particular case.”
Scientists are also trying to determine if the earthquakes indicate whether something more catastrophic may be possible. The largest earthquake ever recorded in Texas occurred in the Big Bend area, near Valentine, where a 5.8-magnitude quake was recorded in 1931.
7:37 a.m. -- The first earthquake, measuring as a 2.3-magnitude, was recorded at 7:37 a.m. Wednesday near the University of Dallas Irving location on East Northgate Drive.
3:10 p.m. -- At 3:10 p.m Tuesday a magnitude-3.5 earthquake was recorded near Texas 183 and Texas 114, east of the former site of Texas Stadium in Irving.
“All of a sudden you hear a 'boom.' That's why we thought somebody hit the building. And then everything started shaking,” said Melissa Lockard, who works in Irving.
6:52 p.m. -- The strongest of the day, was recorded at 6:52 p.m. and was measured as a magnitude-3.6. Its epicenter was in the Trinity River basin near the Bachman Branch in East Irving.
“We said, ‘Oh, my God, this is another one,” said Dania Medina, who works at the Days Inn off Texas 183 in Irving. “And we didn't believe it.”
8:11, 8:12 p.m. -- The fourth and fifth quakes, one of them magnitude-2.9 and the other magnitude-2.7, occurred within seconds of each other, again in Irving near the former site of Texas Stadium. One was located along Loop 12 at Texas 114 while the other was just south of Texas 183 at the Trinity River.
9:54 p.m. -- A sixth earthquake was recorded as a magnitude-1.7 at 9:54 p.m. near the intersection of Harry Hines Boulevard and West Northwest Highway in Dallas.
10:05 p.m. -- The seventh earthquake followed minutes later at 10:05 p.m. and was measured as a magnitude-2.4 on the University of Dallas campus near Texas 114 and Tom Braniff Drive in Irving.
11:02 p.m. -- The USGS recorded an eighth earthquake, measured as a 1.6-magnitude, at 11:02 p.m. near the Trinity River south of Storey Lane.
NBC 5's Johnny Archer, Kevin Young and Alex Russell contribtued to this report.