An earthquake swarm continues to rattle Irving Wednesday as a fourth tremor has been confirmed by the United States Geological Survey, increasing the two-day total to 12 temblors.
Wednesday's quakes weren't as strong as the 3.5- and 3.6-magnitude earthquakes widely felt Tuesday afternoon, but are still significant enough to be felt by many in the area.
The latest quake, a 2.7-magnitude, was recorded at 9:57 a.m. with an epicenter between Texas Spur 482, Texas 114 and Loop 12 in Irving.
The recent swarm of earthquakes are centered in an area 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.
Due to the increase in seismic activity, the Irving Independent School District said Wednesday that they have prepared a video showing students how to "drop, cover, hold on" during an earthquake and that in the future they'll conduct earthquake drills the same way they do for fire, safety and tornadoes.
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.
12:59 a.m. -- Wednesday's first earthquake measured as a 3.1-magnitude quake whose epicenter was observed to be between Loop 12, Texas 114 and Texas 183.
1:24 a.m. -- The USGS confirmed a 2.3-magnitude earthquake adjacent to Loop 12 and Texas 114 near the Trinity River.
8:34 a.m. -- The USGS confirmed a 2.6-magnitude earthquake at 8:34 a.m. between Texas Spur 482, Texas 114 and Texas 183.
9:57 a.m. -- The USGS confirmed a 2.7-magnitude earthquake at 9:57 a.m. between Texas Spur 482, Texas 114 and Loop 12.
Eight Earthquakes Rattle North Texas Tuesday
The USGS recorded eight earthquakes ranging from 1.6- to 3.6-magnitude between 7:37 a.m. and 11:02 p.m. Tuesday.
The second quake of the day, a magnitude-3.5 earthquake, was recorded at 3:10 p.m. near Texas 183 and Texas 114, east of the former site of Texas Stadium in Irving and was widely felt by many across North Texas.
“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.
Another widely felt quake, a 3.6-magnitude earthquake, was recorded at 6:52 p.m. and 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.”
The USGS listed the strongest of the quakes as 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.
Before Tuesday, the largest magnitude quake to occur in the Irving area since October 2014 had been a 3.3-magnitude quake on Nov. 22. The most recent was a 2.4 MMI III at 8:29 p.m. New Year's Day.
Scientists at Southern Methodist University said these are some of the largest quakes to hit North Texas in recent history. Since 2008, more than 100 quakes have hit the area, with 45 in the Irving area alone and more than 20 in the last three months.
SMU researchers are taking a hard look at disposed waste water as a possible cause.
“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.
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.
NBC 5's Frank Heinz and Jeff Smith contributed to this report.