By Marice Richter

DALLAS (Reuters) - Seismologists installed a new earthquake-monitoring device in the Dallas suburb of Irving this week after a series of minor temblors rocked an area near the site of the former Dallas Cowboys football stadium.

Irving was hit by a magnitude 3.5 quake, the U.S. Geological Survey said. It was the strongest in a series of nearly 20 minor quakes to hit around Irving since September.

There were no immediate reports of injury or damage.

But the seismic series has left residents on edge, wondering if the situation will get worse and what has been behind an uptick in quakes over the last several years.

"The safety and security of our residents is paramount for the city of Irving," said Irving City Manager Chris Hillman.

Some residents believe the implosion of Texas Stadium in 2010 may have exacerbated problems. There is also speculation the quakes might be related to hydraulic fracturing, or "fracking," an energy extraction technique that has a long history in north Texas.

Irving, headquarters of Exxon Mobil's XTO unit that helped pioneer hydraulic fracturing in the region, has two gas wells that were fracked in 2010, according to city officials.

The study by seismologists from Southern Methodist University in Dallas is aimed at pinpointing the quakes' epicenters, said SMU seismology professor Brian Stump.

About 40 miles west of Irving, a series of small earthquakes rattled the town of Azle a year ago, causing sinkholes and cracks in house foundations. Residents blamed fracking and injection disposal wells for drilling waste as the cause but an official finding is still pending.

Energy industry officials have said fracking is safe and causes no significant seismic damage

(Additional reporting by Lisa Maria Garza in Dallas; Writing by Jon Herskovitz; Editing by Doina Chiacu and Sandra Maler)