Before 2001 the area averaged one magnitude-3 earthquake every two years, but has since seen the rate jump to eight per year.
A research team from the U.S. Geological Survey is trying to find out what's causing the increase.
Justin Rubinstein, a geophysicist with USGS says the rumbles are likely triggered by wastewater injection wells from drilling projects.
"What we think is happening at least for some of these earthquakes is these fluids more or less are acting as a lubricant on faults in the area, making earthquakes more likely," Rubinstein said.
The full results of the USGS investigation will be published in six to nine months, Rubinstein said.
