Cleveland, Ohio (Reuters) - Alarmed over a string of earthquakes linked to deep wells in nearby Youngstown, authorities in Mansfield, Ohio have threatened to block construction of two similar waste disposal wells planned within their city limits.

Ohio has over 170 active disposal wells, though only recently has it become permissable to use them for disposal of out-of-state waste from fracking, a controversial process to drive gas and oil out of underground rock.

Now, fresh questions about their safety are being raised in the wake of 11 earthquakes that struck Youngstown last year, all centered near wells used for disposal of fracking waste.

In Mansfield, city officials are reconsidering plans to allow two new 5,000 foot waste disposal wells to be built. Last spring, an Austin, Texas-based company, Preferred Fluids Management, obtained a drilling permit for the wells.

The city wants Preferred Fluids Management to pay for the testing of every tanker of fluid previously discarded in the Mansfield wells and a full geological survey of the area. Otherwise, officials said, the city will fight the drilling.

"The city of Mansfield will be the first as a whole to oppose the injection disposal wells," John Spon, the city of Mansfield's law director, told Reuters.

There is a new demand for fracking fluid disposal in Ohio, because Pennsylvania no longer allows fracking companies to treat and then dump the water used in the process. To deal with waste, disposal wells are drilled to specific geological depths, and millions of gallons of leftover fluid are injected or sandwiched into the rock.

One obstacle city officials face is a 2004 law exempting these types of wells from urban zoning rules, essentially giving Ohio Department of Natural Resources exclusive jurisdiction.

"We are not going to concede that the state of Ohio can dictate the operation of these wells," Spon said.

Steve Mobley, owner of Preferred Fluids Management, said he doubts the wells will be drilled anytime soon due to the business climate.

That climate stretches far beyond Mansfield. Statewide, the Youngstown quakes have provided a flashpoint for a number of groups who oppose fracking out of concern about both the safety of the wells and the contents of the fracking fluid.

Teresa Mills of the Center for Health, Environment and Justice - an organizer of an anti-fracking protest at the state capital in Columbus this week -- said there should be more studies on fracking's effects on the environment.

A bill before the Ohio House would establish a moratorium on horizontal stimulation of oil and gas wells until the U.S. Environmental Protection Agency does further testing on the relationship of horizontal drilling to drinking water quality.

Those who favor fracking say it offers potential for industry growth in areas that desperately need it. They claim the controversy is an over-reaction and that strict regulations would hurt the region's domestic energy production.

"Do you know how many injection wells there are in the country?" asked Mobley, of Preferred Fluids Management. "There are 150,000 and there has never been any suggestion of seismic activity associated with them."

But Won-Young Kim, a research professor of Seismology Geology and Tectonophysics at the Lamont-Doherty Earth Observatory at Columbia University, said that circumstantial evidence suggests a link between the most recent Youngstown earthquake and the high-pressure well activity.

Last year the city of Youngstown, about 100 miles north of Mansfield, experienced a series of small earthquakes ending with a 4.0 quake New Year's Eve felt by thousands of residents. As a result, the Ohio Department of Natural Resources announced it would suspend operations at five injection wells in the area.

The agency said there are no permanent decisions yet regarding the status of the Youngstown wells but that it is doing a comprehensive study of the issue.

Earthquakes induced by human activity have been documented in a few locations in the United States, Japan and Canada, according to the U.S. Geological Survey web site. The largest and best known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado, where in 1967 a 5.5 magnitude earthquake followed a series of smaller quakes.

In Mansfield, city officials plan to meet with state environmental regulators prior to a public meeting on injection wells January 19.

"The message to the industry is - Don't mess with the city of Mansfield," Spon said. "We are drawing a line in the sand and we will pursue whatever remedies we have."

(Reporting by Kim Palmer; Editing by Mary Wisniewski and Paul Thomasch)