MORGANTOWN, W.Va. -- Human activity associated with oil and gas production can sometimes cause earthquakes, but the problem is not hydraulic fracturing, a seismologist from the University of Texas told researchers gathered for a two-day conference on Marcellus Shale gas drilling.When the rare quakes do occur, they're typically linked to the disposal of drilling fluids in underground injection wells, Cliff Frohlich said Monday at West Virginia University. And the vast majority of injection wells don't cause quakes either, he said.Frohlich cited six earthquakes since 2008 in Arkansas, Colorado, Ohio, Oklahoma and Texas, ranging from magnitude 3.3 to magnitude 5.7. Their locations show that human-caused earthquakes are geographically widespread and geologically diverse, but "very rare," given the amount of petroleum produced and the amount of waste being disposed of.Why some injection wells cause earthquakes and others don't remains unclear, he said. Frohlich hypothesizes that quakes occur when a "suitably oriented" fault lies near an injection site."Hydraulic fracturing almost never causes true earthquakes," he told the group gathered for the National Research Council workshop. "It is the disposal of fluids that is a concern."Texas has 10,000 injection wells, Frohlich said, and some have been in use since the 1930s. That effectively makes the state a giant research lab for the shale-gas drilling issues now facing Appalachian states including New York, Ohio, Pennsylvania and West Virginia.If injection wells were "hugely dangerous," he said, "we would know.""Texas would be famous as a state that just rocks with major earthquakes," Frohlich said. "That is not true."WVU is hosting the conference (live webcast here) (agenda here) through Tuesday for the National Research Council, which is the operating arm of the National Academy of Sciences.WVU Vice President for Research Fred King said reports the workshop will generate should be available before the start of West Virginia's legislative session in January and could help guide future regulatory discussions.Frohlich urged policymakers to consider cultural and population differences if they are weighing regulation aimed at minimizing the risk of earthquakes through either the spacing between or monitoring of injection wells."There's places in West Texas you could have a 5.2 earthquake and it wouldn't bother anyone," he said. "If you're going to operate in urban areas, I think you need to invest in incredibly stringent regulations. But in other areas, you probably don't."WVU chief of staff Jay Cole said the university has a special obligation to help industry and government identify critical issues as shale-gas development grows and to identify questions that remain to be answered.The workshop features representatives of industry and government, including the National Energy Technology Laboratory, U.S. Geological Survey and U.S. Environmental Protection Agency, as well as researchers from 12 universities.Ray Boswell, technology manager for natural gas technology programs at Morgantown's national lab, said drillers tapping the Marcellus are producing more gas even as they sink fewer wells and are outpacing production estimates made by the U.S. Energy Information Administration.The region's reserves, he said, can easily sustain strong production through 2040.Joseph Frantz, vice president of engineering for Texas-based oil and gas producer Range Resources, said technology is allowing drillers to create more-efficient operations on smaller physical footprints.Deep horizontal wells today disturb only 1 percent of the surface on a 1,000-acre site, he said, compared with 19 percent disruption with conventional vertical wells set 1,000 feet apart.Frantz said drillers in the Appalachian basin are producing nearly 12 billion cubic feet of oil and gas per day from the Marcellus, a figure that has skyrocketed since 2009 as drillers rapidly embrace and deploy technology developed in the nation's other shale-gas fields.Range and other companies are casing their wells with as many as four layers of steel and cement, redundancies that Frantz said dramatically reduce any risk of groundwater contamination. Range is also using rubber containment pads and berms under every piece of equipment to stop pollution from soaking into the ground or migrating offsite, he said.It's an expensive way to do business, Frantz said, but "this is the right thing to do.""We talk about this social license to operate," he said. "We always have to be transparent and honest and open with everybody. ... If we don't do our job, someone's going to come in and tell us how it should be done, and that may not be a pleasant day."