Environmental groups are threatening to sue the US environmental regulator, alleging it is failing in its duty to tackle a surge in earthquakes that they blame on the American shale revolution.The groups on Wednesday said they were preparing a lawsuit against the Environmental Protection Agency for not curbing the disposal of wastewater by oil companies, a practice that scientists say has triggered a spike in seismic activity. Earthquakes linked to oil and gas production have unnerved residents in Oklahoma, Texas and elsewhere, and have become an unexpectedly pressing problem for the US fracking boom that has upended energy markets. The groups threatening to sue the EPA, their usual ally, say it has a legal obligation to update rules on the disposal of wastewater from oil production, which have not changed since 1988.