By Jon Herskovitz

(Reuters) - Fault lines dating back hundreds of millions of years in Oklahoma that have been recently reactivated could lead to a devastating quake in the state where many structures were not built to withstand major seismic activity, a report said.

The state, which has seen several hundred seismic events over the past five years, has "a high degree of potential earthquake hazards," according to the study accepted for publication this month whose authors include researchers from the U.S. Geological Survey (USGS).

"The majority of the recent earthquakes in central Oklahoma define reactivated ancient faults at shallow depths in the crust" of less than 3.7 miles (6 km), said the report for the American Geophysical Union.

The report did not look at whether the reactivation of the faults was linked to the energy extraction technique known as hydraulic fracturing, or fracking.

Daniel McNamara, one of the paper's authors and a research geophysicist at USGS, said on Tuesday the 300 million-year-old subsurface faults that had not been active are suspected to be associated with the recent seismic activity.

"Any one of these fault zones that are producing magnitude 3 or 4 earthquakes could rupture into a larger earthquake. There are as many as 12 different fault zones that are capable of producing a large, 5 to 6 magnitude earthquake," he said.

In November 2011, Oklahoma suffered a 5.6 magnitude quake that damaged more than a dozen homes and several businesses.

Building codes in Oklahoma for seismic events are not as stringent as in quake-prone states such as California.

Wastewater disposal related to the fracking is suspected by many scientists to contribute to the earthquake activity. Millions of gallons of wastewater are typically trucked from a fracking site to wells where the water is injected thousands of feet underground into porous rock layers.

Energy companies deny there is a link between fracking and major seismic activity.

(Reporting by Jon Herskovitz in Austin, Texas; Editing by Eric Beech)