NEW YORK (Reuters) - Oil and gas activities may have caused nearly nine in 10 of the earthquakes Texas has experienced in the past 40 years, and the quakes have become more frequent as oilfield activity has picked up in the past decade, according to a forthcoming study.

Of the 162 Texas earthquakes of magnitude 3 or greater between 1975 and 2015, a quarter were "almost certainly" induced by oil and gas activities, while 33 percent were "probably induced and 28 percent were "possibly induced," researchers led by University of Texas-Austin geoscientist Cliff Frohlich wrote.

A sharp uptick in oil-linked earthquakes has caused popular uproar and regulatory scrutiny in northern neighbor Oklahoma.

While the phenomenon is not nearly as widespread in Texas, the paper, set to be published in Seismological Research Letters on Wednesday, shows the United States' hottest shale plays are not immune to increased seismic risk. Reuters was provided with a copy of the report prior to publication.

The researchers also criticized the Texas Railroad Commission, the agency responsible for regulating petroleum production in the state, for being "slow to acknowledge that induced earthquakes occur in Texas."

Since shale oil and gas fields like the Haynesville and the Permian boomed in 2008 due to the widespread use of horizontal drilling and hydraulic fracturing technologies, the rate of earthquakes exceeding magnitude 3.0 has increased from 2 per year to 12 per year in Texas, the top U.S. oil state.

This so-called unconventional production generates two to three times more wastewater than in conventional oil fields, and has boosted the amount of water that must be injected deep underground into disposal wells, which have been linked to the recent wave of quakes in Texas and Oklahoma.

Oklahoma, where large amounts of water naturally come out of wells along with oil and gas, saw 890 earthquakes of magnitude 3.0 and above in 2015 alone, compared with two to three a year before 2009. There, regulators have asked some companies to reduce wastewater injection rates to try to tame seismic activity.

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The authors acknowledged that the majority of disposal wells and petroleum fields in Texas had not been associated with earthquakes.

"Nevertheless ... we cannot dismiss the correlations in time and space over a long operational history," the authors write.

A Texas Railroad Commission spokeswoman dismissed the study as "arbitrary" and "subjective" and said the agency had also taken steps to reduce injection volumes.

(Reporting by Luc Cohen; Editing by Terry Wade and James Dalgleish)