By Seth Borenstein

AP Science Writer

WASHINGTON (AP) - A new study explains how just four wells forcing massive amounts of drilling wastewater into the ground are probably shaking up Oklahoma.

Those wells seem to have triggered more than 100 small-to-medium earthquakes in the past five years, according to a study published Thursday by the journal Science. Many of the quakes were much farther away from the wells than expected.

Combined, those wells daily pour more than 5 million gallons of water a mile or two underground into rock formations, the study found. That buildup of fluid creates more pressure that “has to go somewhere,” said study lead author Cornell University seismologist Katie Keranen.

Researchers originally figured the water diffused through underground rocks slowly. But instead, it is moving faster and farther and triggers quake fault lines that already were likely ready to move, she said.

“You really don’t need to raise the pressure a great deal,” she added.