A tornado watch was issued Monday for parts of Oklahoma, just one day after more than 20 earthquakes rocked the state.

The National Weather Service issued the watch and it is expected to remain in effect through the evening.

The bulk of the counties affected are in southwest Oklahoma, according to the National Weather Service. Parts of Texas are also included in the watch.

The state is at risk of tornados because of thunderstorms that have moved into the area. Large hail is also a possibility, according to reports.

Strong thunderstorms are expected statewide, dropping as much as six inches of rain in some areas, the National Weather Service said. Residents were warned of potential flooding accompanying the heavy rains.

The latest warnings come on the heels of a tremor-packed weekend for the state.

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A 5.6-magnitude earthquake rocked the region late Saturday night. It was the largest reported earthquake in the state’s history.

More than 20 earthquakes struck the state in the hours before and after the major one. Some of the smaller earthquakes still registered a 4.0-magnitude or higher.

The weekend’s seismic activity is part of a trend of increased activity in the state.

More than 1,000 earthquakes were recorded in 2010 after seismographs were installed in the area, according to officials. Only around 100 of those earthquakes, however, were strong enough to be felt by people in the area.

It is unclear why there is a sudden spike in the number of reported earthquakes in Oklahoma. Before 2010, the state typically had around 50 earthquakes a year, officials said.

Oklahoma has had 10 times more earthquakes than normal since mid-2009, Austin Holland, a research seismologist with the Oklahoma Geological Survey, told the New York Times.

“It could be a natural cycle; we just don’t know,” said Holland.

Only minor injuries and structural damages were reported from the earthquake, officials said.