Magma stores are recharging inside Mount St. Helens, setting off a swarm of small earthquakes since last month, according to the U.S. Geological Survey.

Since the catastrophic eruption of May 18, 1980, scientists have been conducting research and collecting data on the volcano to learn more about its typical behavior.

CBS affiliate KIRO reported over the last eight weeks, there have been more than 130 earthquakes located by the Pacific Northwest Seismic Network and many more that are too small to be located.

Starting on March 14, a number of small earthquakes have occurred beneath the volcano, at a depth between 1.2 to 4 miles. The earthquakes have low magnitudes of 0.5 or less with the largest a 1.3.

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The USGS says the earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes a week, but there are no signs an imminent eruption.

The quakes are too small to be felt, even if you were standing on the surface directly above.

"The earthquakes are volcano-tectonic in nature, indicative of a slip on a small fault. Such events are commonly seen in active hydrothermal and magmatic systems," the USGS said in a statement on its Facebook page.

"The current pattern of seismicity is similar to swarms seen at Mount St. Helens in 2013 and 2014; recharge swarms in the 1990s had much higher earthquake rates and energy release," according to the USGS.

No anomalous gases, increases in ground inflation or shallow seismic activity have been detected with the swarm, and there are no signs of an imminent eruption, the agency said.

As was seen at Mount St. Helens between 1987 and 2004, recharge can continue for many years beneath a volcano without an eruption.

Last year, the state of Oklahoma set a record for earthquakes with 907 registering a magnitude of 3 or more. It's causing anxiety, damage and residents to rethink one of the state's biggest industries -- oil and gas production -- which scientists say is causing nearly all the quakes.

Bill Whitaker goes to Oklahoma, now the capital of earthquake activity in the continental U.S., to report the story for the next edition of 60 Minutes, Sunday, May 8 at 7 p.m. ET/PT.