Next week, Nature reports, 24 holes up to 328 feet deep will be drilled all along the slopes of Mount St. Helens. When they’re finished, industrial strength explosives—the kind used for quarrying—will be dropped in to the bottom and the holes refilled. Then, over the course of four nights, the charges will be detonated, each shaking the ground briefly.
Mount St. Helens the United States’ most infamous volcano. And while it might seem like a strange idea to poke and prod at a volcano with explosives, this endeavor is part of a study by seismologists, who are trying to get a peek at Mount St. Helens' inner workings. The study is called iMUSH (or Imaging Magma Under Mount St. Helens). And the aim is to figure out a few burning questions, including: Where is all the magma under Mount St. Helens stored? How much magma is there? How are local earthquakes related to the volcano?
The explosions will create shaking roughly equivalent to a magnitude 2 earthquake, which is usually not strong enough to be felt by humans. You'd have to be standing within about 200 yards of the explosion to feel anything at all.
The explosions will act kind of like a sonar pulse, imaging the ground under the volcano. The resulting vibrations will be picked up by a network of seismometers scattered across the mountain. By collecting data on the timing and intensity of the shockwaves from the explosions as they reach the seismometers, the researchers will be able to get a better picture of what the inner structure of the volcano is like.
In addition to the thousands of smaller seismometers that are part of the active source seismic program (active, because the researchers are actively setting off the controlled explosions), the research group has installed around 70 larger seismometers all over the area, which will stay there for two years. These larger seismometers are part of the passive source seismic part of the project. Instead of setting off explosions, the iMUSH researchers will wait for natural earthquakes to occur and use their readings from the seismometers to get an even better idea of what’s going on inside the volcano.