Since its last eruptive activity in 1857, Mount St. Helens had been largely dormant. Seismographs were not installed until 1972. This period of 123 years of inactivity ended in early 1980. On March 15, a cluster of tiny earthquakes rocked the area around the mountain. For six days, more than 100 earthquakes clustered around Mount St. Helens, an indication that magma was moving. There was initially some doubt as to whether the earthquakes were precursors to an eruption. By March 20, a magnitude 4.2 earthquake shook the wilderness around the volcano. The next day, seismologists installed three seismic recorder stations. By March 24, volcanologists at the USGS—including Johnston—became more confident that the seismic activity was a sign of an impending eruption. After March 25, seismic activity drastically increased. By March 26, more than seven earthquakes over magnitude 4.0 had been recorded, and the next day, hazard warnings were publicly issued. On March 27, a phreatic eruption took place, ejecting a plume of ash nearly into the air.