MOUNT ST. HELENS NATIONAL MONUMENT, Wash. — A magnitude 3 earthquake rattled Mount St. Helens today, triggering rockfall and sending an ash plume above the crater rim, the U.S. Geological Survey reported.

The quake occurred at 5:22 a.m., the largest recorded at the volcano in several months. Its cause was not immediately known.

In the past two days scientists have placed new Global Positioning System monitors and a seismic station on the east and west sides of the glacier. Gas-emission readings earlier this week showed little change.

The U.S. Geological Survey and the University of Washington have monitored the volcano closely since it rumbled back to life Sept. 23, with shuddering seismic activity that peaked above magnitude 3 as hot magma broke through rocks in its path. Molten rock reached the surface Oct. 11, marking resumption of dome-building activity that had stopped in 1986. On March 8, it shot ash higher than 30,000 feet, and on July 2, rockfall knocked a large piece off the growing lava dome.

Scientists have said a more explosive eruption, possibly dropping ash within a 10-mile radius of the crater, is possible at any time.