ALPINE, N.J. — Last month, passengers on the Metro-North Railroad trains that run along the Hudson River looked up from their tepid coffee and their iPads to find the familiar view transformed. Just south of Hastings-on-Hudson, it looked as if a giant hand had carved a gash in the umber face of the Palisades, the wall of jagged cliffs that towers over the west bank of the river, depositing a huge mound of boulders below.

An enormous slab of the Palisades had crashed to the ground at 7:28 p.m. on May 12, on a warm, otherwise uneventful Saturday evening. A rumble was reportedly heard for miles around, and a large plume of dust was visible from the opposite shore.

The seismometer on the campus of the Columbia University Lamont-Doherty Earth Observatory, which happens to be located a mile north of the rockfall, picked up the motion, registering it with a wild squiggle that lasted for 35 seconds.

“It vibrated the ground by two micrometers, or two-millionths of a meter, which is not very much,” said Bill Menke, a seismologist and professor of earth and environmental sciences at Columbia. “But it’s easily detectable, and we knew something had happened.”