Magnitude 2.1 earthquake 69 km NE of Chignik, Alaska

Modern magnitudes are moment magnitudes (Mw), computed from the seismic moment: the area of the fault that slipped, how far it slipped and the rigidity of the rock. Each whole step up the scale is about 32 times more energy, so a magnitude 7 releases roughly a thousand times the energy of a magnitude 5.

Magnitude is one number for the whole earthquake, describing the energy released at the source. Intensity describes the shaking at a particular place, on the Modified Mercalli scale from I (not felt) to X+ (extreme), and falls off with distance from the epicenter.

An aftershock is a smaller earthquake in the same area after a larger one, as the crust around the ruptured fault adjusts to the new stresses. They are most frequent in the first hours and days and fade over weeks to months, though a large mainshock can produce aftershocks for years.

The first solution uses the nearest stations only. As recordings arrive from more seismographs around the world, USGS revises the magnitude, depth and epicenter, sometimes several times in the first hours. This page reflects the latest revision.

From the U.S. Geological Survey’s real-time earthquake feeds, which combine the USGS National Earthquake Information Center with regional seismic networks worldwide. EarthBrowser ingests the feed every minute.