Most earthquakes follow a simple scaling law: The duration in seconds of slip on the fault is proportional to 10 M/2, where M is the earthquake’s moment magnitude, a dimensionless number logarithmically related to the energy release. A typical M = 5 earthquake lasts 2 seconds; an M = 7 one, 20 seconds. The short duration gives rise to the efficiently radiated high-frequency seismic waves most often used to detect and locate earthquakes around the world.
But earthquakes exist with durations—from tens to thousands of seconds, say—much longer than the scaling law would suggest. And because they radiate more of their energy as long-period waves, the events often go undetected. In 2003 Göran Ekström, a seismologist at Columbia University, was looking for such oddballs. He systematically mined three years of archival data from the global seismographic network, which monitors about 200 seismometers distributed worldwide. His algorithm filtered the...
