Scientists in California have found that earthquakes can occur much deeper below the Earth’s surface than originally believed, a discovery that alters their understanding of seismic behavior and potential risks.

Seismologists have long believed that earthquakes occur less than 12 to 15 miles underground in the planet’s brittle, rocky crust. But new research has found evidence of quakes deeper than 15 miles under the surface, in the upper mantle, an area where the rock is so hot that it is no longer brittle but creeps, moving around like an extremely hard honey.

Three scientists at Caltech in Pasadena studied data collected over six months from 5,000 state-of-the-art sensors installed in Long …