Scientists have discovered a new type of earthquake that occurs at deeper-than-expected levels in Earth's upper mantle, according to a new finding published in the Science journal. "The discovery of unusually deep seismicity shed light on the physical mechanisms that promote earthquakes in the high temperature and pressure conditions at the top of Earth's mantle," Lingsen Meng, a seismologist at the University of California, Los Angeles, told Xinhua on Friday. According to the new study, Earth's surface deforms in part as a result of ruptures along brittle crustal faults that generate earthquakes. To detect rock deformation in the ductile lower crust and mantle can be very difficult. In order to better monitor deeper or smaller seismic activity, researchers installed one of the densest seismic arrays in the world with thousands of state-of-the-art sensors stretching nearly 80 km from Culver City to Newport Beach in Southern California. Seismologists had long believed that earthquakes occur less than 12 to 15 miles (19-24 km) underground in the planet's brittle, rocky crust.
However, after analyzing data collected over six months from about 8,000 sensors, researchers have 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 extremely hard honey. The quakes were so deep that they cannot be felt at the surface by conventional seismic sensors. The findings may help us understand how ductile deformation operates in this part of Earth.