Researchers from Japan were able to study for the first time exactly how seismicity developed in the lower layers of the mantle. To do this, they used the rear projection method.

On May 30, 2015, an earthquake with a magnitude of 7.9 struck the Japanese islands of Ogasawara, located about 1000 km south of Tokyo. Seismic activity occurred at a depth of more than 660 km below the Earth’s surface, near the transition between the upper and lower mantle. The mechanism of deep-focus earthquakes, like the 2015 earthquake, has long remained mysterious – extremely high pressure and temperature at such depths should lead to deformation of rocks, and not to their destruction, as in smaller earthquakes.

Using a 4D rear projection method, the researchers traced the trajectory of the earthquake and for the first time identified seismic activity that began in the lower mantle. They relied on measurements from a network of high-sensitivity seismographs (Hi-net), a network of seismic stations located throughout Japan. The data obtained with these instruments is similar to the ripples in a pond from a falling pebble: by calculating how seismic waves propagate, the researchers were able to accurately determine the path of a deep-focus earthquake.

The team found that the main shock began at a depth of 660 km, then spread west-northwest for at least eight seconds, decreasing in depth. Analysis two hours after the main shock revealed aftershocks (aftershocks) at a depth of 624 to 751 km.

The generally accepted model for deep-focus earthquakes is a transformational fault: instability causes the transition of olivine in the subducting plate to a denser form – spinel. However, aftershocks below 700 km occurred outside the zone where this transition occurs. The authors suggest that deep seismicity may have resulted from stress changes caused by the subsidence of the subducting plate segment in response to the main shock, although this hypothesis requires further study.