The reverse projection method allowed for the first time to study seismicity in the lower mantle

On May 30, 2015, a magnitude 7.9 earthquake occurred near the Japanese Ogasawara Islands, located

about 1000 km south of Tokyo.The seismic activity occurred 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 - the extremely high pressure and temperature at such depths should lead to deformation of rocks, and not to their destruction, as in smaller earthquakes.

Using 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.

Scientists have found evidence of an earthquake that occurred 2,800 years ago

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

The generally accepted model of deep focusearthquakes are a transformational fault: instability causes the transition of olivine in the subducting plate into 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.

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