Deep-focus earthquakes are earthquakes that occur at depths of several hundred kilometers in the mantle. Such earthquakes can cause serious disasters, such as the 1994 Bolivian earthquake, which occurred at a depth of 638 kilometers and had a magnitude of 8.3.
The cause of deep-focus earthquakes, on the other hand, has remained a mystery because earthquakes are caused by the rapid sliding of a fault, which is difficult under the great pressures of the deep mantle. Laboratory deformation experiments have been used to try to understand the mechanism of deep-focus earthquake occurrence, but experiments under deep mantle conditions have not been conducted due to technological limitations.
For the first time, a team used our state-of-the-art, large-volume deformation apparatus in conjunction with synchrotron X-ray observations to perform deformation experiments on natural olivine, the major mineral of the mantle and subducting oceanic lithosphere (slab). They observed major faulting in the sample using X-ray imaging under deep mantle conditions, as well as associated “earthquakes” using ultrasonic acoustic emission measurements.
After careful examination of the recovered sample, they discovered that the faulting was caused by the formation of “new” olivine with ultra-fine grains of tens of nanometers as a result of the phase transformation of “old” olivine, which served as a lubricant for the fault’s rapid sliding. They also discovered evidence that the sample had been locally melted along the fault as a result of the extremely high temperature caused by the rapid sliding. Based on these laboratory experiments, their model well explains the distribution of deep-focus earthquakes, which increase with depths ranging from 400 km to 600 km, where metastable “old” olivine is expected to form ultra-fine-grained “new” olivine.
The findings were published in Nature Communications.
