The 2016 Kumamoto earthquakes shocked the inhabitants of the western island of Kyushu. It caused hundreds of casualties and serious damage to vital infrastructure. The epicenter of the quake was traced to the Futagawa fault in a region neighboring Mount Aso which is an active volcano in Kumamoto Prefecture that last erupted in October 2021.
An investigation of earth displaced by the series of quakes has offered potentially new clues into seismic activity in regions close to volcanoes. The study has also attributed the root cause of the 7.3-magnitude mainshock to specific geological damage.
The Futagawa strike-slip fault is a vertical break in the ground tracing a line southwest originating from Mount Aso. There the two sides of the fault point straight down and brush against each other side to side during an earthquake.
Scientists had anticipated that any rupturing would occur exclusively near the strike-slip fault system. Scientists drilled a series of boreholes which includes one that measured more than 600 meters deep.
By extracting and analyzing the types of rocks present in these cores. Scientists were able to reconstruct the different layers of earth around the fault.
They noticed that the same layer of rock sediment was appearing at different depths and separated by more than 200 meters vertically. This large gap could only be explained by the current strike-slip motion. There the two sides of the fault move up and down with respect to each other.
Researchers attributed this dramatic change in the fault slip mode to eruption activity of Aso occurring around this time. Such observations from the Aso volcanic region could apply more broadly to similar volcanic areas near other subduction zones.