yamada and his team suspect that the theory didn't explain everything.hey dug a hole on the slopes a meter or two deep and found volcanic ash, the result of an explosion 9,000 years ago. further research revealed the ashy soil retains moisture in all kinds of weather. >> translator: when water retention is high, the soil loses its strength when suddenly shaking. >> reporter: yamada ran a condition recreating the conditions of the 2018 quake sending vibrations toughhr the wet soil. ordinary soil was not affected, butt the moist ash-ladenn soil liquefied in an instant. when soil of this sort was in a slope of 13 degrees it flowed out. that's why he thinks disaster struck in unexpected areas. >> translator: volcanic ash underlies many slopes in japan. we need to reconsider our guidelines and either reenforce the ground or avoid building houses in those places. >> reporter: yamada is workingn to locate the slopes most prone to collapse knowing disaster can strike at any moment. >>> defense chiefs from the u.s. and south korea have reaffirmed their commitment