A record of ancient rupture
Along Japan's Median Tectonic Line (MTL), the nation's longest fault system, geologists find veins of dark, glassy rock inside granite and gneiss. This material is pseudotachylyte, the frozen remnant of an ancient earthquake. The Median Tectonic Line itself formed roughly 70 million years ago, during the Late Cretaceous, as tectonic forces joined separate landmasses into the beginnings of the Japanese archipelago.
The glass veins document moments of intense violence. During a seismic slip, the friction between the fault walls generated incredible heat. In seconds, temperatures on the slip surface exceeded 1200°C. This was hot enough to melt the surrounding rock instantly. This molten rock, or magma, was then injected into nearby fractures. Because the surrounding rock was much cooler, the magma quenched almost immediately, solidifying into a glass before crystals could form. These thin veins, often just millimeters thick, are direct evidence of prehistoric earthquakes, earning them the name "fossil earthquakes."