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."
The signature of steam
The formation of this earthquake glass happened deep within the Earth's crust, at depths estimated between 10 and 20 kilometers. The ambient temperature was already 250 to 350°C, and water existed not as a liquid but locked inside the crystal structures of minerals like biotite and hornblende.
The sudden temperature spike from frictional heating had a dramatic effect on this trapped water. It flash-boiled, turning to highly pressurized steam in an instant. This process, known as thermal pressurization, leaves distinct chemical fingerprints in the rock. Analysis of the pseudotachylyte shows that its chemical makeup is enriched with components from water-bearing minerals, which melted preferentially during the event. In some examples of pseudotachylyte worldwide, scientists find microscopic vesicles—tiny bubbles frozen into the glass—which are the preserved signs of water vapor escaping the melt. This evidence shows how fluids deep within a fault zone behave during a rupture.