砂川 地震

Sixteen kilometres west‑south‑west of Shikotan island lies a remote stretch of the Pacific Ocean off the southeastern coast of the Kuril chain, in the northern part of the Sea of Okhotsk. This shallow offshore sector sits on the subduction zone where the Pacific Plate is being thrust beneath the Okhotsk (or North American) Plate, a process that generates frequent megathrust earthquakes and associated tsunamis along the Kuril‑Kamchatka arc. The area is characterized by relatively young, basaltic oceanic crust that is being subducted at a convergence rate of roughly 70 mm yr⁻¹, producing a thickening of the continental margin and a complex network of thrust and normal faults that extend both beneath the seabed and into the island’s volcanic flank. The seafloor here is largely sedimentary, built from volcanic ash and marine deposits that accumulate from Shikotan’s volcanic activity, and it is periodically reshaped by submarine landslides triggered by seismic shaking. Because of the high seismicity of the zone—most notably the 2006 Kuril Islands 7.1‑magnitude event that generated a 1‑m tsunami—and the lack of permanent human settlements in this offshore sector, the primary concerns are geophysical hazards for nearby coastal communities and potential impacts on marine ecosystems.