Great 1700 Cascadia Earthquake Shakes Pacific Coast

A magnitude 9.0 megathrust earthquake ruptured the entire Cascadia Subduction Zone on January 26, 1700, generating a tsunami that devastated the Pacific Northwest coast and crossed the ocean to strike Japan ten hours later. The earthquake was one of the largest in North American history, comparable in magnitude to the 2011 Tohoku earthquake that caused the Fukushima disaster. For nearly three centuries, the event was unknown to Western science. The Pacific Northwest appeared seismically quiet. There were no written records from the indigenous peoples who experienced it, though oral histories of the Makah, Yurok, and other coastal nations preserved accounts of massive waves and land subsidence. The breakthrough came from Japanese historical records. Japanese officials in 1700 documented what they called an "orphan tsunami," waves that arrived without an accompanying local earthquake. By matching the timing and characteristics of the Japanese records with geological evidence along the Pacific Northwest coast, including drowned forests, sand deposits, and subsidence patterns, scientists were able to date the Cascadia earthquake precisely to the evening of January 26, 1700. The discovery, published in a landmark 1996 paper, fundamentally changed seismic hazard planning for the entire region. Before the evidence was assembled, the Pacific Northwest was considered a low-risk earthquake zone. After it, engineers and emergency planners had to confront the reality that the region faces the same catastrophic subduction zone earthquake risk as Japan, Chile, and Indonesia. The Cascadia Subduction Zone stretches 700 miles from Cape Mendocino, California, to Vancouver Island. Geologists estimate that major ruptures occur roughly every 300 to 500 years. The last one was 326 years ago.