Unearthed earthquakes

THROUGH MOST OF THE 20TH CENTURY, North America’s Cascadia region was thought incapable of generating earthquakes larger than magnitude 7.5. Any tsunami striking the region’s coasts would come from afar, leaving hours for warning and evacuation. Yet by century’s end, Cascadia had its own recognized source of earthquakes of magnitude 8 to 9 and of tsunamis that would reach its shores in a few tens of minutes. That recognition began in the early 1980s. Earth scientists were then beginning to debate Cascadia’s potential for great earthquakes—shocks of magnitude 8 or higher. Despite hints from oral histories of native peoples,