Renewal of tidal forests in Washington state after a subduction earthquakein AD 1700

With few exceptions, today's tidal trees near Washington's Pacific coast po stdate an earthquake that lowered the region by 1 m or more. The earthquake , which occurred in A.D. 1700, is the most recent to have ruptured much of the plate boundary at this central part of the Cascadia subduction zone. Be cause of the coseismic subsidence, lowland forests became tidal flats where thousands of trees died. Most of the trees killed were Sitka spruce (Picea sitchensis). In the centuries since the earthquake, tidal deposits have bu ilt new land that has been colonized by new Sitka spruce. All but several t ens of the region's tidal spruce consequently postdate 1700, as shown by co unts of annual rings in 121 of the largest spruce in tidal forests at Copal is River, Grays Harbor, and Willapa Bay. Forests began to return to each of these estuaries in the early 1700s and spread seaward in the late 1700s an d 1800s. Annual rings in the oldest of the trees thus record a large fracti on of the earthquake-recurrence interval that began with the 1700 earthquak e. (C) 2001 University of Washington.