Drowned tree stumps in intertidal zone. Sunset Bay State Park, Oregon. These tree stumps indicate that this beach was somewhat above sea level when the trees grew, but dropped to below sea level as the result of an earthquake. Carbon dating of the wood indicates the trees died in the year 1700. In fact, there are drowned tree trunks in the intertidal zone all up and down the Oregon coast, all of which died at about the same time. They are one of the main lines of evidence for the magnitude 8.7-9 earthquake on the Cascadia Subduction Zone in the year 1700.