Paleomagnetic directions of estuarine mud provide additional evidence that individual earthquakes, or rapid series of earthquakes, caused widespread coseismic land-level changes during the past 2000 years in western Washington and Oregon. Most of the paleomagnetic measurements were made on mud dating from the first decades after coseismic subsidence from plate-boundary earthquakes at the Cascadia subduction zone. Mud deposited soon after the A.D. 1700 Cascadia earthquake has similar remanent directions among all five sites (k = 171) sampled along 80 km of Pacific coast between Grays Harbor and the mouth of the Columbia River. Likewise, internally consistent directions were obtained along this stretch of coast from mud deposited soon after a plate-boundary earthquake (or earthquake series) in A.D. 340–410 and soon after another such event in A.D. 680–720. Also analyzed were remanent magnetizations of mud deposited shortly before (or shortly after) land-level changes from seismicity in the North America plate beneath Puget Sound. A mean direction for sites on the Snohomish River delta, near Everett, from the time of an earthquake on the Seattle fault in A.D. 900–930 is statistically identical (95% confidence level) to a mean direction in mud that was uplifted in A.D. 800–1000 at potentially correlative sites near Tacoma and Olympia. The paleomagnetic direction from Everett for the upper-plate earthquake of A.D. 900–930 differs substantially from that for a plate-boundary earthquake in A.D. 810–1190. This difference implies that the upper-plate earthquake preceded the plate-boundary earthquake by a century or two on the basis of comparisons of their paleomagnetic poles with a previously reconstructed path of geomagnetic paleosecular variation in western North America.
