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Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact.

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Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since ca. A.D. 800. The 45 measured stratigraphic sections consist mostly of 2-4 m of olivegray, intertidal mud containing abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.5؊3-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2؊10 cm of gray clay. We correlated the couplet, which is generally ϳ2 m below the modern marsh surface, across an ϳ20 km 2 area. Sand dikes and sand-filled cracks to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50-75 cm. Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south. We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position