The main beach in Seaside is broad and sandy, with a wide band of low grassy dunes between the promenade and the beach - enough to keep storm waves out of town, but not enough to shelter the town from a significant tsunami. The plate boundary is not far offshore - twenty minutes would be barely enough time to get to high ground under the best of conditions.
Seaside lies just north of Tillamook Head, at the south end of the Clatsop Dunes. This segment of beach, from Seaside north to the Columbia River is a sub-cell of the larger Columbia River cell and most of the sand on these beaches came out of the Columbia River. The other three sub-cells all lie north of the river mouth, in Washington.
The flat sandy beach in these photos is a big contrast with the cobble beaches at the very south end which I'll describe in the next post. But as you walk south - to the vicinity of "U" Avenue - a cobble berm begins to assert itself.