The clear signal — the geodetic fingerprint — of a large subduction earthquake would be the abrupt lowering of land behind the beaches when the upper plate got stretched like taffy, snapped to the west, and then sank below the tide line. (discovermagazine.com)
In the violent, shuddering release of strain during an earthquake, the upper plate would snap to the west, toward its original shape. (discovermagazine.com)
On the other hand, if the two plates were stuck together by friction, strain would build up in the rocks and the upper plate would bend down along the outer edge and thicken inland, humping upward until the rocks along the fault failed. (discovermagazine.com)