Image acquired by Sea Marc 1A side-scan sonar, with sound coming from bottom of image. (Elvis’s head and upper body, holding a guitar, are at top of image.) Figure 4-2. For three decades after the first discovery that Cascadia is a subduction zone, the fault itself could only be viewed by seismic-reflection profiles (Figure 4-2) and by relatively crude depth soundings.
They were cut during the last few hundred thousand years, in the Pleistocene, by slurries of water and sand brought to the sea by the Columbia River, swollen with floodwater from melting ice sheets in British Columbia and the Rocky Mountains, and by other major rivers draining melting glaciers in Puget Sound and flowing west through the present Strait of Juan de Fuca and down a broad valley south of the Olympic Mountains of southwest Washington.
At the top of the slope is the continental shelf, a flat surface carved during lower sea-levels of the Ice Ages. Elvis image at the Wecoma Fault (Figure 4-1) is between letters PR and PU.
Wecoma Fault continues up continental slope (WF) toward slope break.
Unfortunately, that name doesn’t stick, and it is formally named the Wecoma Fault for the ship that found it. Elvis image, marking point of discovery of the Wecoma Fault, which offsets a deep-sea channel about one hundred meters (more than three hundred feet).
Slip rate on this left-lateral strike-slip fault is about one inch every four years.