Diego Melgar 

Coastal subsidence, dating of soil samples and tree rings, and sedimentological evidence of a tsunami point to coseismic activity on a sizable portion of the Cascadia subduction zone circa 1700. Documents from Japan reveal that on January 26th of that year there were tsunami impacts across distant locations in the country and past modeling shows that a large Cascadia earthquake is the most likely source. The prevailing hypothesis is that only a single large event rupturing the entire plate boundary can explain these observations. Here we model tens of thousands of ruptures and simulate their coastal subsidence and far-field tsunami signals and show that it is possible that the 1700 earthquake was instead part of a sequence of several earthquakes. Partial rupture of as little as ~40% of the along-strike extent of the megathrust in one large M>8.7 earthquake can explain the tsunami in Japan and a part of the coastal subsidence. As many as four more earthquakes with M<8.6 can complete the coseismic subsidence signal without their tsunamis being large enough to be recorded in Japan. Given the spatial gaps in the presently available geologic estimates of coastal subsidence data it is also possible that short segments of the megathrust have remained unbroken. The findings have significant implications for Cascadia geodynamics and how earthquake and tsunami hazards in the region are quantified.