The first ocean floor evidence of great Cascadia earthquakes



[1] It can be argued that Japan's giant 11 March 2011 Tohoku earthquake and its tsunami should not have come as geophysical surprises. GPS had shown that the entire subduction zone along the Japan Trench behaves as one enormous unit rather than segmented sections that rupture with different frequencies and strengths [Suwa et al., 2006]. Historical documents and coastal sand deposits had shown a history of Sendai area tsunamis larger than those of recent centuries [Sawai et al., 2008]. Such hindsight brings to mind the Cascadia subduction zone, with its history of great earthquakes that, despite being literally in the backyards of many Pacific Northwest geologists and geophysicists, went unrecognized until late in the twentieth century. In 1969, at the dawn of the development of plate tectonic theory, scientists lacked a record of significant historical earthquakes along the U.S. northwestern coast and had no reason to believe that there were any seismic hazards in the region. What was known was that deep-sea cores taken offshore in 1965–1967 contained extensive turbidity current deposits— graded beds of sand and silt— interbedded with pelagic clays, sequences that by 1969 I had just finished studying for my doctoral dissertation. At the time, those turbidites (the sediments deposited by turbidity currents or submarine mudflows) posed a great enigma. What process or disturbance could have generated these recurring turbidity currents?