THE NATURE OF CASCADIA SUBDUCTION ZONE EARTHQUAKE CYCLES DRAWN FROM COASTAL PALEOSEISMOLOGY

Coastal wetlands and lakes register paleoseismic evidence for great Cascadia earthquakes that implies repeated cycles of strain accumulation and release by slip during megathrust earthquakes, which gives rise to high tsunamis. Beneath intertidal wetlands, widespread buried soils have been likened to a barcode that reflects the vertical component of elastic strain and coastal lakes capture and preserve tsunami deposits.

However, unlike many other subduction zones, Cascadia’s seismic cycle is enigmatic due to a lack of historical seismicity. In lieu of historical observations, often too brief to characterize the seismic cycle, coastal paleoseismology has failed to rule out two competing, end-member hypotheses. In the first, great earthquakes rupture the entire 1000-km-long margin every 400-500 years on average. In the second alternative, a complete seismic cycle for Cascadia may entail, initially, "large" earthquakes that do not release all of the accumulated strain followed by "great" earthquakes, at the end of the cycle, responsible for major release of accumulated seismic energy. Great Cascadia earthquakes may be serial ruptures similar to major ruptures that terminated strain accumulation cycles in southern Chile in 1960 and 2010 and in the Mentawai segment in Sumatra in 1797 and 1833. Cascadia’s coastal paleoseismic record potentially overlooks large earthquakes that have too little slip to leave sedimentary evidence. Instead, ¹⁴C-based chronologies reflect mostly great earthquakes and usually cannot distinguish a cascading series of two or more ruptures within decades from a single giant rupture.

Some of Cascadia’s earthquake cycles may exhibit patterns similar to the second hypothesis, which cannot be rejected by coastal paleoseismology. One possible example of serial ruptures ending a seismic cycle comes from a 4300-yr record of Cascadia tsunamis archived in an Oregon coastal lake and evidence of coseismic subsidence at adjacent estuaries. One tsunami that inundated the lake around 1600 yr BP was closely followed, about two decades later, by another tsunami. We infer that rupture of the subduction zone north of Cape Blanco in southern Oregon generated the first tsunami and a subsequent rupture of the margin south of Cape Blanco spawned the second tsunami, ending an earthquake cycle ~1600 years ago.