CASCADIA AS AN END-MEMBER PRODUCER OF GREAT SUBDUCTION EARTHQUAKES

In the global spectrum of great earthquake producers, the Cascadia subduction zone is an end member in several respects and appears to break many “rules”. The recurrence of great earthquakes of Mw ~ 9, against the backdrop of very low background seismicity, is a spectacular example of characteristic earthquake sequence that does not fit the Gutenberg–Richter distribution. In terms of the thermal regime, Cascadia has about the warmest megathrust not only because of the young age (~4 – 15 Ma) of the subducting plate and moderate convergence rate (3–4 cm/yr), but also because of the very thick insulating sediment cover (2–3 km) on the low-relief incoming plate. For a warm megathrust, the rheologically controlled seismogenic zone must be quite shallow and therefore have a narrow width in the dip direction. To release sufficient moment to produce an Mw 9 earthquake, the rupture has to be very long as seen in the interpretation of coastal and offshore paleoseismic observations, resulting in an odd “long-narrow” aspect ratio. The geodetically seen locked zone of the megathrust is indeed very narrow if we assume the locking is about 100% on the basis of extreme seismic quiescence of the interface at present. Given these peculiarities, the only explanation for Cascadia’s propensity for Mw 9 events is that the megathrust must have an extraordinary ability to allow seismic rupture to propagate along strike. Two minimum conditions are needed for long-range rupture propagation: The entire fault must be sufficiently close to the failure stress prior to rupture initiation, and the fault zone must have a relatively simple structure. Both conditions can result from a very smooth subducting seafloor. Globally, all the giant earthquakes that we know of have occurred at subduction zones with smooth subducting seafloor, with Cascadia as an end-member example. The megathrusts at these subduction zones are weaker than those that have rugged incoming seafloor. The weakness is reflected in the very little frictional heat they produce, for which Cascadia is also an end member. In summary, Cascadia has an end-member smooth and weak megathrust that produces characteristic great earthquakes. Low-level structural and stress heterogeneities may result in rupture patchiness but may not fully stop rupture propagation.