The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile

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Abstract
This study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes. Data on past great earthquake seismic limits have been compiled for four continental subduction zones, Cascadia, SW Japan, south Alaska, and Chile. These limits have then been compared to the predictions of several models for what constrains great earthquake rupture. Temperatures on the subduction thrusts have been estimated by finite element numerical models. The landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100°C, that is, depths of about 2 to 10 km for the subduction zones studied. This temperature agrees with the dehydration of stable sliding smectite clays to illite‐chlorite. The temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow. The downdip limits correspond to the depth on the thrust where either (1) the temperature reaches about 350°C, which corresponds to thermally activated stable‐sliding behavior for crustal rocks (with a transition to 450°C), or (2) about 40 km depth if 350°C is reached at greater depth. Depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc Moho, and this downdip limit may be a consequence of stable‐sliding serpentinite or talc and other hydrated forearc mantle rocks. The primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile. Secondary control comes from the thickness of incoming sediment, the convergence rate, and the radioactive heat generation in the overlying forearc. The 100°C updip limit occurs near the trench for young subducting plates with a thick sediment section such as Cascadia (6–8 Ma), and up to 80 km landward for older oceanic crust such as south Alaska (∼50 Ma). The 350°C downdip thermal limit is applicable for young oceanic plates (e.g., Cascadia and Nankai), whereas the forearc mantle limit applies for older plates (e.g., south Alaska and Chile except near the Chile Rise). For the margins studied that have experienced great earthquakes, there is generally good agreement between the postulated thermal and Moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform, tsunami and dislocation modeling. The downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits.