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

This study examines thermal and structural controls of the updip and downdi p rupture limits of great subduction thrust earthquakes. Data on past great earthquake seismic limits have been compiled for four continental subducti on zones, Cascadia, SW Japan, south Alaska, and Chile. These limits have th en been compared to the predictions of several models for what constrains g reat earthquake rupture. Temperatures on the subduction thrusts have been e stimated by finite element numerical models. The landward limits of the obs erved updip aseismic zones correspond to the position where the thrust temp erature reaches about 100 degrees C, that is, depths of about 2 to 10 km fo r the subduction zones studied. This temperature agrees with the dehydratio n of stable sliding smectite clays to illite-chlorite. The temperatures in this region are controlled mainly by the thickness of sediment on the incom ing crust and by the crustal age and thus heat flow. The downdip limits cor respond to the depth on the thrust where either (1) the temperature reaches about 350 degrees C, which corresponds to thermally activated stable-slidi ng behavior for crustal rocks (with a transition to 450 degrees C), or (2) about 40 km depth if 350 degrees C is reached at greater depth. Depths of a bout 40 km approximately correspond to the intersection of the thrust with the continental forearc Moho, and this downdip limit may be a consequence o f stable-sliding serpentinite or talc and other hydrated forearc mantle roc ks. The primary temperature controls on the downdip region are the age of t he subducting oceanic plate and the thrust dip profile. Secondary control c omes from the thickness of incoming sediment, the convergence rate, and the radioactive heat generation in the overlying forearc. The 100 degrees C up dip limit occurs near the trench for young subducting plates with a thick s ediment section such as Cascadia (6-8 Ma), and up to 80 km landward for old er oceanic crust such as south Alaska (similar to 50 Ma). The 350 degrees C downdip thermal limit is applicable for young oceanic plates (e.g., Cascad ia 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 s tudied that have experienced great earthquakes, there is generally good agr eement between the postulated thermal and Moho limits and the rupture or se ismogenic zone as defined by the distribution of aftershocks and by wavefor m, tsunami and dislocation modeling. The downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limit s.