SHEAR-ZONE THICKNESS AND THE SEISMICITY OF CHILEAN-TYPE AND MARIANAS-TYPE SUBDUCTION ZONES

Chilean-type convergent margins have many large (M > 7.6) earthquakes, whereas Marianas-type ones do not, This dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement, whereas i t becomes understandable if it is viewed as a relatively thick, sedime nt-filled shear zone, which thins or thickens arcward depending on sub duction speed and sediment supply, Chilean-type margins have thick tre nch fills, and their shear zones generally thin arcward from inlets as much as several thousand metres high, the most pronounced thinning be ing located near backstops, Tall (up to several kilometres) seamounts are subducted essentially intact to relatively great depths and confin ing pressures before jamming into the roof of the channel and becoming seismogenic asperities, Their near-basal ruptures can generate large thrust-type earthquakes, mainly concentrated in seismic fronts near ba ckstops, Marianas-type margins, in contrast, have thin trench fills, a nd their shear zones generally thicken arcward from inlets that can be as little as 300 m high, Seamounts are truncated near the inlet at lo w confining pressures and generate only small earthquakes, After passi ng the inlet, they do not touch the roof and therefore cannot generate large earthquakes. A similar mechanism may explain seismic gaps at se diment-poor regions of subduction zones.