POSSIBLE MORPHOLOGIC INDICATORS FOR THE LOCATION OF LARGE SLOW EARTHQUAKES IN SUBDUCTION ZONES

Global observations of convergent margin morphology may identify regions more likely to produce tsunami earthquakes. Earthquake observations and laboratory modeling show that subduction of seafloor relief influences the accretionary prism (AP), interplate coupling and the occurrence of large interplate earthquakes. The amplitude, wavelength, and direction of convergence of seafloor roughness appear to relate to the scale of the disruption of the subduction process. Sandbox models of AP deformation caused by subduction of relatively small seafloor relief causes a displacement of the active decollement into the AP, rather than along its base. If the relief has a “wake” (e.g. seamount), passive entrance of the toe of the AP, in the wake of the descending seafloor relief, down into the subduction channel occurs, leaving an indented toe. More importantly, when seafloor relief is sufficiently deep in the subduction zone, the decollement descends to the base of the AP. However, this new base has been little faulted if at all, as it was carried passively down the subduction channel. Therefore, conditions for normal earthquake rupture may not exist, and a tsunami earthquake may be more likely. The morphology of an indented AP can be easily identified globally in ETOPO-2 bathymetry. As most convergent margins define small circles and to first order, the bathymetry of the inner wall of the trench is nearly smooth, trench inner wall bathymetry should define conic sections. Residual bathymetry would be the difference between a regionally predicted conic section and observed bathymetry. Anomalously deep residual bathymetry successfully identifies the location of the Tonga, Nicaragua, 1963 Kuril Islands, and eastern Java slow earthquakes. Both slow earthquakes in Peru (1960, 1996) occur along non-accreting margin segments presently subducting rough seafloor with sediment filled troughs. The 1968 and 1994 “hybrid” earthquakes off Honshu are anomalous in that their ruptures began updip as slow earthquakes and later became regular earthquakes. Their initial ruptures lie in the source region of the 1896 slow earthquake.