Where do tsunami earthquakes occur?

True cause for tsunami earthquakes

I have stated that a slow slip at the fault plane is one reason for generating a larger tsunami. However, even if a seismic moment is doubled due to the slow slip, the moment magnitude increases only by 0.2. It seems to me that slow slips are not enough to explain anomalous tsunami excitations (See Ide et al., 1993 for the case of the 1992 Nicaraguan earthquake). This means that a tsunami earthquake should have deformation of the trench wedge sediment like seen in the NW corner of the Chichi earthquake fault zone. Satake (1994) and other studies, which used tsunami amplitudes and arrival times, all indicate that source areas are located near the trench axis and very localized.

Where do tsunami earthquakes occur?

If we acknowledge the above mechanism as true, we can predict locations where tsunami earthquakes occur in the future. It is the place where faults branching from the decollement cut the trench wedge sediment. Case a) No tsunami event occur. The trench wedge sediment does not cover the thrust zone. In the Nankai Trough, huge out of sequence thrusts meet the sea water directly (Kuramoto et al., 1999). Historically, we do not know tsunami earthquakes in the Nankai Trough, except for the 1605 Keicho tsunami earthquake, which might have broken the shallowest portion of the decollement and the forntal thrust (personal comm. with Dr. Katsuhiko Ishibashi, Kobe Univ). Case b) Tsunami events occur. The trench wedge sediment pushed by the faulting deform severely. This situation can be seen in the northern Japan Trench (off-Sanriku) and the south Kuril Trench, and would probably seen in the trench off Nicaragua and Papua New Guinea. Case c) No tsunami event occur. Thrusts cut an accretionary prism and a thick sedimentary wedge above, but the deformation at the surface would not be localized. Barbados might be an example. References Ide, S., F. Imamura, Y. Yoshida, and K. Abe, Source characteristics of the Nicaraguan tsunami earthquake of September 2, 1992,Geophys. Res. Lett., 20, 863-866, 1993. Kuramoto, S. et al., Japan-US collaborated study on western Nankai accretionary prism -evolution of accretionary prism and seismogenic zone imaging -, Abstr. Seism. Soc. Jpn 1999.