Tsunami earthquakes are characterized by (1) slow slip and (2) tsunamis larger than expected from seismic slip. We examine whether the 2004 Sumatra– Andaman earthquake had these features and thus involved a component of tsunami earthquakes. Fitting the effective moment versus frequency curve, we obtain characteristic times τ (Kanamori, 1972) of 70 and 290 sec. The former is on the scaling for normal earthquakes; the latter is longer. In the area off northern Sumatra, by back- projecting the wavefront recorded on the sea surface height (ssh) by the satellite altimetry, we estimate the seaward edge of the fault plane to be located at the deformation front. We then invert the ssh, assuming that the seaward edge of the fault is at this location, and obtain a maximum slip of 40 m, which was estimated to be 15 m previously. This amount of slip, approximately twice the slip estimated seismologically and geodetically, suggests that additional crustal deformation, such as inelastic uplift of the trench sediments, might have occurred near the deformation front. We propose that a similar, possibly slow, slip occurred in the shallow subduction boundary, extending from all over the rupture zone derived by the body-surface waves, with a smaller northward propagation velocity, corresponding to the longer τ. This rupture mode would solve the enigmatic features of this event seen in the seismological and tsunami data. The 2004 Sumatra–Andaman earthquake, having both features (1) and (2) that characterize tsunami eathquakes, likely had a component of tsunami earthquakes.

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