Different depths of near-trench slips of the 1896 Sanriku and 2011 Tohoku earthquakes

The 1896 Sanriku earthquake was a typical ‘tsunami earthquake’ which caused large tsunami despite its weak ground shaking. It occurred along the Japan Trench in the northern tsunami source area of the 2011 Tohoku earthquake where a delayed tsunami generation has been proposed. Hence the relation between the 1896 and 2011 tsunami sources is an important scientific as well as societal issue. The tsunami heights along the northern and central Sanriku coasts from both earthquakes were similar, but the tsunami waveforms at regional distances in Japan were much larger in 2011. Computed tsunamis from the northeastern part of the 2011 tsunami source model roughly reproduced the 1896 tsunami heights on the Sanriku coast, but were much larger than the recorded tsunami waveforms. Both the Sanriku tsunami heights and the waveforms were reproduced by a 200-km × 50-km fault with an average slip of 8 m, with the large (20 m) slip on a 100-km × 25-km asperity. The moment magnitude Mw of this model is 8.1. During the 2011 Tohoku earthquake, slip on the 1896 asperity (at a depth of 3.5–7 km) was 3–14 m, while the shallower part (depth 0–3.5 km) slipped 20–36 m. Thus the large slips on the plate interface during the 1896 and 2011 earthquakes were complementary.