Re-examination of the 1896 Sanriku Earthquake Source

Abstract:

Comparison of tsunamis from the 2011 Tohoku earthquake and the 1896 Sanriku earthquake shows that the runup heights along the northern and central Sanriku coasts were similar (Tsuji et al., 2015, Pageoph), but the 1896 tsunami waveforms recorded on tide gauges at Hanasaki, Ayukawa and Choshi show much smaller (< 1/5) amplitudes. These three tsunami waveforms were previously used to estimate the 1896 fault model by Tanioka and Satake (1996, GRL) and Tanioka and Seno (2001, GRL). Computed tsunami from a northeastern part (150 km x 50 km or 200 km x 50 km near the trench axis) of the 2011 Tohoku earthquake slip model (Satake et al., 2013, BSSA) roughly reproduces the 1896 runup heights on the Sanriku coast, but the tsunami waveforms on the tide gauges are much larger than the observed ones. Inversion of the 1896 tsunami waveforms indicates that the largest slip is 20 m on the second subfault (50 km x 25 km) from north and from the trench axis, and the slip on other subfaults ranges 3 to 7 m. However, this slip distribution model produces coastal tsunami heights smaller than the observed. If the length of the large slip subfault is extended to 100 km, then both observed tsunami waveforms and coastal heights are reproduced. Thus the 1896 tsunami model is 200 km long, 50 km wide with an average slip of 8 m, but the very large slip (20 m) was located on a 100 km x 25 km fault at a depth range of 3.5 – 7 km. Assuming a rigidity of 2 x 10¹⁰N/m², the total seismic moment is 1.6 x 10²¹ Nm and the corresponding moment magnitude is Mw 8.1. During the 2011 Tohoku earthquake, the slips on these subfaults which largely slipped 20 m in 1896 were 3 and 14 m, but the shallower parts slipped 20 and 36 m, where the 1896 slip was 3 m. Hence the large slip of the 2011 Tohoku earthquake occurred even shallower part (a depth range of 0 – 3.5 km) of the 1896 Sanriku earthquake.