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

Geoscience Letters, Dec 2017

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 M w 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.

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

Satake et al. Geosci. Lett. Different depths of near-trench slips of the 1896 Sanriku and 2011 Tohoku earthquakes Kenji Satake 0 Yushiro Fujii 2 Shigeru Yamaki 1 0 Earthquake Research Institute, The University of Tokyo , 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032 , Japan 1 Seamus Ltd , 2235 Kizaki, Kita-ku, Niigata 950-3304 , Japan 2 International Institute of Seismology and Earthquake Engineering, Building Research Institute , 1 Tachihara, Tsukuba, Ibaraki 305-0802 , Japan 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. Background The 11 March 2011 Tohoku earthquake (Mw 9.0) was the largest instrumentally recorded earthquake in Japan and caused devastating tsunami damage including ~  18,500 casualties. The ground shaking was felt throughout the Japanese Islands with the maximum seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale, or 11–12 on the Modified Mercalli scale (Fig. 1a). Huge slip (>  50  m) on the plate interface up to the Japan Trench axis was estimated near the epicenter (~  38.5°N) from seismic waves (Ide et  al. 2011) , inland and submarine geodetic data (Iinuma et  al. 2012) , and tsunami waveforms (Fujii et al. 2011; Satake et al. 2013b) . On the contrary, the largest tsunami heights on the Sanriku coast, ~  40  m, were recorded ~  100  km north (near 39.6°N). This enigma was explained by a delayed tsunami generation in the northern part of tsunami source through the tsunami waveform analysis (Satake et  al. 2013b; Tappin et  al. 2014) . However, the cause of the delayed tsunami generation is still controversial, either due to slip on shallow plate interface (Satake et  al. 2013b) or submarine landslide (Tappin et al. 2014) . In the northern part of the 2011 tsunami source, the 15 June 1896 Sanriku earthquake occurred and caused the worst tsunami disaster in Japan, with casualties of ~  20, 000 (Shuto et al. 2007) . The 1896 Sanriku earthquake was a typical example of a ‘tsunami earthquake’ (Kanamori 1972; Tanioka and Satake 1996b) . The origin time: 19  h 32 m (local time), the epicenter: 144°E, 39.5°N, and magnitude: M = 6.8 were estimated from Japanese seismological data (Utsu 1979) . The surface wave magnitude MS  =  7.2 was assigned from global data (Abe 1994) . The moment magnitude Mw was estimated as 8.0–8.2, from a comparison of aftershock activity with other large earthquakes (Utsu 1994) . The tsunami magnitude Mt was determined 11 March 2011 Tohoku (M 9.0) JMA MM b 15 June 1896 Sanriku (M 8.2) Hanasaki Ayukawa Choshi 1896 Sanriku 2011 Tohoku 8 Ayukawa −2 2 0 6 4 2 0 −)2 m ( e2 d u t li 0 p m −A2 2011 Tohoku 1896 Sanriku Choshi 2011 Tohoku 1896 Sanriku 0 30 60 90 120 150 180 Time (min) as 8.6 from global data (Abe 1979) and 8.2 from Japanese data (Abe 1981) . The ground shaking was weak (2–3 on the JMA seismic intensity scale, corresponding to 4–5 on the Modified Mercalli scale; Fig.  1b). However, the tsunami heights on the Sanriku coast from the 2011 and 1896 earthquakes were roughly similar (Fig. 1c, Tsuji et al. 2014) as detailed in the “Tsunami data of the 1896 earthquake.” Tsunami waveform modeling of the 1896 Sanriku earthquake has shown that slip occurred on a narrow fault located near the trench axis (Tanioka and Satake 1996b; Tanioka and Seno 2001) . This is a common feature of ‘tsunami earthquakes’ such as the 1992 Nicaragua or 2010 Mentawai earthquakes (Satake and Tanioka 1999; Satake et  al. 2013a) . Tanioka et  al. (1997 ) further proposed that the 1896 Sanriku ‘tsunami earthquake’ occurred in a region where the ocean bottom topography is rough, characterized by well-developed horst and graben structures. Polet and Kanamori (2000) extended this model to global subduction zones, based on the examination of the sour (...truncated)


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Kenji Satake, Yushiro Fujii, Shigeru Yamaki. Different depths of near-trench slips of the 1896 Sanriku and 2011 Tohoku earthquakes, Geoscience Letters, 2017, pp. 33, Volume 4, Issue 1, DOI: 10.1186/s40562-017-0099-y