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)