Interplate coupling in the Ryukyu Trench: possibility …...M8.0, 1911 M7.4, 1771 Thrust-type large...
Transcript of Interplate coupling in the Ryukyu Trench: possibility …...M8.0, 1911 M7.4, 1771 Thrust-type large...
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Interplate coupling in the Ryukyu Trench: possibility of large
earthquakes and mega tsunamis
Mamoru Nakamura
University of the Ryukyus
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M8.0, 1911
M7.4, 1771
Thrust-type large earthquakes: 1771 Yaeyama tsunami
Damaged earthquakes in the Ryukyu area 1771 Yaeyama earthquake (M7.4?) Mt8.5 1911 Kikaijima earthquake (M8.0) (Philippine Sea plate: intra-plate earthquake)
Historical large earthquakes in the Ryukyu trench (1700-2010)
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The 1771 Yaeyama tsunami
Kawana,2000; Goto et al., 2010
Run-up heights
Ishigaki Is. Iriomote Is.
Tarama Is.
Hateruma Is.
Miyako Is.
Irabu Is.
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Inundation area of the 1771 Yaeyama tsunami in the south Ishigaki Island
Goto et al., 2010 4
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Tsunami boulders
Tsunami boulder (coral) (not moved by the 1771 tsunami)
Tsunami boulder (moved by the 1771 tsunami)
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Inundation of the 1771 event in the Tarama Island
660±60
1890±80
4730±200
2440±160
1020±80
2610±90
The house where the tsunami reached Tsunami boulder and its age (coral fossil)
A
B
A B
Tsunami inundation area :1~3 km from shoreline
(Nakata, 1990, Kawana & Nakata, 1994)
Alt
itu
de
(m
)
Distance (km) 6
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Trace of tsunami at the ruins in the Miyako Island
Shimabukuro, 2008
Tsunami sediments of the 1771 event
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Trace of ground shaking of the 1771 Yaeyama earthquake
the tsunami attacked between 9 century and 1771
Cracks and sand layer (Okinawa Archeological center, 2009)
Karadake shell mound Karadake old coffins
cracks by the ground shaking
inundation of tsunami
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Source of the 1771 Yaeyama tsunami
A
B
C
A:Intraplate eq.+landslide (Imamura et al., 2001,2008) No evidence for landslides B:Intraplate eq. (Nakamura, 2006) M8 intraplate earthquake? C:Interplate eq. (Nakamura, 2009)
★:epicenter by the earthquake catalogue.
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Mega-tsunami before the 1771 event
Goto et al., 2010
Occurrence of mega tsunami : 200, 500, 1000,2000,2500 yrs. BP.
5 times for 3000 years.
Age of coral fossil at the tsunami boulder
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Evidence of mega-tsunami before 1771 event
14C age of the coral fossil of the tsunami boulder(TK-2) 2110~2390 yBP
The tsunami boulder moved from Koruse-shrine to east and north direction by the 1771 tsunami. (written in the old documents)(大波之時各村之形行書)
Goto et al., 2010
2000 yrs BP. : the boulder moved from reef to land.
1771: the boulder was broken and moved. 11
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Tsunami Folklores in the Miyako area
Tsunami Folklores : 13~15 century
Extinction of Kidomari village by big wave
Kadegaru village Trama
Sunagawa village
Obi-Iwa (tsunami boulder)
documented in the old literature (Miyako-kyuki, 1748) 12
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5cm/yr GPS horizontal velocity (GSI, 1997-2006) Tsushima is fixed
coupling model
observed
Stress accumulation Southward movement of Ryukyu Islands -> weak coupling Inter-seismic coupling: 5% (Paterson & Seno, 1984)
The Ryukyu Trench is coupled?
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Width of the coupled zone W
Horizontal velocity relative to the Amurian plate (NW direction: positive)
Ho
rizo
nta
l dis
pla
cem
en
t b
y th
e in
terp
late
co
up
ling
(mm
/yr)
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Ocean Bottom Crustal Movement Measurement System
Combination of (1) Kinematic GPS (5Hz sampling) (2) Acoustic ranging system
Position determination of ocean-bottom benchmarks
University of the Ryukyus Nagoya University IES, Academia Sinica
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GPS(sampling interval:0.2s)
Satellite compass
Transducer
Tonan-Maru (176t) (Okinawa Prefectural Fisheries and Ocean Research Center )
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Benchmarks 35km land-side from Ryukyu Trench interval: 2km Reference of kinematic GPS Univ. Ryukyus
2km
35km
110km
Univ. Ryukyus
Ocean-bottom crustal deformation measurement in the central Ryukyu trench
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Result of the observations
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Velocity of the benchmark
Horizontal velocity of the benchmark relative to the Amurian plate
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Width of the coupled zone W
Horizontal velocity of the benchmark relative to the Amurian plate (NW direction: positive)
Ho
rizo
nta
l dis
pla
cem
en
t b
y th
e in
terp
late
co
up
ling
(mm
/yr)
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Backslip model (2)
Coupled zone from trench to 50~70 km widths
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Model of coupling area
New point ( Nov. 2011~)
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Vertical cross-section of the central Ryukyu Trench
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Backslip by the coupled area in the south Ryukyu Trench
W=50km W=100km
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Tonga region
(Bevis et al., 1995, Nature)
Backarc spreading
Weak interplate coupling ?
61%
37%
14%
Seismic coupling (motion of the backarc spreading is not included in the calculation) (Pachenco& Sykes, 1993 JGR)
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The 2009 Samoa-Tonga earthquake
(Beavan et al., 2010, Nature)
(Satake, 2010, Nature)
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2011 Tohoku earthquake
Large slip near the Trench axis
Coupled zone in the shallow part of Japan Trench
(Lee et al., 2011)
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Distribution of coupled area, splay faults, and source of earthquake in the Ryukyu Trench
Heki & Kataoka, 2008
Nakamura, 2009
Coupled area
Splay faults (Hsu et al., 2009)
Tsunami earthquake (Nakamura, 2009)
Slow slip events
VLFEs (Tu et al.,2009,2010)
15c tsunami?
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Tsunami hazard mitigation in Okinawa
Display the elevation of location
School (opened on Apr. 2011)
Started to evacuation drill to the tsunami safe zone
School: moved from terrace area to landfill area. People did not pay attention to tsunami disaster.
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Conclusions
• Occurrence of Mega-tsunami in the southern Ryukyu Trench: – 5 times during 3000 years
• 200, 500, 1000,2000,2500 years BP
– Source faults: Ryukyu Trench
• Shallow part of the coupled area in the central Ryukyu Trench – 50~70 km in width – 100~ km in length
• Shallow part of the Ryukyu Trench – Full-coupled? – Cause of large earthquakes and tsunami?
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Distribution of travel time residuals
Jan. 2008 Feb. 2008 Jul. 2008
May. 2009 Nov. 2009
Rms: 70 micro-sec 31
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Backslip model (1)
No slips from trench to 50 km width
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Calculation of displacements
Block rotation Of the Ryukyu arc
Backslip motion by the coupling
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Seismic gap (?) in the central Ryukyu Trench
Low seismicity ( > M 5) for 100 years.
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The locations of VLF events in 2007 (Tu et al., 2009)
VLFs: thrust fault type (Tu et al., 2010)
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Interplate coupling and possibility of large earthquakes in the Ryukyu region
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