2018 3rd International Conference on Design, Materials and Manufacturing (ICDMM 2018)

University of the Ryukyus

Some Thoughts on The Risk of Natural Disasters in RyukyuArchipelagoÖmer AYDAN
University of the Ryukyus, Dept. of Civil Engineering,
Disaster Prevention Research Center for Islands Regions(Pamukkale University, Denizli, Turkey)
2018 August 12

Ryukyu Archipelago

Ryukyu Archipelago is situated on the Ryukyu Arc and consists of achain of islands for a length of 1300 km between Kyushu Island of Japanand Taiwan. The main islands are Amami-Oshima, Okinawa, Miyako,Ishigaki, Iriomote and Yonaguni from north to southNatural Disasters•Earthquake•Tsunami•Geotechnical Disasters(Slope failure, Liquefaction, Debris flow, sinkholes)•Typhoon, Tornadoes, Torrential rains

TECTONICS
Ryukyu arc is considered
to be a convergent platemargin where thePhilippine Sea plate issubducting beneath theEurasian plate
The arc is a rifting
fragment of continental
crust and it is oriented
NE-SW direction andthe convergence rate
between the Philippine
sea plate and the Eurasia
plate varies from 5 to 7
cm/yr.

CRUSTAL DEFORMATIONSIn the region, the Philippine Sea plate moves towards WNW with respect
to the interior of the Eurasia plate, having a relative velocity of
approximately 60 mm/yr. This figure shows the crustal deformations
measured from GPS network of GEONET of Japan

Earthquake Risk

1958 Ishigaki Earthquake, Toppled
Block wall (from JMA)

Past Seismicity of Ryukyu Archipelago

From Goto et al. 2011

It is almost unknown compared to other regions of Japan

Seismicity for the last 100 years
from Ömer AYDAN

The region is very seismically active and there are more than 5 large
events having a magnitude greater than 7. It is not an aseismic region

Seismicity on vertical cross section A-A'As seen from the figure, the Philippine Sea plate subducts beneath theOkinawa plate-let with an inclination of about 30-35 degrees. On thewest side, the seismic activity in Okinawa trough is also clearly seen.

Research on Active Faults are not sufficient compared other regionsof Japan
NIEDKisaki

Okinawa Main Island

Miyako Island
Kisaki

NIEDEstimation of Magnitude of Past Earthquakes from geologicalevidences and archaeological remains

Aydan (2012, 2018)

Segmentation and anticipated seismic parameters of mega earthquakes

Japan Nuclear Regulation Authority

Mw=9.6
Chijima & Japan
TrenchesIzu & Ogasawara
Trenches
Mw=9.6
NankaiTrough &Ryukyu
Archipelago

Mw=9.2

Archeological Evidences of Past Earthquakes in Okinawa Island
1) Minami-Uebaru Fault (Offset 10m)

Mw=8.1

2) Toppled rock block in Nakagusuku
Castle remains about 400 years ago

Estimated Maximum Acceleration > 0.7g
Intra-plate Earthquake: Mw8.1
Inter-plate earthquake: Mw9.1
3) Taira-Bridge: Bridge and well & settlement
Deformation anddamage to remains
by an earthquake
several hundreds
ears ago（Aydan, 2015, 2016)

Mw=？

Damage to Karstic caves can be used for inferring past earthquakes

Original idea by Gilli1771 Meiwa Earthquake

Tsunami run-up height in Ishigaki, Miyako and Tarama Island
Values are incompatible with an earthquake of Magnitude 7.4

Ishigaki CaveKarstic Cave in thearea of New Ishigaki
AirportC CaveNakabari
Karstic Cave

Miyako Island

2005 Off-Sumatra Earthquake (Nias Island) Mw 8.5)

Tögindrawa

Tögindrawa

Tögindrawa.

Estimationg of seismic
characteristics from stalactites
and stalagmites
HORIZONTAL SEISMIC COEFFICIENTLayers of sedimentation ofstalactites and stalagmites

2Stalactite Stalagmite

L/a681020

102468STRENGTH RATIO (t/L)

10

Crustal Deformation (2005 Off-Sumatra Earthquake (Nias Island)

PierEstimation from corals: Sumatra

TSUNAMIPOSSIBILITY OF TSUNAMI IN RYUKYU ARCHIPELAGO

The tsunami by the 1771 Meiwa earthquake is well documented.
However, the other tsunamis are fundamentally unknown

New Ishigaki Airport
Tsunami boulders and deposits observed on rock cuts ofRyukyu Limestone Formation

Tsunami Boulder

Tsunami depositsTsunami Boulders

Tsunami Boulder in Ohama district of Ishigaki Island.

The ages of the coral fossils on some tsunami boulders in the vicinity ofIshigaki and were determined by Goto et al [14] and they identified
several periods: about 200, 500, 1000, 2000, and 2500 years ago on thetsunami boulders.

Geometrical Parameters of Major Tsunami Boulders in Ryukyu
Archipelago

ShimojiIshigaki

MiyakoOkinawa

The largest tsunami boulder exist
in Shimoji Island

Investigation of tsunami boulders through drones

Estimation of Magnitude of Mega EarthquakesSee Aydan and Tokashiki (2018) for details

Geotechnical Disasters
Slope failure & Debris flow
Sinkholes,
LiquefactionCliff Failures

Aydan et al., 1985

Kyanmisaki Shore

Route 331 in Nanjo City
Tokashiki & Aydan, 2010

Kita-Uebaru Landslide and Debris FlowEvaluation of Causes of Kita-Uebaru Landslide & Debris Flow

Heavy rainfall

Tokashiki & Aydan, 2010

120m relative slip and post-failure profile were well estimatedSinkholes
2009 L’Aquila Earthquake, Italy
Toriike Shimoji

Futenma Base

From Ginowan City

Ground Liquefaction

Recently many artificial landfills

Itoman City

Damage to Katsuren Castle
Remains bythe 2010 Off the Okinawa Island
KATSURENCASTLE
earthquake
Epicenter

Locations of major damages

Damage at Katsuren Castle

Historical structures may be damaged
KumamotoCastle

MakishiMiyakoMakishiMany Piloti Structures

Regional seismic coefficient is 0.7,
Which is the lowest in Japan.
Kumamoto was 0.8.
Retrofitting is necessary

Infra structures should be retrofitted

Typhoons(Hurricanes) and Tornadoes
Coriolis
Force

Increase in Size and Originates more northward
Alley ofTyphoons
Strong wind,heavy rainand floods

Typhoon No.12

Structural Damage-1

Structural Damage-2

Monitoring of the effects of typhoon on a apartment (2014 July 8)

CONCLUSIONS
Some of conclusions from this study are as follow:
1) The risk of mega-earthquakes and mega-tsunamis is quite high inRyukyu Archipelago despite the common belief that the Okinawa
Prefecture is an seismic region. The mega-earthquakes up toMagnitude 9.7 is possible in view of the tsunami boulders distributed
over the Ryukyu Archipelago while the segmentation of the Ryukyu
trench implies risk of mega-earthquakes with a magnitude greaterthan 9.5.
2) The risk geotechnical natural disasters in Ryukyu Archipelago islandslides in Shimajiri formation and, cliff failures and sinkholes inRyukyu limestone formation.
3) The Ryukyu Archipelago is also known as the " Typhoon Alley".
The risk of natural disasters due to typhoons is quite high. The
global warming is expected to increase the risk of super-scaletyphoons in Ryukyu Archipelago. This may also result in themodification of design wind loads on structures in Japan.

THANK YOU FOR YOUR ATTENTIONFault outcrop appeared in2016 Kumamoto earthquake