Possible Mega-thrust earthquake along theRyukyu trench:
event the historical data can’t tell
Kuo-Fong Ma

馬國鳳 教授Department of Earth Sciences,
National Central University

中央大學地球物理研究所
Earthquake Physics and Seismotectonics Laboratory

Damaging Earthquakes in Taiwan since 1700s
M>7 inland
earthquake~ every 100 years

Relative quiet in seismicity

WesternTaiwan~every30-40years

Three M>7earthquakes
in twomonths

Earthquake Source Model• Subduction Zone Earthquake• Inter- and Intra-slab events• Study of Historical
Earthquakes- 1909 Taipei M6.8 Earthquake- 1920 Hualian M8.0 Earthquake

Historical Earthquakes-Location, Magnitude(Documents)-Focal Mechanism(Historical records)-Earthquake and Tsunami
Potential(Ground Motion Prediction,Tsunami Simulation)

Bathymetry and Tectonic Setting near Taiwan

Seismicity M>5.0 earthquakes with historical M>7.0 earthquake since 1900

Lay et al. (2011)

Ryukyu and Manila subduction behave similarly as this model?
Possible trench ruptured mega-thrust events?
No long historical data to tell.
(Historical literature records of only about 300 years.)

The 1920 Hualien Earthquake
1920 Hualien Earthquake
12:21 on June 6, 1920
Location:24oN, 122oE, Dep=20 km
Magnitude

MGR=8.0(Gutenberg and Richter,1954)
MS=8.0(Abe,1981,1984, Lee et al.1978
M=8.3(Bath and Duda, 1979)
MH=8.3(Hsu, 1971,1979, 1980, 1989)
ML=8.0(Cheng et al., 1996)
ML=7.5, MW=8.0(Cheng et al., 2010)(鄭世楠)

HongoSource Duration ~ 120sec
Rupture for about 300km

Osaka

ZikaweiP-S-HongoSource Duration ~ 120sec ?
Rupture for about 300km

Osaka

P-S-~M8 EQ

1920 M8.0

Zikawei4o16o

W=73kmScaling using AREATaiwan
h : seimogenic depthβ: scaling parameter related
to the effective fault width- Small-Moderate earthquakes
Mw ~ LogA- Large earthquakes
Mw ~ 4/3logA- Extra largest earthquakes
Mw ~ 2/3logAA)
22
HM  log A  log
 const.
A3[1  max( 1, 2 )] / 2
H max( 1,California
Mw8.0
Mw7.7

W~73km
We~50km
Le~80-120 km
Ae=4000 km2-6000km2(Shaw, 2009)

Const.: Stress drop related constant(Yen and Ma, 2011)

Seismicity with focal depth < 30 km

MLSeismicity concentrates
at the lower-edge of the fault

Epicenterof 331 EQ~M8 EQ

GaguaRidge~M8 EQ122.5ºE
Plausible
rupture zone

123ºE

1990-2010
Yaru Hsu (personal communication)
124ºE

1920?

Seismicity concentrates
at the lower-edge of the fault

Seismicity is absent to theeast of the Gagua ridge

Distance from trench (km)

4o
16o

W=73km
W=144 km

Scaling using AREA
Taiwanh : seimogenic depthβ: scaling parameter related
to the effective fault width- Small-Moderate earthquakes
Mw ~ LogA- Large earthquakes
Mw ~ 4/3logA- Extra largest earthquakes
Mw ~ 2/3logAA)
22HM  log A  log
 const.
A3[1  max( 1, 2 )] / 2H max( 1,(Shaw, 2009)

Const.: Stress drop related constant

Mw8.6

California

Mw8.0
Mw7.7

W~73km
We~50km
Le~80-120 km
Ae=4000 km2-6000km2

断層最短距離
福島第一原子力発電所：46.1km ：550gal
福島第二原子力発電所：44.5km : 288gal
女川原子力発電所：46.3km:607gal
東海第二発電所：44.6km:225.4gal
女川原発

2011/03/11 14:46 Mw9.0
Str: 201 Dip:9 23.7km
Dat points
NE Taiwan
N Taiwan
PGA M8.6103

福島第一東海村PGA (cm/s/s)

福島第二

10

PGA M8.0

2Obs
Shi Mw9.0
Shi Mw8.0
Shi Mw7.5
Shi Mw7.0

101

100

110100Distance1000

Manila Trech
Tectonic setting
GPS horizontal velocity w.r.t theSunda Plate gradually decrease
from north to south along thewestern Luzon at rates of 81 to50 mm/yr in the westnorthwest direction.
(1996-2008)

Convergence rate along the Manila Trench
Velocities w.r.t to the Sunda Plate
The fault locking effect on thePhilippine Fault is removed fromGPS velocity field (Yu et al. 2010)• GPS horizontal velocities decrease from
north to south suggesting a southward
decrease of convergence rate along theManila Trench .

Historical Tsunami=>
Paleo-tsunami Studies (古海嘯)時間地點 出處描述
備註
文中提海水退卻1781 高雄、 台灣采訪 「鳳港西里有加藤港，……乾隆四年4、 屏東 冊「祥異。 十六（1781）年四、五月間，時甚 及第二海嘯波，5月間地震」
晴霽，忽海水暴吼如雷，巨湧排空， 深具科學意義。
水漲數十丈，近村人居被淹，皆攀由內文描述可了援而上至尾，自分必死，不數刻，解第一海嘯波波
水暴退，人在竹上搖曳呼救，有強
高約3m，第二海力者一躍至地，兼救他人，互相引
嘯波約4-5m，與援而下。間有牧地甚廣及附近田園
情境分析雷同，溝壑，悉是魚蝦，撥刺跳躍，十里可信度高。
內村民提籃挈筒，往爭取焉。……
漁者乘筏從竹上過，遠望其家已成
巨浸，至水汐時，茅屋數椽，已無有矣。

19Field Studies: 相關地點調查結果
•••
•
•

東隆宮
鎮海宮
鎮海公園
嘉蓮宮
南隆宮

From: Prof. T.R. Wu’s student, Mr. Tsai

東港鎮

1781 ?
1894

20

鎮海宮
•

崙仔頂(鎮海宮)、汕仔寮、太監府(東隆宮)
三個村落在1894年遭海嘯襲擊，盡沉海底。

• What happened in 1894?

圖片
來
源：鎮
海宮
沿
革史誌

Conclusions• Earthquake Scaling on Length, Width and Slip for Ground
Motion Prediction Simulation is useful for crustal events,
however, width extension of the Subduction zone eventsneeds to be addressed differently.
But, how? What is the Mmax of a subduction zone?
• How to deal with extreme event which was not yet occurred?
e.g. 1920 M8.0 Hualiean earthquake => rupture to the trench• How to implement this historical not yet occurred event into
PSHA or SHA?
Shall be included or not to be included?
• Continuing the literature search on paleo-earthquakes/tsunamis
and modeling of the historical earthquakes

THANK YOU!

Seismicity between 1973 and 2010
NEIC (Mw 4.6-7.7)

Extension

Plate coupling ratio derived from inversions of GPS data
Observations v.s. Predictions

Hsu Yaru et al., 2012
Residuals of horizontal displacements

ScarboroughSeamountχ2 = 4.6

coupling ratio (c ) slip deficit rate
plate convergenc e rate