Ryuku.pdf

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Tectonophysics 466 (2009) 255 – 267

www.elsevier.com/locate/tecto

Subducting oceanic high causes compressional faulting in southernmost

Ryukyu forearc as revealed by hypocentral determinations of

earthquakes and reflection/refraction seismic data

Yvonne Font a,

⁎ , Serge Lallemand b

a Géosciences Azur, UMR IRD – CNRS – UPMC – UNSA 6526, 06235 Villefranche-sur-Mer, France

b Géosciences Montpellier, UMR CNRS – UM2 5243, CC.60, UM2, place E. Bataillon, 34095 Montpellier, France

Available online 22 November 2007

Abstract

Absolute earthquake hypocenter locations have been determined in the area offshore eastern Taiwan, at the Southernmost Ryukyu subduction

zone. Location process is run within a 3D velocity model by combining the Taiwanese and neighboring Japanese networks and using the 3D

MAXI technique. The study focuses on the most active seismic cluster in the Taiwan region that occurs in the forearc domain offshore eastern

Taiwan. Earthquakes distribute mainly along 2 active planes. The first one aligns along the subduction interface and the second one, shallower

affects the overriding margin. Focal mechanisms within the shallow group indicate that nodal planes are either compatible with high-angle back-

thrusts or low-angle thrusts. The active seismic deformation exclusively indicates reverse faulting revealing that the forearc basement undergoes

trench-perpendicular strong compression. By integrating the seismological image into the regional context, we favor the hypothesis in which the

dense seismicity occurring offshore marks the activity of en-échelon high-angle reverse faults accommodating the uplift of a broken piece of

Ryukyu Arc basement, called Hoping Basement Rise. The uplift is inferred to be caused by the subduction of an oceanic relief, either exotic block,

seamount or oceanic crust sliver. Our favored solution satisfies the narrowness of epicenter's cluster along the Hoping Canyon, and the

observation of high-angle active faults on seismic lines crossing the area. Furthermore, this solution is compatible with the active uplift of the

Hoping Rise demonstrated from morphological and sedimentological data. We do not exclude the branching of the high-angle reverse faults

system onto a splay fault connected with the subduction interface but further investigations are needed to map precisely the 3D distribution of

active faults that break the margin.

Keywords: Splay fault; Ryukyu subduction; Taiwan; Earthquake location

1. Introduction

offshore area, hypocentral determinations based on local

seismological observation are usually poorly resolved. Conse-

quently, the subduction thrust fault zone

Great earthquakes mostly generate on plate interface of

subduction zones. Near Taiwan, the southernmost Ryukyu

subduction zone exhibits, in the Ryukyu forearc domain, the

densest seismic activity of the whole area. This area, located at a

few tens of kilometers from Taiwanese and Japanese coasts, has

generated historical earthquakes as the June 5th 1920, magnitude-

8 event ( Wang and Kuo, 1995 ) or more recently the Mw 7.1

earthquake, on February 31st 2002 ( Fig. 1 and Table 1 ). In this

–

the most destructive

earthquakes and tsunamis generator

–

is badly imaged and

seismic hazard inefficiently evaluated.

This paper reviews independent studies carried on the Nanao

forearc region with the purpose of better understanding the high

concentration of seismicity occurring there ( Fig. 1 C). We first

present the morphological and tectonic structure of the south-

ernmost Ryukyu forearc essentially based on marine reflection

and refraction seismic data. In this framework, we then describe

the hypocentral distribution of a refine earthquake dataset

whose location has been reprocessed using appropriate hetero-

geneous velocity model ( Font et al., 2003 ) and 3D location

technique. We will briefly summarize the technique used to

⁎ Corresponding author.

E-mail addresses: font@geoazur.obs-vlfr.fr (Y. Font),

Serge.Lallemand@dstu.univ-montp2.fr (S. Lallemand).

doi: 10.1016/j.tecto.2007.11.018

256

Y. Font, S. Lallemand / Tectonophysics 466 (2009) 255 – 267

obtain the refine data set. More detail on the MAXI technique

can be found in Font et al. (2004) . Finally, this study reveals the

geometry of active faults that present a potential seismogenic

risk for neighboring coastal cities.

1997 ). North-East of Taiwan, the PSP subducts beneath the

rifted Eurasian plate margin (i.e. the Ryukyu Arc located south

of the opening South Okinawa Trough) along the Ryukyu

Trench ( Fig. 1 B,C). East of Taiwan, the deformed Eurasian

continental margin collides against the Luzon volcanic arc,

originated from the Manila east-dipping subduction system

(southwest of Taiwan). The Taiwan orogen is often regarded as

the result of this active collision (e.g. Suppe, 1981; Ho, 1986 ).

Due to the Okinawa Trough extension ( Sibuet et al., 1995,

1998 ), the westernmost Ryukyu Arc segment is presently

moving southward, 1.4 cm/year faster than NE Taiwan ( Fig. 1 ;

2. Geodynamic background

Taiwan is located at the boundary between the Philippine Sea

plate (PSP) and the continental margin of the Eurasian plate

( Fig. 1 A). Near Taiwan, the PSP converges toward the Eurasian

plate at a rate of 8

–

9 cm/year along N306°

–

N312° ( Yu et al.,

Fig. 1. A. Plate boundaries around Taiwan. B. Geodynamic context of Taiwan (modified after Lallemand and Liu, 1998 ). C. Seismicity map from 1991 to 1999

(hypocenter catalog from Central Weather Bureau). D. Instrumental seismicity of magnitude earthquake bigger than 7 (from Engdahl and Villasenor, 2002 ) and studied

focal mechanism (see also Table 1 ).

Y. Font, S. Lallemand / Tectonophysics 466 (2009) 255 – 267

257

Table 1

Earthquakes of magnitude bigger than 7.5 since 1700 and bigger than 7.0 since 2000 occurring offshore eastern Taiwan

Time (year/month/day)

Latitude

Longitude

Depth

Magnitude

Reference

1811/03/17

23.8°N

121.8°E

7.5 (based on intensity)

Tsai (1985)

1815/10/13

24.0°N

121.7°E

7.7 (based on intensity)

Tsai (1985)

1920/06/05

24.0°N

122.0°E

8.1 (Ms)

Wang and Kuo (1995)

8.0 to 8.3 (ML)

IES web site

23.8°N

122.1°E

7.9 (Mw)

Engdahl et al. (1998)

1922/09/01

24.5°N

122.0°E

7.5 (Mw)

Engdahl et al. (1998)

1951/10/21

23.7°N

121.5°E

7.5 (Mw)

Engdahl et al. (1998)

1966/03/12

24.1°N

122.6°E

7.9 (Ms)

Wang and Kuo (1995)

24.3°N

122.7°E

7.4 (Mw)

Engdahl et al. (1998)

7.6 (Ms)

Engdahl and Villasenor (2002)

1978/12/23

23.2°N

122.0°E

7.2 (Ms) 7.0 (Mw)

Engdahl and Villasenor (2002)

122.6°E

7.0 (Ms) 7.0 (Mw)

CMT Harvard

1986/11/14

23.9°N

121.6°E

7.8 (Ms)

Wang and Kuo (1995)

24.0°N

121.7°E

7.4 (Mw)

Engdahl et al. (1998)

7.7 (Ms)

Engdahl and Villasenor (2002)

24.0°N

121.8°E

7.8 (Ms)

CMT Harvard

2001/12/18

24.0°N

122.7°E

7.3 (Ms)

CMT Harvard

122.8°E

6.3 (Ms) 6.8 (Mw) 7.2 (mb)

Engdahl and Villasenor (2002)

2002/03/31

24.3°N

122.1°E

7.1 (Mw) 7.2 (mb)

Engdahl and Villasenor (2002)

122.2°E

7.4 (Ms)

CMT Harvard

References are indicated in the table.

Imanishi et al., 1996; Lallemand and Liu, 1998 ). As a

consequence, the relative convergence rate increases across

the southernmost Ryukyu Trench and reaches

∼

–

11 cm/year.

slab deformation near Taiwan (between 50 and 100 km depth). The

slab deformation, as proposed by Kao (1998) , could result from

horizontal E

3. Review on the Ryukyu subduction structures from

previous works

3.3. Shallow part of the subducting PSP (

40 km depth)

3.1. Ryukyu

–

Taiwan junction

MCS refraction performed along 2 arc-

parallel lines ( Table 2 , Line EW-14 and EW-16), many authors

( Wang and Chiang, 1998; McIntosh and Nakamura, 1998;

Hetland and Wu, 2001; Wang et al., 2004; McIntosh et al.,

2005 ) have shown that the PSP, at latitudes ranging from the

accretionary prism to the Ryukyu Arc, also presents highs-and-

lows geometry. Near Taiwan, the subducting crust presents 2

rises which location is not well determined as it differs

regarding the refraction modeling ( McIntosh and Nakamura,

1998; Wang et al., 2004 ). The eastern rise is basically located

along the northward prolongation of the Gagua Ridge. The

western rise location varies, regarding authors, between the

western extremity of the Nanao Basin and the Hoping Basin.

Along line EW-14 ( McIntosh and Nakamura, 1998; Wang et al.,

2004 ), both rises have an elevation of about 4 km compared to

surroundings and, in average, the interplate contact zone is

between 15 and 18 km in depth ( McIntosh and Nakamura, 1998

or Wang et al., 2004 ). Wang et al. (2004) interpreted this high-

and-low configuration as the buckling of the subducted slab due

to increasing lateral compression. Font et al. (2001) proposed

that it might correspond to the subduction of an oceanic relief.

–

West of 123°E, less than 200 km from the Taiwanese collision

zone, the E

–

SE azimuth at its southwestern ex-

tremity. East of Taiwan, the southernmost extremity of the Ryukyu

Trench was mapped in 1996 using swath bathymetry ( Lallemand

et al., 1997a; Liu et al., 1998 ). The Ryukyu deformation front can

be clearly followed until 122°E ( Figs. 1 and 2 ) with the onland

suture zone (marked by the Longitudinal Valley Fault, e.g.,

Angelier et al., 1997 ) is unclear and themodalities of this transition

are still controversial (e.g., Angelier et al., 1990; Hsu and Sibuet,

1995; Chemenda et al., 1995; 1997; Lallemand et al., 1997a,b;

Sibuet and Hsu, 1997; Teng et al., 2000; Lallemand et al., 2001 ).

–

3.2. Deep part of the subducting PSP (

40 km depth)

The PSP slab, near Taiwan, plunges more steeply than east of

the Gagua Ridge and the subducting oceanic crust is estimated to be

Early Cretaceous in age, from gabbro dating of the Huatung basin

( Deschamps et al., 2000 ). From seismicity observation, the north-

ward dipping PSP slab is observed down to 300 km depth ( Kao,

1998; Font et al., 1999 ). Tomographic image agrees with seismicity

that the southwesternmost extremity of the slab reaches about

120 km depth under northern Taiwan ( Rau and Wu, 1995; Wu

et al., 1997 ). Near the slab termination, the slab (between 40 and

120 km) becomes shallower and folded ( Font et al., 1999 ). From

recent seismological investigation, Chou et al. (2006) confirm the

3.4. Overriding active margin: Ryukyu Arc basement and forearc

sedimentary basins

The analysis of 45 seismic reflection profiles, acquired across

the westernmost Ryukyu forearc region, has allowed Font et al.

(2001) to map the Ryukyu Arc basement. Beneath the forearc

W compression related to the collision between the

subducting slab and the root of the Eurasian lithosphere.

–

Based on OBS

W trend of the Ryukyu subduction system turns

northward, reaching a NW

258

Y. Font, S. Lallemand / Tectonophysics 466 (2009) 255 – 267

basins, the Ryukyu basement also displays a high-and-low

morphology. The two major basement highs are the Nanao and

Hoping Basement Rises ( Fig. 2 A). Both are flat-top rises about

2 km higher than the neighboring depression. The Nanao

Basement Rise is located along the northward prolongation of

the Gagua Ridge. The Hoping Basement Rise stands at about

122.2°E. Both Ryukyu basement rises stand above the rises

observed in the subducting PSP. Seaward, the Ryukyu basement

arc terminates at the northern rear of the accretionary prism, against

a set of transcurrent faults ( Fig. 2 BandC, Font et al., 2001 ).

Both the Nanao and Hoping Basement rises generated highs

on the bathymetry surface, called the Nanao Rise and the

Hoping Rise. Those rises delimit the 3 forearc basins ( Fig. 2 B

and C), shallowing westward ( Lallemand et al., 1997b; Font

et al., 2001 ). From east to west, they are the East Nanao Basin,

the Nanao Basin and the Hoping Basin. The older and highly

deformed Suao Basin underlies the Hoping Basin and lies on

top of the Hoping Basement Rise. In this study, we focus

specifically on the Hoping

Nanao area. Based on bathymetry

(see Lallemand et al., 1997b ), the Hoping Rise marks a 600-to-

700 m elevated surface between the forearc basins. The Hoping

Basin is connected to the Nanao Basin by the Hoping Canyon

that deeply incises the sedimentary cover of the Hoping Rise

( Fig. 2 C and B). At this location, based on multichannel seismic

reflection lines across the Hoping Rise, the Ryukyu Arc

basement is buried beneath 1000 to 2000 m of deformed

sediments (Hoping and Suao basins

–

—

Figs. 3 and 4 ).

3.5. Mapped near surface faults

In the studied region, from bathymetry and seismic reflection

analyses, several modes of deformation affect the forearc

structures.

First, the forearc region is dissected by major crustal strike-

slip faults (see Figs. 1 and 2 ). Those faults accommodate the

obliquity of the convergence, in a hand, and the rifting of the

southern Okinawa trough, in the other hand. Indeed, the

convergence obliquity between the PSP and the Ryukyu Arc

increases near Taiwan from 40°, east of 123°E, to 60°, west of

122.75°E ( Lallemand and Liu, 1998 ). As a partitioning result,

an aseismic trench-parallel transcurrent fault zone is observed

along the rear of the Ryukyu accretionary wedge, allowing the

southern part of the wedge to be dragged laterally toward

Taiwan ( Fig. 2 B, Dominguez et al., 1998 ). On seismic profiles,

the seaward termination of the Ryukyu basement is obscured by

diffractions that are caused by this transcurrent faulting. Since

the Ryukyu basement is never recognized seaward of this

obscured area, Font et al. (2001) suggested that transcurrent

faults developed at the seaward termination of the basement.

The transcurrent fault zone would therefore develop within the

sedimentary prism. The opening of the southern Okinawa

Trough has been quantified by GPS, evidencing the differ-

ential motion between the Ilan Plain and the Yonaguni Island

(123°E

Fig. 1 B). To accommodate the differential motion

Fig. 2. A. Ryukyu basement map below sea-level (sedimentary cover has been

removed, Font et al., 2001 ). HBR = Hoping Basement Rise; NBR = Nanao

Basement Rise. B. Regional geodynamic context of the southernmost Ryukyu

subduction system (modified after Font et al., 2001 ). H.R. = Hoping Rise; N.R. =

Nanao Rise; East N. B. = East Nanao Basin. C. Detailed bathymetry map of the

same area.

—

Y. Font, S. Lallemand / Tectonophysics 466 (2009) 255 – 267

259

Table 2

List and location of published multichannel and refraction seismics in the southernmost Ryukyu forearc region

between the two blocks, Lallemand and Liu (1998) have

proposed a N

Hoping and Suao sedimentary basins evidenced this N

–

S transform fault zone cutting through Ryukyu

Arc from the Southern Okinawa Trough to the accretionary prism

( Fig. 2 B). This fault zone offsets clockwise the Ryukyu Arc,

causing a bayonet-shape in the arc slope. A flower structure in the

transform fault on several seismic profiles (e.g. Figs. 3 and 4 ).

A second mode of deformation affects the Ryukyu Arc

basement. NNE

SSW normal faults are observed on the flanks

of both Ryukyu basement highs, offsetting the top of the

Fig. 3. Section of seismic profile EW-14 (after Font et al., 2001 ).

–

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