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Birth of a major strike-slip fault in SW Japan Marc-Andre Gutscher* and Serge Lallemand Laboratoire de GeÂophysique, Tectonique et Sedimentation, University of Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 05, France

ABSTRACT A 500 km-long strike-slip fault, the North Chugoku Shear Zone Tectonic Line (MTL), near the Miocene volcanic arc, the central (NCSZ) is identified in SW Japan, responsible for four M 4 7 segment in Shikoku has shown little seismicity for the last 1000 earthquakes in the past 130 years. A new geodynamic mechanism years. Pure dextral strike-slip focal mechanisms for 15 modern of increased interplate coupling above an obliquely subducting and historical events indicate that part of the transcurrent motion flat slab is presented to explain the transfer of trench parallel is being transferred from the MTL to the NCSZ. motion 400 km inland from the , to the level of the Quaternary adakitic volcanic arc. While Quaternary dextral strike- Terra Nova, 11, 203±209, 1999 slip motion is widely believed to have occurred along the Median

10 6 greater). This agrees with qualita- Takemura, 1993; Kanaori et al., 1994), Introduction tive correlations established by pre- continuing to the SW to Kyushu (Ka- There has been much debate on the vious workers (Jordan et al., 1983; mata and Kodama, 1994; Itoh et al., causes and location of slip partitioning Smalley et al., 1993). In cases of oblique 1998), where the fault trace intersects in obliquely convergent tectonic set- convergence, this increased interplate the active volcanic arc (Fig. 1). Recent tings (Fitch, 1972; Jarrard, 1986; De- coupling can detach a major forearc GPS studies also indicate modern slip Mets, 1992; McCaffrey, 1992, 1996; sliver and entrain it parallel to the along the MTL (Itoh et al., 1998; Le Pinet and Cobbold, 1992; Lallemand, trench. Seismicity along the major Pichon et al., 1998; Ozawa et al., 1999). 1999; Chemenda et al., in press). We bounding strike-slip fault can be quite However, in the last 1000 years, the present a model of strain partitioning high, with shallow M 4 7 events occur- central segment of the MTL on Shiko- near the volcanic arc based on observa- ring on average 2±3 times per century. ku has shown little to no seismicity tions from the North Andean margin This is the case in Ecuador, where (Research Group for Research Group (Gutscher et al., 1999a), and apply it to Carnegie Ridge, an oceanic plateau for Active Faults of Japan, 1980; Tsut- explain the pattern of seismicity in SW representing part of the Galapagos hot- sumi et al., 1991). In contrast to the Japan. A recent study of oblique sub- spot track, subducts beneath the MTL, western Honshu (Chugoku) has duction using analogue modelling Northern Andes (Pennington, 1981; been seismically active (Wesnousky et (Chemenda et al., in press) demon- Gutscher et al., 1999a). It produces an al., 1982; Oike and Huzita, 1988; Shen- strates interplate friction to be an im- intermediate-depth seismic gap, and Tu et al., 1995) (Fig. 2). Similar to portant factor controlling strain parti- apparently supports a 400 km-wide flat Ecuador (Gutscher et al., 1999a), shal- tioning. It also showed that interplate slab segment. Approximately 400 km low seismicity is aligned along a Qua- pressure (e.g. caused by steep or flat inland from the trench, major dextral ternary adakitic volcanic arc (Morris, subduction) is the primary factor con- strike-slip motion occurs along several 1995) far inland. trolling the tectonic regime of the upper fault splays of the Dolores±Guyaquil plate. Subduction angle, and more spe- megashear at about the level of the Flat subduction beneath SW Japan cifically the unusual subduction geome- adakitic volcanic arc (Kellogg and try known as `flat subduction' (Bara- Vega, 1995; Gutscher et al., 1999a). Along the Shikoku margin and below zangi and Isacks, 1976; Sacks, 1983; adjacent SW Honshu, flat subduction Cahill and Isacks, 1992; Gutscher et occurs (Nakanishi, 1980; Hirahara, Oblique convergence and al., 1999b), can modify interplate cou- 1981) and has been attributed to the transcurrent motion pling by up to an order of magnitude. A relatively young age (5 20 Myr) of the statistical analysis of shallow (5 70 km Plate kinematic data indicate the con- Shikoku Basin lithosphere (Sacks, 1983) depth) upper plate seismicity per- vergence vector between the Philippine as well as buoyancy effects from the formed along the length of the entire Sea Plate and SW Japan at the Nankai flanking Izu Bonin Arc to the east and Andean chain (Gutscher and Mala- trough to be N518W at 4.6 cm yr71 the Palau-Kyushu Ridge (the proto Izu- vieille, 1999; Gutscher et al., in press), (Seno et al., 1993) (Fig. 1). Geodetic Bonin arc) to the west (Hirahara, 1981). reveals that seismic energy released studies suggest a vector of N568W The flat slab beneath SW Japan is not 250±800 km from the trench is on aver- (Shen-Tu et al., 1995) to N718W (Ha- well constrained by hypocentre data age 3±5 times higher above flat slab shimoto and Jackson, 1993). Conver- (Fig. 2), since the segments than for adjacent steep slab gence is thus 20±408 oblique to the slab beneath Shikoku and Chugoku is segments (in some extreme cases over margin normal direction. Geological essentially aseismic below 60±80 km data suggest that dextral strike-slip mo- depth. However, seismicity cross-sec- Correspondence and present address: tion has occurred throughout the Qua- tions beneath Kyushu and Shikoku, GEOMAR, Marine Geodynamics, Wisch- ternary along the Median Tectonic reveal dramatic differences (Fig. 2). hofstr. 1±3, D-24148 Kiel, Germany. E- Line (MTL) in Shikoku (Kanaori, While the oceanic plate beneath Kyushu mail: [email protected] 1990; Tsutsumi et al., 1991; Itoh and reaches depths of 200 km at a distance of

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Birth of a major strike-slip fault, Japan . M.-A. Gutscher and S. Lallemand Terra Nova, Vol 11, No. 5, 203±209 ...... Flat Slab 126ûE 130ûE (aseismic) 140ûE

Japan Sea Honshu S. Korea Arc NCSZH Volc. Y N 35ûN Ch A Adakitic

MTL Bonin - ArcIzu Sh

Unz.V Ky

Nankai Trough

Palau Kyushu- Philippine Ridge Sea Plate 30ûN

(backarc basin)

Ryukyu Arc

Ryukyu Trench

Fig. 1 Tectonic setting of the SW Japan region showing oblique, NW-directed subduction of the Philippine Sea Plate and active volcanoes (shaded triangles) and the zone of flat subduction (shaded light grey). Dextral strike-slip motion is partitioned along the North Chugoku Shear Zone (NCSZ), parallel to the adakitic arc, as well as along the Median Tectonic Line (MTL), the former Miocene arc. Sh = Shikoku Island, Ky = Kyushu Island, Ch = Chugoku peninsula, A = Arima-Takatsuki Tectonic Line, H = Hanaore Fault, Y = Yoro Fault, N = Neodani Fault., Unz.V = Unzen Volcano.

& 300 km from the trench, the slab 1995) also place the Philippine Sea coast (along the southern margin of the beneath Shikoku has only reached 60± Plate at a shallow depth consistent with Japan Sea) with four large M 5 7earth- 80 km depth at the same distance. The low-angle subduction. These observa- quakes in the last 130 years (Fig. 3). arc-trench gap is also much greater (400 tions, combined with the adakitic vol- Another 8 strong earthquakes (6.0 4 M km) for the Shikoku section. canic arc, parallel to, but 400 km dis- 4 6.9) have occurred along this zone since Tomographic data image the higher tant from the Nankai trough (Morris, 1710. There are historical records of M velocity Philippine Sea Plate slab des- 1995), provide strong evidence for a 600 4 7 earthquakes along this part of the cending steeply beneath Kyushu (Fig. km-wide flat slab region beneath SW Japan Sea coast dating back to AD700 2, Sect. A±A') and lying horizontally Japan in agreement with previous inter- (Oike and Huzita, 1988). The global, between 70 km and 100 km depth be- pretations (Nakanishi, 1980; Hirahara, modern (1964±95) ISC teleseismic dataset neath Shikoku and Chugoku (Fig. 2, 1981; Sacks, 1983; Oda et al., 1990; of Mb 4 4.0 earthquakes relocated by Sect. B±B') (Hirahara, 1981). The latter Morris, 1995). Engdahl et al. (1998), images this trend agrees with interpretations of S-wave (Fig. 2). Extensive regional catalogues phase data attributed to a flat-lying (Wesnousky et al., 1982; Shen-Tu et al., Seismicity in SW Japan slab at 50±70 km depth beneath 1995; Japan Meteorological Agency± Shikoku and Chugoku (Nakanishi, Historical and modern catalogues reveal a JMA data set 1926±96) and modern mi- 1980; Oda et al., 1990). Seismic data from 30 km-wide 600 km-long band of high croseismicity (M 5 1) surveys, spanning the eastern Nankai trough (Ishida, seismicity parallel to the north Chugoku July 1965±June 1984 (Oike and Huzita,

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36ûN B' 500 flat slab ad. arc 300 Shikuku Chugoku 200 34ûN high p-wave velocities 100 trench

A' 0 BB' 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200 500 Unzen

32ûN steep slab 400 Kyushu

A/B c.-a. arc 200 100 trench 30ûN high p-wave velocities 0

130ûE 132ûE134ûE 136ûE AA' surface historical seismicity M modern seismicity Mb -6000 -4000 -2000 0 300 1000 3000 elevation [m] earthquake 7.0 6.5 5.0 lvto nm in elevation 8.0 0 50 100 150 200 depth [km] 6.0 6.0 4.0

Fig. 2 Seismicity and relief (Smith and Sandwell, 1997) of SW Japan, with modern seismicity 1964±95 (Engdahl et al., 1998) and historical seismicity. Black triangles = active volcanic arc. Location and sampling boxes shown for two seismological sections illustrating different subduction styles of the Philippine Sea Plate. Boundaries of high p-wave velocities (oceanic slabs) from a tomographic study (Hirahara, 1981) are shaded grey. Note shallow upper plate seismicity along arc. A±A', WNW-orientated section beneath Kyushu, with calc-alkaline arc (c.-a. arc) above a steep slab at 120±150 km depth; B±B', NNW-orientated section beneath Shikoku and westernmost Honshu (Chugoku), showing a flat slab at 50±80 km depth. Adakitic arc located 400 km from the trench.

1988) and July 1976±June 1987 (Imoto, from Chugoku onto Kyushu and join 1 The overall distribution of seismicity 1991) all confirm this pattern of seismicity the prolongation of the MTL near the is orientated along a N708E-trending as well. While isolated segments of this Unzen volcano, where seismic activity line. All available focal mechanisms for band of seismicity have been studied and is also high (Fig. 2). In eastern Chugoku this region (Wesnousky et al., 1982; identified (the eastern segment by Ishika- prominent fault splays (Hanaore, Yoro Shen-Tu et al., 1995; Harvard CMT wa, 1995; the western segment by Ka- and Neodani fault systems and the Catalogue; Dziewonski et al., 1981) naori, 1998a, 1998b) this activity has never Arima-Takatsuki Tectonic Line) con- show nearly pure strike-slip motion, been attributed to a continuous large- nect the two systems (Figs 1, 3). The with one fault plane aligned parallel scale structure. We propose that this seis- latter dextral splay was activated dur- to the coast (roughly N708E) (Fig. 3). micity is due to a transcurrent fault sys- ing the destructive 1995 Kobe earth- 2 Aftershock distribution and fault tem, hereafter named the North Chugoku quake (Pollitz and Sacks, 1997). plane motion from two large historical Shear Zone (NCSZ). earthquakes: The region between the NCSZ and 1943 M=7.4 Tottori earthquake: After- the MTL is marked by conjugate Evidence for dextral slip along shocks are distributed along a & 40 N508E dextral and N608W sinistral NCSZ km-long, N708E-trending zone (Kana- morphological fault fabric (Oike and mori, 1973). The displacement of trian- Huzita, 1988; Kanaori, 1990; Itoh and Several lines of evidence indicate a pri- gulation points indicates 2.50 m of dex- Takemura, 1993) (Fig. 3 inset). The marily dextral sense of motion along tral slip along the N708E focal plane NCSZ system appears to cross SW the NCSZ. (Kanamori, 1973).

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M6.8 1952 in this region) are well documented M6.9 1858 (Otsuki and Ehiro, 1978; Itoh and Ta- M7.3 1948 kemura, 1993), and clearly seen in the M 6.9 1963 M6.5 M7.8 morphology (Fig. 2) and satellite M7.4 1949 1927 M6.2 1943 images (Kanaori et al., 1994). These 1983 Cretaceous age, sinistral strike-slip M5.5 1991 NCSZ MitF M5.6 1978 faults with 10±40 km of displacement prior to 80 Ma (Otsuki and Ehiro, M7.4 M5.2 1997 YamF 1872 M6.1 1930 1978) show evidence for present day M5.8 Kobe dextral reactivation based on field stu- 1997 Chugoku M6.9 1995 dies of several fault outcrops (Kanaori, 1998a, b). MTL

M5.2 Geodynamic model 1991 Shikoku Because the interplate contact area (5 400 km down-dip length) is greater MTL for flat slab regions (Fig. 4) stress is transferred to the upper plate at Unzen volcano NCSZ fixed greater distances (400±800 km) from the trench (Gutscher and Malavieille,

s1 s1 1999; Gutscher et al., in press). Flat ? subduction also perturbs the typical Kyushu subduction-related calc-alkaline mag- matism and commonly shuts off volcan- ism completely (Pilger, 1981, 1984; MTL McGeary et al., 1985). Alternatively, a broad adakitic arc can develop such as Phil. Sea Plate in Ecuador (Abbott et al., 1994; Morris, 1995; Monzier et al., 1997; Gutscher et al., Fig. 3 Focal mechanisms for all recent, shallow (depth 5 70 km), M 4 5 earthquakes in 1999a). Prior to cessation of volcanism, SW Japan (Harvard, CMT). Available focal mechanisms for large historical (pre 1976) adakitic magmatism extending into the earthquakes also shown (Wesnousky et al., 1982; Shen-Tu et al., 1995). Events foreland also occurred above the Chile interpreted to be related to the NCSZ (see Table 1) shown in black with magnitude and flat slab region (Kay and Abbruzzi, 1996). year given. Others are shaded grey, incl. the M=6.9 Kobe 1995 event. YamF, Yamasaki In cases of oblique subduction, strain Fault; MitF, Mitoke Fault. Inset shows roughly E±W orientation of s1 (maximum partitioning commonly occurs at the compressive stress) in the Chugoku block due to its position between the MTL and level of the volcanic arc which is an NCSZ, producing sets of conjugate N508E dextral and N608W sinistral faults. inherent lithospheric weakness. One of the best examples is the Great Suma- tra fault (Bellier and Sebrier, 1994, 1927 M=7.8 Tango earthquake: Post quake: This event reactivated the wes- 1995). In the Philippines (Philippine seismic surveys indicate both conjugate tern end of the Tottori 1943 rupture Fault), S. Colombia (Dolores Guaya- fault planes were activated simulta- zone (Oike and Huzita, 1988; Imoto, quil Megashear) and Kyushu (Kyushu neously (Kanamori, 1973). The 1991). Microseismicity recorded over a Tectonic Line), strike-slip motion also N208W trending Gomura fault showed two year (Oike and Huzita, 1988) and occurs along the calc-alkaline volcanic 2.50 m of sinistral motion, while the four year (Imoto, 1991) period shows a arc. When flat subduction perturbs the N708E-trending Yamada fault showed N708E trend. system causing volcanic activity to mi- 0.70 m of dextral motion. However, 28 August 1991 M=5.5 W Chugoku grate inland (Kay and Abruzzi, 1996) post-seismic uplift was greatest for the Mtns. earthquake: Aftershocks (JMA and when the increased interplate cou- Yamada fault (5 cm vs. 1 cm) and Data Base) are orientated roughly E± pling causes upper plate deformation to persisted for over 20 years (compared W and thus favour activation of the be localized further from the trench, to & 3 years for the Gomura fault) N708E focal plane in a dextral sense. part of the transcurrent motion may (Kanamori, 1973). 25 June 1997 M=5.8 N. Yamaguchi be transferred from the extinct volcanic 3 Aftershock distribution and fault Pref. earthquake: Aftershocks (JMA line to the new volcanic line further plane motion from recent M 4 5 Data Base) are orientated NE±SW inland. This evolution is illustrated earthquakes: and focal mechanism shows dextral slip schematically in Fig. 4. This geody- 3 June 1978 M=5.6 (Mb5.2) earth- (Kanaori, pers. comm. 1999). namic model succeeds in explaining quake: 3 earthquakes of nearly equal 4 Shear-sense indicators in fault rocks the geological and seismological obser- magnitude Mb = 5.2, 4.9 and 5.1 in westernmost Chugoku: vations in SW Japan. In the Miocene, struck over the span of 1 h along an Prominent N508E-trending linea- the calc-alkaline volcanic arc was lo- ENE trend 20 km long. ments in westernmost Chugoku (paral- cated near the MTL (Stein et al., 1994) 30 Oct. 1983 M=6.2 Tottori earth- lel to our proposed trace for the NCSZ and transcurrent motion was concen-

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strike-slip fault trench

arc /continental lithosphere

oblique c. 60 km thick subduction steep slab

flat slab

oceanic lithosphere

c. 30 km thick upper plate

steep active calc-alkaline arc slab relic calc-alkaline arc

active adakitic arc

Fig. 4 Geodynamic model of strain partitioning related to oblique and flat subduction. The interplate interface is shaded light grey and is 3±4 times greater for flat slab than for adjacent steep slab regions. Viscous coupling thus acts over a greater surface area. Deformation is commonly concentrated at the level of the volcanic arc. An adakitic arc can develop at a greater distance from the trench and transcurrent motion can thus be transferred further inland thereby entraining a major forearc sliver. trated here. In the Quaternary an ada- deed, both high-resolution GTOPO30 to several hundred years). Considering kitic volcanic arc developed 400 km (Fig. 2) topography, as well as satellite the four M 4 7 earthquakes in the last from the trench, presumably in re- (SPOT) imagery image N708E linea- 150 years and a fault which is probably sponse to the change in style of subduc- ments near the NE coast of Chugoku. locked during the interseismic period, tion. The increased interplate coupling 3 The NCSZ appears to be a nascent several decades of GPS data would between the two horizontal litho- fault initiated during the Quaternary be necessary to obtain a statistically spheres has transferred part of the and has not yet produced substantial meaningful sampling of the total slip transcurrent motion to the NCSZ (4 20 km) displacement. (coseismic + postseismic) representa- whose pure dextral strike-slip activity tive of long-term deformation and plate follows the trend of the adakitic volca- A second question to pose is why tectonic motion. nic arc exactly. movement along the NCSZ has not Long-term triangulation and survey- If the NCSZ is such a major struc- been detected in numerous geodetic ing studies attempted to determine the ture, one can ask why little evidence of a studies. These include short-term GPS steady-state strain after seismic slip and large-scale fault has been identified at (Le Pichon et al., 1998; Ito et al., 1999; therefore `exclude the data taken before the surface. Several factors may contri- Ozawa et al., 1999) and long-term stu- large earthquakes' (Hashimoto and bute to explain this deficiency: dies integrating surveying data gathered Jackson, 1993). These studies thus con- over nearly one century (Hashimoto clude that `velocities of blocks on the 1 Movement along the NCSZ is caused and Jackson, 1993; Shen-Tu et al., Japan Sea coast and on the north of the by a velocity discontinuity at the base of 1995). Recent GPS studies have a typi- MTL do not exceed estimated errors' the lithosphere. Surface deformation cal period of observation of 1±2 years (Hashimoto and Jackson, 1993). Thus, may thus be distributed along a fault and the Japanese nationwide network, both short-and long-term geodetic stu- system on the order of 10±30 km wide. registering since 1994, has grown from dies suggest little to no postseismic slip 2 Along most of its length, the NCSZ & 200 to more than 900 stations (Ito et along the NCSZ. closely follows the trace of the Quatern- al., 1999; Ozawa et al., 1999). Despite However, the 15 M 4 5 strike-slip ary adakitic arc. Thus, lava flows may this excellent and dense coverage, the earthquakes, during the past 150 years, have masked the surface expression at critical factor remains the period of as well as historical records of M 4 7 several places, disrupting the long fea- observation (5 5 years) compared to earthquakes dating back to AD700 ture into several shorter segments. In- the seismic cycle (on the order of one (Oike and Huzita, 1988) indicate that

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Birth of a major strike-slip fault, Japan . M.-A. Gutscher and S. Lallemand Terra Nova, Vol 11, No. 5, 203±209 ...... substantial, dextral, coseismic slip oc- Tectonics Meeting, Royal Holloway, the inner belt of Southwest Japan. curs. We consider these large earth- London, April 1999, pp. 214±217. Tectonophysics, 177, 381±399. quakes (Fig. 3) to be genetically related Gutscher, M.-A., Malavieille, J., Lallemand, Kanaori, Y., Kawakami, S. and Yairi, K., to strain partitioning due to the oblique S. and Collot, J.-Y., 1999a. Tectonic 1994. Seismotectonics of the Median segmentation of the North Andean Margin. Tectonic Line in Southwest Japan; and flat subduction beneath SW Japan. Impact of the Carnegie Ridge collision. implications for coupling among major Earth Planet. Sci. Lett., 168, 255±270. fault systems. Pure Appl. Geophys., 142, Acknowledgements Gutscher, M.-A., Olivet, J.-L., Aslanian, D., 589±607. Maury, R. and Eissen, J.-P., 1999b. The Kanaori, Y., 1998a. Risk assessment of We thank Philippe Huchon and Yuji `lost Inca Plateau': Cause of flat destructive inland earthquakes by the Kanaori for constructive and critical re- subduction beneath Peru? Earth Planet. average release-rate of seismic moments views which helped improve the manu- Sci. Lett., 171, 335±341. ± an example from active fault systems in script. Figures 1, 2, 3 were constructed Gutscher, M.-A., Spakman, W., Bijwaard, the western Chugoku District, using GMT software. M.-A. Gutscher was H. and Engdahl, E.R., in press. southwest Japan. J. Japan Soc. Eng. supported by a Marie Curie TMR Research Geodynamics of Flat Subduction: Geol., 39, 287±297 (in Japanese with Grant from the European Union. Seismological and Tomographic English Abstract). Evidence From Peru and the Andean Kanaori, Y., 1998b. Seismic risk assessment References Margin. Tectonics. of active fault systems in the western Hashimoto, M. and Jackson, D.D., 1993. Chugoku district of southwest Japan. J. 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