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Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] Subduction zones are generally characterized by large earthquakes that contribute about 90% of the total seismic moment worldwide, and can cause great damage from tsunamis such as the 2004 Sumatra Mw 9.0 earthquake. Here, we report recurrent large subduction zone earthquakes revealed by extensive submarine liquefactions formed in the Nankai-Suruga trough where the Philippine Sea plate is subducting beneath the Eurasian plate. Field evidence and grain-size analysis of liquefied deposits show that the multiple-liquefactions were repeatedly induced by strong earthquake shaking during the Pleistocene in the eastern Nankai-Suruga subduction zone. The recognition and analysis of submarine liquefactions provide reliable paleoseismic evidence in the Nankai-Suruga subduction zone.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] The Nankai (Southern Sea, Japan)-Suruga trough is well-known as a Chilean-type subduction zone characterized by its low dip, enormous accretionary prime, and presence of great earthquakes (Figure 1) [Uyeda and Kanamori, 1979; Uyeda, 1991]. Historical and archeological evidence documents more than twenty large earthquakes occurred in the past thirteen centuries, including nine instrumentally-recorded earthquakes of M ≥ 6.9 along the Nankai-Suruga subduction zone [Ando, 1975; Kondo, 1985; Shiono, 1988; Samgawa, 1992; Seno et al., 1993; Sugiyama, 1994; Kamiichi et al., 2003]. The eastern Nankai-Suruga trough suffered an M 7.9 earthquake in 1923 which produced high-intensity shaking felt throughout the central Japan island of Honshu including Tokyo. A major gap in the great interplate earthquakes is identified in the eastern segment of the Nankai-Suruga trough where an M 8 class earthquake is expected to occur along the eastern segment of the Nankai-Suruga subduction zone in the near future [Ishibashi, 1981]. However, the absence of co-seismic ruptures and compelling geological evidence associated with past large earthquakes in this gap area has hindered further assessment of past long-term behavior of subduction zone earthquakes and related seismic hazard for the densely populated Nankai-Suruga coastal region. Seeking for the direct seismic record, we conducted a paleoseismic study by a field search for earthquake-induced liquefaction feature generated in the eastern Nankai-Suruga trough. Here, we report the field evidence that multiple-liquefactions were repeatedly induced by large earthquakes during the Pleistocene in the eastern Nankai-Suruga subduction zone and discuss the possible magnitude of the paleoseismic events.

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Figure 1. Geologic and tectonic maps of study area. (a) Geologic map of Udo Hill showing the liquefied sedimentary sequences (data from Kondo [1985]). Loc.1: Location of typical outcrop of liquefaction. (b) Tectonic setting of central Japan Island (plate movement rates from Seno et al. [1993]). (c) Idealized cross section of the Nankai-Suruga subduction zone at the study area.

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2. Tectonic Setting

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[3] The study region, Udo Hill, is located in the northeast end of the Nankai-Suruga trough where the Philippine Sea plate is subducting northwestward beneath the Eurasian plate at a rate of 3–5 cm/yr (Figure 1). The plate boundary extends from the Nankai trough through the Suruga trough to pass beneath Mt. Fuji from there, it follows the Sagami trough down a submarine canyon to the triple junction with the Pacific plate (Figure 1b). There are many onshore active faults and folding structures in the Udo Hill with a general trend parallel-subparallel to the northeastern segment of the Suruga trough (Figure 1a). The Udo Hill is mainly composed of Miocene-Pleistocene submarine sedimentary deposits which consist of interbedded gravel, silt-sand, and mudstone [Kondo, 1985]. All these sedimentary layers are weakly consolidated and have been folded as a dome structure with a general anticline axis striking northeast (Figure 1a).

3. Occurrence of Liquefied Deposits

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[4] Liquefaction features of previous large earthquakes are found in the Udo Hill over a large area of > 10 km2 in the Pleistocene submarine sedimentary sequences (Figures 12) . Numerous silt-sand network veins and dikes yellowish in color were developed in the grey, weak consolidated mudstone layers, and locally in the sand-gravel layers, that generally can be traced downward to the source silt-sand layers (Figures 24, auxiliary material Figures S1–S3). Most veins have a steep dip angle of >50° and sharp boundary with the country sediments. More than three liquefied silt-sand layers are interbedded with unliquefied mudstone and sand-gravel layers (Figure 2). Each liquefied silt-sand layer is overlain by a mudstone layer 1–3 m thick and then capped by a weak- or undeformed sand-gravel layer that is 0.3–2 m thick. In a single liquefied layer, silt-sand veins formed by different events can also be recognized by its color and texture that old veins are cut irregularly by younger veins (Figure 3, auxiliary material Figure S2a). These textural features indicate that the liquefactions were repeatedly produced during the deposition of one sedimentary sequence of silt-sand-mud-gravel before the overlying liquefied silt-sand layer was deposited. Many silt-sand veins and dikes extend upward and blindly terminate in the overlying mudstone layer, occurring as much as 3 m above the source silt-sand layer (Figure 2, auxiliary material Figures S1–S3). An interval between single veins or dikes is generally less than 1 m. A single vein or dike varies from a few millimeters to 50 centimeters in width and >5 m long in horizontal on the outcrops. Sediment within these veins is generally structureless although local flowing lamellas parallel to sidewalls of vein can be observed. The ends of veins generally show a round shape and sharp contact with little or no evidence of weathering or erosion after formation (Figure 2, auxiliary material Figure S2b). Some gravels of 3–10 cm in diameter are involved in the silt-sand injection veins, which are oriented parallel to the sidewalls of the vein. Locally, silt-sand veins of up to 50 cm wide were developed along echelon-oriented faults, and these veins can be traced for >30 m in horizontal (Figure 4, auxiliary material Figure S3). The sidewalls of the vein bounded by the fault plane are generally straight and dip steeply. Such geometric features of the faults are similar to those of co-seismic surface ruptures produced by recent large inland earthquakes [Lin, 1997; Lin et al., 2001a, 2002]. This structural relationship between the faults and injected veins may indicate that these echelon-oriented silt-sand veins formed by injection of liquefied silt-sand along the co-seismic ruptures during faulting and strong earthquake shaking.

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Figure 2. (a) Photograph and (b) its sketch showing typical outcrop of liquefied sedimentary sequences in the study area. There are at least three liquefied silt-sand layers which are interbedded with the unliquefied mudstone and sand-gravel layers.

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image

Figure 3. (a) Photograph and (b) its sketch showing the networks of silt-sand veins and dikes formed by liquefaction. The older veins are cut by the later veins and dikes.

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image

Figure 4. Photograph showing the occurrence of silt-sand veins and dikes formed by liquefaction and developed along en echelon-oriented faults. Arrows indicate the fault traces. The hammer (33 cm long) in the center indicates the scale.

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[5] The liquefaction-related sediments are composed of a series of silt-sand-mud-gravel sequences, which are considered to have been deposited along a delta front trough in the forearc basin of the Nankai-Suruga subduction zone during the Pleistocene at a shallow depth of a few tens of meters under sea level owing to the presence of submarine fossils such as coral and clam (Figure 1c). The grain-size distribution of silt-sand shows that the liquefied silt-sand layers consist of fine-grained silt and sand, 85 wt% of which are <0.1 mm in grain size (Figure 5). In contrast, the unliquefied mudstone is composed of up to 98 wt% silt-sand of <0.1 mm in grain size (Figure 5). The fine-grained materials of the mudstone layer may play a role of less-permeable cap in preserving high pore pressures within underlying cohesionless silt-sand layer to produce liquefaction.

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Figure 5. Cumulative frequency curve of grain size of the liquefied silt-sand and the unliquefied mudstone layers.

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4. Discussion and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[6] On the basis of the liquefaction features described above, we conclude that the enormous silt-sand veins and dikes were repeatedly produced by liquefaction caused by strong earthquake shaking in the submarine unconsolidated sediments during the Pleistocene. Other non-earthquake-origins considered for the submarine liquefactions including floods, sea floor landslides, storms, and tsunamis can be rejected owing to the following six main features of the silt-sand veins, namely: (1) extensive networks and dikes injected in weak- or un-deformed overlying beds, (2) upward injection which can be traced back to the source liquefied layer, (3) multiple-veins formed in discrete events in the same location, (4) multiple liquefied layers developed in different sedimentary sequences, (5) injection veins followed the echelon-oriented faults, and (6) similar features observed over a large area of >10 km2. Such earthquake-induced liquefaction features are consistent to those reported as others [Obermeier et al., 1985; Marco et al., 1996; Tuttle and Schweig, 1996].

[7] The above stratigraphic evidence shows that the submarine liquefactions formed in the Nankai-Suruga trough at shallow depths of <10 m under the sea floor which is equal to the thickness of one silt-sand-mud-gravel sequence. The submarine strata, including the liquefied sedimentary sequences formed in the forearc basin of the Nankai-Suruga trough, have been uplifted to ∼300 m in elevation (top of the Udo Hill) and folded with NE-striking anticlinal ridges during the past 100 ka due to northwestward subducting of the Philippine Sea plate beneath the Eurasian plate (Figure 1) [Kondo, 1985]. Historical records and paleoseismic studies document that the southwestern Japan island coast of the upper plate along the Nankai-Suruga subduction zone was widely uplifted by large earthquakes during the Holocene [Ishibashi, 1985; Maemoku and Tsubono, 1990; Sugiyama, 1994]. Large earthquake intervals of 1–2.5 centuries have been estimated during the past thirteen centuries in the Nankai-Suruga trough [Sugiyama, 1994]. The uplift of the Udo Hill was probably caused by co-seismic movement during the late Pleistocene accompanying large subduction zone earthquakes in the Nankai-Suruga trough. The extensively and densely distributed silt-sand network veins and dikes described here are indicative of such large subduction zone earthquakes.

[8] The most important factors contributing to earthquake-induced liquefaction are the amplitude of cycle shear stresses and the number of applications of these shear stresses related to earthquake magnitude [National Research Council, 1985]. Although liquefaction features can develop at magnitudes as low as 5 [Audemard and de Santis, 1991], the earthquake-induced liquefactions reported in Japan are mainly associated with magnitudes of M ≥ 6 [Kuribayashi and Tatsuro, 1975]. Recently two large earthquakes in Japan, the 1995 Kobe Mw 7.2 and 2000 Western Totori Mw 7.3 earthquakes, each caused sand boils and flowages of artificially-buried sand-soil in lowlands that were mainly concentrated in the deformation zone associated with the seismic fault zone [Lin, 1997; Lin et al., 2001b]. The characteristics of large scale, densely distributed network veins and dikes indicate that their formation requires strong shaking that increased pore water pressure and reduced effective stress to produce liquefactions in a large submarine area. The fault-related injection veins indicate that the liquefactions were probably accompanied with distinct co-seismic surface ruptures that were mainly produced by large earthquakes of M ≥7 in Japan island [Matsuda, 1975]. Magnitude 7 therefore is considered as a conservative lower bound for the submarine liquefactions described here. There are three historic earthquakes of M > 8, occurring in 1498, 1707, and 1854, each ruptured the entire Nankai-Suruga subduction zone up to the eastern end of the Nankai-Suruga trough for a length of >500 km [Sugiyama, 1994]. The instrumental-recorded 1944 M 8.1 earthquake is also documented to rupture throughout the same area as that of 1498, 1707, and 1854 events [Ishibashi, 1981]. The extensive submarine liquefactions probably record similar great subduction zone earthquakes of magnitude ≥8 during the Pleistocene as those of the historic earthquakes. This study shows that large earthquakes occurred repeatedly in the Nankai-Suruga trough during Pleistocene. Additional paleoseismic studies are requested to infer a more precise magnitude and recurrent interval for the liquefaction-induced great subduction zone earthquakes in the Nankai-Suruga trough.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[9] The author thanks J. Suppe of the Princeton University and K. Kano of Shizuoka University for constructive discussion and T. Maruyama for assistance in the field. Z. Wang and an anonymous reviewer improved this paper with their valuable and critical suggestions. This contribution was inspired by the discussions with students and staffs of the Shizuoka University and Princeton University where the author spent one year as a visiting scholar. This work was supported by the Science Project (1818340158) of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information
  • Ando, M. (1975), Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan, Tectonophysics, 27, 119140.
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonic Setting
  5. 3. Occurrence of Liquefied Deposits
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

Auxiliary material for this article contains three figures showing the detail occurrence of silt-sand veins and dikes observed in the Udo Hill, central Japan.

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grl22365-sup-0002-fs01.jpgimage/pjpeg2828KFigure S1. Photograph and sketch showing the occurrence of the liquefied silt-sand layer.
grl22365-sup-0003-fs02.jpgimage/pjpeg5279KFigure S2. Photographs showing the occurrence of the liquefied silt-sand layer.
grl22365-sup-0004-fs03.jpgimage/pjpeg3641KFigure S3. Photograph showing the occurrence silt-sand veins injected in the fractures.

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