Variation of physical properties beneath a fault observed by a two‐ship seismic survey off southwest Japan

Tsuru, Takeshi Go; Miura, Seiichi; Park, Jin-Oh; Ito, Aki; Fujie, Gou; Kaneda, Yasufumi; No, Tetsuo; Katayama, Takuya; Kasahara, Junzo

doi:10.1029/2004jb003036

Cited by 13 publications

(18 citation statements)

References 32 publications

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“…One of the characteristic structures off the Kii Peninsula is a plutonic rock body with high seismic velocity and high density, which is coincident with the segment boundary of megathrust earthquakes along the Nankai Trough (Kodaira et al 2006). A topographic high in the TOC can be recognized to the west of the aftershock area in the MCS reflection survey obtained along the strike direction of the subducting plate (line COP2 in Tsuru et al (2005)). A boundary of the ocean floor topography (the purple dashed line in Fig.…”

Section: Earthquake Cycle Simulation Based On the Hierarchical Asperimentioning

confidence: 99%

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The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

Hyodo

Nakanishi

Yamashita

et al. 2018

Prog Earth Planet Sci

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Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (M w 5.9, M JMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (M w 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region.

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Section: Earthquake Cycle Simulation Based On the Hierarchical Asperimentioning

confidence: 99%

“…5. The thin black line represents the location of MCS survey line (COP2), and the thick black segment represents the location of a topographic high (Tsuru et al 2005). The purple dashed line indicates the boundary of the topographic high in the basin.…”

Section: Earthquake Cycle Simulation Based On the Hierarchical Asperimentioning

confidence: 99%

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

Hyodo

Nakanishi

Yamashita

et al. 2018

Prog Earth Planet Sci

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“…Data processing for the 2D seismic reflection data involved filtering, velocity analysis, stacking, deconvolution, and post-stack migration (Yilmaz and Doherty 2001). These 2D seismic profiles show the geological structures of the accretionary prism (inner and outer wedges) off Kumano (ODKM-B; Figure 6a The deeper part of the mega-splay fault system (>6 km below the seafloor) is well defined as a prominent single reflector on the analyzed seismic profiles off Kumano, as shown in Figures 2 and 6a,b (Park et al 2002;Tsuru et al 2005;Bangs et al 2009;Moore et al 2009). The seismic velocity derived from waveform tomography (Figure 2b) demonstrates that the footwall of the deep mega-splay fault has low P-wave velocity due mainly to an overpressured state and a lithology difference (Figure 2c; e.g., Tsuji et al 2014).…”

Section: Seismic Datamentioning

confidence: 99%

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

2014

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We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.

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“…Seismic reflection surveys Tsuru et al, 2002Tsuru et al, , 2005Kodaira et al, 2012;Nakamura et al, 2013] combined with previous drilling expedition results [Arthur and Adelseck, 1980] and the results of the JFAST expedition [Chester et al, 2013b;Nakamura et al, 2013] show that the incoming Pacific Plate is composed of a basement of basaltic rocks overlain by a seismically resolvable unit of chert and a sequence of pelagic clay and hemipelagic mudstone (Figure 2). Both the basement and the entire sedimentary section are cut by normal faults.…”

Section: Tectonic Settingmentioning

confidence: 99%

Structure and lithology of the Japan Trench subduction plate boundary fault

et al. 2015

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The 2011 Mw9.0 Tohoku‐oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite‐rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay‐rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary décollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity‐strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity‐weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin.

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Variation of physical properties beneath a fault observed by a two‐ship seismic survey off southwest Japan

Cited by 13 publications

References 32 publications

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

Structure and lithology of the Japan Trench subduction plate boundary fault

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