Tetsuo No

The 2011 Tohoku-Oki earthquake: displacement reaching the trench axis.

Vertical and horizontal displacement that occurred up to the Japan trench likely contributed to formation of the tsunami. We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (MW) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.

Seismic imaging of a possible paleoarc in the Izu-Bonin intraoceanic arc and its implications for arc evolution processes

Crustal evolution processes in intraoceanic arcs, including crustal accretion and rifting, have been long discussed. To examine crustal evolution in the Izu-Bonin intraoceanic island arc, we conducted an active-source wide-angle seismic study along a north-south profile (500 km long) within a possible paleoarc in the rear arc (i.e., the Nishi-shichito ridge) about 150 km west of the present-day volcanic front. In this study, the seismic velocity and reflectivity images are obtained using the wide-angle seismic data. For the seismic velocity imaging, we applied refraction tomography in which 93,535 picks were used. The overall root-mean-square (rms) misfit calculated from the initial model of the refraction tomography was 483.1 ms, and those calculated from the final model were reduced to 66.7 ms. The resultant seismic image shows marked variations of crustal thickness along the seismic profile: thin crust (10–15 km thick) in the northern part, three discrete thick crustal segments (20–25 km thick) in the central part, and a moderately thick crust (∼15 km thick) in the southern part. These variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6.0–6.8 km/s. This variation of crustal thickness does not correlate with seafloor topography, which is characterized by post-Miocene across-arc seamount chains. It does correlate well with crustal variations observed along the present-day volcanic front of the Izu-Bonin arc. These findings suggest that the magmatic activity that created the across-arc seamount chains had little effect on the rear-arc crust and that the main part of the rear-arc crust was created before the rear arc separated from the volcanic front. By correlating the structural variations along the rear arc (i.e., the variation of the average seismic velocity as well as the thickness of the middle crust) and those along the present-day volcanic front, we found that the direction of rifting to separate the rear arc (paleoarc) from the present-day volcanic front was north-northeast.

Seismic constraints of the formation process on the back‐arc basin in the southeastern Japan Sea

To clarify the formation process of the back-arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa-shima Island. The crust beneath the northern Yamato Basin off Awa-shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4–6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high-velocity lowermost crust in the northern Yamato Basin with 7.2–7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25–26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

In situ stress state from walkaround VSP anisotropy in the Kumano basin southeast of the Kii Peninsula, Japan

To reveal the stress state within the Kumano basin, which overlies the Nankai accretionary prism, we estimated seismic anisotropy from walkaround vertical seismic profiling (VSP) data recorded at Site C0009 during Integrated Ocean Drilling Program (IODP) Expedition 319. We obtained the following anisotropic parameters: (1) P wave velocity anisotropy derived from azimuthal normal moveout (NMO) velocity analysis, (2) P wave amplitude variation with azimuth, and (3) axes of symmetry of S wave splitting. Azimuthal variations of P wave velocity by ellipsoidal fitting analysis showed that P wave velocity anisotropy within sediments of the Kumano basin was ∼5%. Both the directions of fast P wave velocity and strong amplitude are aligned with the convergence vector of the Philippine Sea plate. Furthermore, S wave splitting analysis indicated that S wave polarization axes were parallel to and normal to the direction of plate subduction. These results indicate that the maximum horizontal stress at Site C0009 in the Kumano basin is in the direction of plate subduction. The horizontal differential stress estimated from the P wave velocity anisotropy (2.7∼5.5 MPa) indicates that the maximum horizontal stress is similar in magnitude to (or a little higher than) the vertical stress.

Subsurface structure of the Myojin Knoll pumiceous volcano obtained from multichannel seismic reflection data

The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

Characteristics of deformation structure around the 2007 Niigata-ken Chuetsu-oki earthquake detected by multi-channel seismic reflection imaging

A multi-channel seismic reflection (MCS) survey was conducted to investigate the tectonic structure off Niigata, which caused the 2007 Niigata-ken Chuetsu-oki earthquake, using the research vessel (R/V) KAIREI of the Japan Agency for Marine-Earth Science and Technology. Based on the results of data processing and interpretation of available data, three areas are identified according to seismic characteristics. The most deformed area is located on the continental shelf near the source region of the 2007 Niigata-ken Chuetsu-oki earthquake, i.e., the area east of the Yoneyama-Ogi Uplifts. A remarkable growth of folds, including fault-related folds, and a strong reflector dipping east is identifiable by localized strain concentrations. The second area is located between the Yoneyama-Ogi Uplifts and the Jouetsu Knoll in the Toyama Trough. Although the deformation of deposits in the second area was smaller than in the first area, folds are identified. The third area is located toward the west of the Jouetsu Knoll in the Toyama Trough. No significant deformed structures developed in this area. Based on the interpretation of stratigraphy obtained in previous studies, seismic characteristics, and well data, the development of an anticline was initiated by a compression field after about 3.6 Ma. In particular, the deformation of sedimentary layers by the compression field occurred rapidly after about 1.3 Ma. Folds have grown larger toward the east after about 1.3 Ma. In addition, subsidence of about 0.2 s in sedimentary layers can be seen at the western margin of the Yoneyama-Ogi Uplifts, suggesting that tectonic movement related to reverse faulting has advanced there very recently. From aftershock distribution on a depth section on line S-2 near the hypocentral region, most hypocenters were determined to be below the strong reflector. This result suggests that fold growth has accompanied past large earthquakes, such as the 2007 Niigata-ken Chuetsu-oki earthquake.

Geophysical imaging of subsurface structures in volcanic area by seismic attenuation profiling

Geophysical imaging by using attenuation property of multichannel seismic reflection data was tested to map spatial variation of physical properties of rocks in a volcanic area. The study area is located around Miyakejima volcanic island, where an intensive earthquake swarm was observed associated with 2000 Miyakejima eruption. Seismic reflection survey was conducted five months after the swarm initiation in order to clarify crustal structure around the hypocenters of the swarm activity. However, the resulting seismic reflection profiles were unable to provide significant information of deep structures around the hypocenters. The authors newly applied a seismic attribute method that focused seismic attenuation instead of reflectivity to the volcanic area, and designed this paper to assess the applicability of this method to subsurface structural studies in poorly reflective volcanic areas. Resulting seismic attenuation profiles successfully figured out attenuation structures around the Miyakejima volcanic island. Interestingly, a remarkable high-attenuation zone was detected between Miyakejima and Kozushima islands, being well correlated with the hypocenter distribution of the earthquake swarm in 2000. The high-attenuation zone is interpreted as a fractured area that was developed by magma activity responsible for the earthquake swarms that have been repeatedly occurring there. The present study can be one example showing the applicability of seismic attenuation profiling in a volcanic area.

Depth-varying structural characters in the rupture zone of the 2011 Tohoku-oki earthquake

Seismic, geodetic, and tsunami data of the 2011 Tohoku-oki earthquake (offshore Japan; moment magnitude, Mw 9.0) have revealed that large coseismic slip reached the trench axis. Moreover, a clear, depth-dependent variation in the source location between highand low-frequency seismic energy radiation was observed. However, depth-varying structural features in the rupture zone have not been well examined. We therefore processed seismic reflection data acquired along five profiles in the rupture zone and examined depth-varying structural characteristics. In the resultant seismic images were interpreted a low-velocity frontal prism, a reflective zone at the trenchward tip of the continental block, and subducted horst and graben structures. The frontal prism, which was imaged as a low-velocity (Vp 2.0–3.5 km/s) wedgeshaped unit with seafloor widths of 13.5–18 km north of 37.5°N, changed abruptly to an elongate sedimentary unit south of 37.5°N. Landward of the frontal prism, 30–80 km from the trench axis, a reflective zone was imaged above the subducted oceanic basement. Subducted horst and graben structures were clearly imaged beneath the frontal prism and the reflective zone, and they could be found to a depth of 25 km. The throws of the normal faults delineating the horst and graben structures become larger landward to as much as 2 km. Comparison of the seismic images, earthquake seismicity, and slip behaviors showed that slips of tsunami earthquakes occur along the plate interface where the frontal prism is well developed. Background seismicity along the plate interface may extend downward to the landward end of the frontal prism and it becomes active around 25 km depth extending down the subduction zone. INTRODUCTION The rupture process of the 2011 Tohoku-oki earthquake (offshore Japan) has been examined extensively using data from state of the art observational networks deployed both globally and locally. In the Japanese islands, these include dense seismic, geodetic, and tide-gauge station networks (e.g., Fujii et al., 2011; Ide et al., 2011; Lay et al., 2011; Sato et al., 2011; Iinuma et al., 2012). Although rupture models differ in detail among studies, a common feature of the slip distribution is that fault displacement of more than 50 m occurred in the shallowest part of the subduction interface beneath the middle slope of the Japan Trench. In addition to earthquake, tsunami, and geodetic studies, marine geological and geophysical studies, including time-lapse bathymetry and controlled source seismic studies, have presented clear evidence that the rupture along the plate boundary reached the seafloor at the trench axis (Fujiwara et al., 2011; Kodaira et al., 2012; Nakamura et al., 2013). Moreover, a clear difference in source location between highand low-frequency seismic energy radiation has been reported (e.g., Hara, 2011; Ishii, 2011; Koketsu et al., 2011; Koper et al., 2011; Simons et al., 2011). For example, inversion studies of tsunami waveforms, which reflect the source region of low-frequency radiation, show a concentration of large slip immediately landward of the trench axis (e.g., Fujii et al., 2011; Satake et al., 2013), whereas back-projection studies in which teleseismic data were used to map the source region of high-frequency radiation have shown that high-frequency energy primarily radiated from the deeper part of the rupture zone to the west of the hypocenter (e.g., Ishii 2011; Wang and Mori, 2011). Similar spatial variation in seismic wave radiation sources has also been observed in the rupture zones of other large megathrust earthquakes, such as the 2004 Sumatra-Andaman (moment magnitude, Mw 9.2) and the 2010 Chile (Mw 8.8) earthquakes; that is, the source region of high-frequency radiation was distributed mostly in the deeper portion of the megathrust fault, whereas large slip occurred in the shallow portion where little high-frequency seismic energy was emitted (Lay et al., 2012). Along the Japan Trench, the 1896 Sanriku earthquake is another well-known example; during this earthquake, low levels of short-period seismic wave radiation emanated from the area close to the trench axis where the large fault slip occurred (e.g., Kanamori, 1972). Even though these depth-varying slip behaviors have been well documented, the structural features that control them have not been well examined. We therefore used newly processed prestack depth migrated images for deep seismic reflection data across the rupture zone of the 2011 Tohoku-oki earthquake and examined structural features along and around the plate boundary (Fig. 1). TECTONIC SETTING The structure and lithology of the Japan Trench subduction zone have been intensively investigated with seismic surveys and ocean drilling for the past three decades. In studies of seafloor topography and seismicity, the Japan Trench forearc region was divided into four areas: a deep-sea terrace, a steep GEOSPHERE GEOSPHERE; v. 13, no. 5 doi:10.1130/GES01489.1

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal‐faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

Visualization of attenuation structure and faults in incoming oceanic crust of the Nankai Trough using seismic attenuation profiling

Seismic attenuation properties were tested as indicators of lateral variation in geological structures and detection of faults within poorly reflective oceanic crust, on a seismic survey line along the Nankai Trough. We can specify both sedimentary structures by configuration of reflections and faults by offsetting of reflections on seismic reflection profiles. This procedure is often applied to analyze geological structures and existence of faults within sedimentary layers; however, it is almost impossible to analyze them within igneous oceanic crust because seismic reflections are inherently invisible there. Therefore, we applied seismic attenuation profiling to visualize geological structures and faults within poorly reflective oceanic crust. As a result, oceanic crust altered by late-coming volcanisms as well as damaged by intraplate earthquakes was imaged as extremely high-attenuation property, which was clearly distinguished from normal oceanic crust. Many faults were observed in the sedimentary unit on the seismic reflection profile, whereas possible lower segments of the faults were imaged as high-attenuation stripes in the oceanic crust on the seismic attenuation profile. Thus, the effectiveness of seismic attenuation profiling to structural and fault imaging within poorly reflective oceanic crust was successfully demonstrated.