Yuka Kaiho

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.

Real-time geophysical measurements on the deep seafloor using submarine cable in the southern Kurile subduction zone

A permanent real-time geophysical observatory using a submarine cable was developed and deployed to monitor seismicity, tsunamis, and other geophysical phenomena in the southern Kurile subduction zone. The geophysical observatory comprises six bottom sensor units, two branching units, a main electro-optical cable with a length of 240 km and two land stations. The bottom sensor units are: 1) three ocean bottom broadband seismometers with hydrophone; 2) two pressure gauges (PGs); 3) a cable end station with environmental measurement sensors. Real-time data from all the undersea sensors are transmitted through the main electro-optical cable to the land station. The geophysical observatory was installed on the continental slope of the southern Kurile trench, southeast Hokkaido, Japan in July 1999. Examples of observed data are presented. Sensor noises and resolution are mentioned for the ocean bottom broadband seismometers and the PGs, respectively. An adaptable observation system including very broadband seismometers is scheduled to be connected to the branching unit in late 2001. The real-time geophysical observatory is expected to greatly advance the understanding of geophysical phenomena in the southern Kurile subduction zone.

PRELIMINARY RESULTS OF A THREE-YEAR CONTINUOUS OBSERVATION BY A DEEP SEAFLOOR OBSERVATORY IN SAGAMI BAY, CENTRAL JAPAN

Abstract A comprehensive long-term deep seafloor observatory was deployed at the plate boundary between the Philippine and North American plates in Sagami Bay, central Japan in 1993 in order to investigate the relations among geophysical parameters associated with earthquake swarms and eruptions of submarine volcanoes that have occurred repeatedly from at least 0.01 Ma to the present. It is also located within the habitat range of the vesicomyid clam, Calyptogena soyoae , the presence of which suggests seepage from below. The observatory, at a depth of 1174 m off Hatsushima Island, is equipped with multi-sensors, such as a seismometer, hydrophone, heat flow temperature probes, color video cameras, a CTD and a current meter. The data and power to and from the land station at Hatsushima Island are sent in real time through an 8-km-long electro-optical cable. More than 3 years of continuous geophysical and environmental data on the deep seafloor were collected up to 1996. In this paper, we describe the system and report on significant changes in ground temperature associated with three earthquake swarms in the adjacent area.

Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench

It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15–18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.

Off Hatsushima Island observatory in Sagami Bay: multidisciplinary long term observation at cold seepage site with underwater mateable connectors for future use

On the seafloor at the depth of 1175 m off Hatsushima Island in Sagami Bay, Central Japan,a cable-connected multi-disciplinary observatory was installed in 1993. Since then long-term real time observation has been carried out, experiencing replacement for upgrades in 2000, recovery and re-deployment for repair in 2002. This site is known as one of the most significant cold seepage sites with large chemosynthetic biological communities consisted mainly of Vesicomyid clams (Calyptogena). The upgraded second observatory is equipped with underwater mateable connectors (optical/electrical). The observatory revealed geophysical and biological events occurred on the seafloor, such as the mudflows and sedimentation generated by swarm earthquakes, spawning of clams triggered by water temperature change. However, several kinds of phenomena and technological problems yet to be neither identified nor solved still remain. As a next step, the observatory is planning to be utilized as a test bed, by using the underwater mateable connectors.

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.

Controlling factor of incoming plate hydration at the north-western Pacific margin

Hydration of the subducting oceanic plate determines the amount of water transported from Earth’s surface into its interior, and plate bending-related faulting (bend faulting) just prior to subduction is considered to promote hydration. Bend faulting shows significant spatial variation, but its contribution to hydration is still poorly understood. Here we present the results of controlled-source seismic surveys around the junction of the Japan and Kuril trenches. We found structural changes caused by bend faulting before subduction differed distinctly between both trenches and were well correlated with plate hydration after subduction, suggesting the bend faulting controls spatial variations in plate hydration. Differences in bend faulting are closely related to the angle between the current trench and the ancient spreading ridge, and the hydration is more extensive where this trench-ridge angle is oblique in the study area. Thus, we propose this angle is a major factor controlling plate hydration.Bend faulting prior to subduction is considered to promote hydration, yet the relationship remains ambiguous. Here, via analysis of seismic surveys from the Japan and Kuril trenches junction, the authors show that pre-subduction bend faulting and post-subduction plate hydration are strongly correlated.

Seafloor borehole broadband seismic observatories in the western pacific and performance of recovered seismic data

In 2000 and 2001, the seafloor borehole seismological observatories WP-1 and WP-2 in the northwestern Pacific were installed. The WP-1 site is in the west Philippine Basin, and the WP-2 observatory is situated on a normal oceanic Mesozoic crust in the northwestern Pacific Basin. The WP-1 observatory was activated in March 2002 using the ROV KAIKO and long-term observation was started. In June 2006, the new ROV KAIKO-7000II dived to the WP-1 (fourth visit) and recovered the data. At present, seismic records of 692-days (Mar. 2002 - Feb. 2004) have been obtained from the WP-1. The WP-2 observatory was activated in October 2000 using the KAIKO. In June 2005, the KAIKO-7000 made fourth visit to the WP-2 and recovered the data. Recording at the WP-2 has been suspended from the fourth ROV visit. In total, 436-days data (Oct. 2000 - Jan. 2001, Aug. 2001 - July 2002) were retrieved. The long-term variations of broadband seismic noise spectra (3 mHz -10 Hz) in oceanic basins were revealed. The stable ambient seismic noise levels enable us to record many teleseismic events uniformly in time. Especially, events with magnitudes greater than 6.5 were clearly recorded with a good signal to noise ratio.