Tetsuya Takeda

Induction of inflammatory arthropathy resembling rheumatoid arthritis in mice transgenic for HTLV-I

Human T cell leukemia virus type-I (HTLV-I) is the etiologic agent of adult T cell leukemia and has also been suggested to be involved in other diseases such as chronic arthritis or myelopathy. To elucidate pathological roles of the virus in disease, transgenic mice were produced that carry the HTLV-I genome. At 2 to 3 months of age, many of the mice developed chronic arthritis resembling rheumatoid arthritis. Synovial and periarticular inflammation with articular erosion caused by invasion of granulation tissues were marked. These observations suggest a possibility that HTLV-I is one of the etiologic agents of chronic arthritis in humans.

Mapping pressurized volcanic fluids from induced crustal seismic velocity drops

Seismic noise reveals volcanic plumbing Monitoring the way in which seismic noise passes through Earths crust after a large earthquake can clarify how volcanoes erupt. Japan has the highest-density seismic network in the world. Brenguier et al. observed reductions in seismic velocity below volcanic regions of Japan from before, to the weeks and months after the 2011 Tohoku-Oki earthquake (see the Perspective by Prejean and Haney). This indicates that pressurized fluids below volcanoes can weaken in response to dynamic stress perturbations. Science, this issue p. 80; see also p. 39 The stress response from a large earthquake can identify crustal areas with pressurized volcanic fluids. [Also see Perspective by Prejean and Haney] Volcanic eruptions are caused by the release of pressure that has accumulated due to hot volcanic fluids at depth. Here, we show that the extent of the regions affected by pressurized fluids can be imaged through the measurement of their response to transient stress perturbations. We used records of seismic noise from the Japanese Hi-net seismic network to measure the crustal seismic velocity changes below volcanic regions caused by the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake. We interpret coseismic crustal seismic velocity reductions as related to the mechanical weakening of the pressurized crust by the dynamic stress associated with the seismic waves. We suggest, therefore, that mapping seismic velocity susceptibility to dynamic stress perturbations can be used for the imaging and characterization of volcanic systems.

Extensional structure in Northern Honshu Arc as inferred from seismic refraction/wide‐angle reflection profiling

A recent extensive seismic wide-angle experiment revealed a new image of crustal and uppermantle structure across Northern Honshu Arc, Japan. The western part of the arc recorded the crustal deformation by the Miocene back arc spreading of the Sea of Japan. The crust is composed of highly deformed Tertiary sedimentary layers, a relatively low velocity (5.75–5.9 km/s) crystalline basement and a 15-km thick lower crust with a velocity of 6.6–7.0 km/s. Clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading. The crust attains the maximum thickness (32–35km) east of the backbone range for which the magmatic intrusion/underplating since 10–15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crust, probably remaining a stable block since the time of the backarc spreading.

Crustal structure in the northern Fossa Magna region, central Japan, modeled from refraction/wide-angle reflection data

The northern Fossa Magna (NFM) is a back-arc rift basin filled with thick Tertiary sediments, which show strong NW-SE shortening deformation. In the NFM, there exist two major active fault systems, the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL) and the Western Nagano Basin active fault system (WNB), both of which have great potentials to cause destructive earthquakes. By reanalyzing five sets of refraction/wide-angle reflection data, we successfully obtained detailed and consistent models of the crustal structure in the NFM region. It was a very effective modeling procedure to incorporate vicinal seismic reflection data and geologic information. The geometries of the active faults in the NFM region were revealed. The ISTL is east dipping, and the WNB is northwest dipping. The Tertiary sedimentary layer (<4.0 km/sec) west and adjacent to the ISTL extends to a depth of 4–5 km. The basement rocks below the Central Uplift Belt (CUB) form a wedge structure, which suggests the westward movement of the CUB basement rocks.

Seismic Evidence for Active Underplating Below the Megathrust Earthquake Zone in Japan

Quake Control Large earthquakes occur at the margins of two colliding plates, where one plate subducts beneath the other at a shallow angle. These megathrust earthquakes often cause destructive tsunamis owing to the displacement of large volumes of water at the fault along the plate boundary. Two related studies of the seismic structure of subduction zones attempt to reveal the underlying mechanisms of megathrust earthquakes (see the Perspective by Wang). Kimura et al. (p. 210) compared seismic reflection images and microearthquake locations at the Philippine Sea plate where it subducts obliquely beneath Japan. The locations of repeating microearthquakes correspond to active transfer of material from the subducting plate to the continent—a process only previously assumed from exhumed metamorphic rocks. Dean et al. (p. 207) observe an expansive structure in the sea-floor sediment near the location of the 2004 and 2005 Sumatra earthquakes in Indonesia that suggests sediment properties may influence the magnitude of megathrust ruptures and their subsequent tsunamis. Microearthquakes correlated to seismic data reveal the real-time dynamics of a subduction zone. Determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes. The peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate (underplating) at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging, but direct seismic evidence of this process is lacking. By comparing seismic reflection profiles with microearthquake distributions in central Japan, we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting Philippine Sea plate. This region coincides with the location of slow-slip events that may serve as signals for monitoring active underplating.

Fluid-driven seismicity activation in northern Nagano region after the 2011 M9.0 Tohoku-oki earthquake

The dynamic triggering of earthquakes is well documented; however, the underlying physical mechanisms remain obscure. Here we analyze the seismicity in northern Nagano, central Japan, following the Tohoku-oki quake, until the occurrence 13 h later of an Mw6.2 local earthquake. We use waveform detection techniques to identify 17 times more earthquakes than those in the Japan Meteorological Agency catalog. The activation of seismicity in the epicentral region of the Mw6.2 event is weak and delayed, culminating with the occurrence of the moderate shock preceded by two small foreshocks. The seismicity activation to the south is shallower, abundant, and starts during the passage of Tohoku-oki surface waves of high dynamic stresses. The early activation occurs in areas of relatively high near-surface fluid temperature, indicating that the dynamic triggering is likely caused by excitation of geothermal crustal fluids. The Mw6.2 Northern Nagano earthquake might have been delay-triggered by fluid migration from a deep source.

Migration of low‐frequency tremors revealed from multiple‐array analyses in western Shikoku, Japan

[1] Multiple-array observation above a belt-like tremor zone was conducted to investigate the detailed location and migration of tremor activity in western Shikoku, Japan. In March 2007, an episodic tremor and slip event occurred, and highly coherent waveforms were recorded at three arrays. Multiple signal classification analysis for the data from each array enabled measuring precise arrival directions. The majority of tremor signals suggested relatively low slowness. The arrival directions of tremor signals were used to locate tremor sources by the grid search method. Tracking the tremor activity showed that the tremor migrated within several hours in the northeast-southwest direction over a distance of 12-15 km, and its migration velocity was 1-2.5 km h -1 . This migration velocity is more rapid than the mean velocity of 0.5 km h -1 over the whole tremor episode lasting several days. Such a short-timescale migration may represent fluctuation of slip acceleration during the slow slip event. Whenever a tremor migrates southwestward, very low frequency earthquakes occur in the vicinity of the tremor migration terminus. This indicates that the tremor migration is related to the occurrence of very low frequency earthquakes and slow slip events.

Well-resolved hypocenter distribution using the double-difference relocation method in the region of the 2007 Chuetsu-oki Earthquake

The 2007 Chuetsu-oki Earthquake (Mj = 6.8) occurred in the eastern margin of the Japan Sea where high strain rates have been observed and many large earthquakes have occurred. The main shock was located near the Western Nagaoka Basin active fault system dipping in the westward direction. We estimated the well-resolved hypocenter location around the main shock rupture zone from the differential arrival times obtained by both manual picking and waveform cross-correlation analysis. From the relocated aftershock distributions, we successfully resolved the detailed fault structures activated by the main shock. The estimated fault model resolves four individual fault segments. The fault model suggests that the main shock predominantly ruptured the southeastward dipping fault planes. On the other hand, the aftershocks around the hypocenter of the main shock occurred on both the northwestward and southeastward dipping fault planes. Both fault planes around the hypocenter of the main shock may have been nearly coincidentally ruptured during the main shock.

Seismological and geological characterization of the crust in the southern part of northern Fossa Magna, central Japan

The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.

Fine fault structure of a moderate earthquake in the 2007 earthquake sequence of Northern Mie, Japan

A moderate crustal earthquake (Mj = 5.4) occurred in the northern part of Mie Prefecture, Central Japan, 2007. In order to clarify the fault structure of the main shock and its relation to the active faults, we estimated the precise hypocenter locations and focal mechanisms. For the relocation of the hypocenters, we used the differential time obtained by both manual picking and waveform cross-correlation analysis. The estimated detailed fault structure suggests that the main shock ruptured the southwestward dipping fault plane. We found that the faults of the moderate earthquake possessed complex fault segments. Around the largest aftershock hypocenter, a subsidiary fault plane parallel to the main shock fault was identified. This result suggests that the fault structure around the deep part of the active fault is complicated. The hypocenters of the foreshock and main shock were located in the offset part of the aftershock alignment, implying that they did not occur on the same fault plane. The Chisato Fault and Yokkaichi Fault were located around the upward extension of the aftershock alignment. The main shock probably occurred in the deep parts of these faults.