Keiji Kasahara

Recent progress of seismic observation networks in Japan —Hi-net, F-net, K-NET and KiK-net—

After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismo-graph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan Meteorological Agency since October 1997 to monitor the seismic activity in and around Japan. The same data are shared to university group in real time using satellite communication for their research work. The data are also archived at the National Research Institute for Earth Science and Disaster Prevention and stored in their database system for public use under a fully open policy.

A densely distributed high-sensitivity seismograph network in Japan: Hi-net by National Research Institute for Earth Science and Disaster Prevention

Seismic observations to retrieve various information from the Earth are the basis of seismology. A seismic observation system requires various technologies for vibration sensors, analog-and-digital measurement, data transmission, and computing for mass data analysis, for example. New developments in technology are adopted whenever possible in the construction of seismic observation systems. In Japan, after the disastrous Kobe Earthquake in 1995, a high-density and high-sensitivity seismograph network was constructed. The seismic network, called the National Research Institute for Earth Science and Disaster Prevention (NIED) Hi-net, uniformly covers the Japanese Islands with a spacing of 20–30km. As a result, the detection capability for microearthquakes has been greatly improved, and various research using Hi-net data has indicated that this seismic network has a great potential to resolve the underground structure and various geophysical phenomena as a radar-array oriented toward the Earth. Equipped with...

Low-velocity oceanic crust at the top of the Philippine Sea and Pacific plates beneath the Kanto region, central Japan, imaged by seismic tomography

[1] We construct fine-scale three-dimensional P and S wave velocity structures beneath the Kanto region, central Japan, by seismic tomography with a spatial correlation of velocities. The Philippine Sea and Pacific plates subduct beneath the Eurasian plate in this area and are imaged with high velocities. Oceanic crust at the uppermost part of these subducting plates is a low-velocity layer. Low-velocity oceanic crust of the Philippine Sea plate subducts to a depth of approximately 80 km. There are also two low-velocity bodies with relatively high V P /V S ratios of 1.80-1.90 in the mantle wedge above the oceanic crust of the Philippine Sea plate. We speculate that the westernmost of these two low-velocity bodies consists of 20% partially serpentinized peridotite, continuous with gabbro in the oceanic crust of the uppermost Philippine Sea plate, while the eastern body is composed of 30% partially serpentinized peridotite. We trace the subducting oceanic crust of the Pacific plate to a depth of ∼120 km. The estimated V P /V S ratio of this layer is 1.85-1.90, which indicates a low probability of molten rock; the gabbroic oceanic crust may have been metamorphosed to garnet-granulite.

Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988

Abstract A moderate earthquake (M 6.7) occurred off eastern Chiba, central Japan, on December 17, 1987 as a consequence of the internal deformation of the Philippine Sea plate. This event helped in clarifying the geometry of the easternmost part of the Philippine Sea plate as well as the mode of earthquake occurrence in this region. Three months later, another moderate earthquake (M 6.0) took plate beneath eastern Tokyo. This event was generated at the slab-slab collision zone at about 90 km depth, and is considered to be triggered by the former earthquake. A similar phenomenon was observed for the Izu (M 6.7) of June 29, 1980 and the mid Chiba (M 6.1) of September 25, 1980 earthquake pair, and the off Ibaraki (M 7.0) of July 23, 1982 and the southern Ibaraki (M 6.0) of February 27, 1983 earthquake pair. These are the only three earthquake pairs of M ≥ 6 which occurred in the Tokyo metropolitan area since 1974. Thus, over these 15 years, all M-6 interplate earthquakes in this specific region are preceded by nearby M-7 earthquakes. This suggests that M-7 shallow events enhance stress in the deeper plate collision zone beneath the Tokyo metropolitan area and trigger M-6 interplate events.

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.

Seismic reflection profiling across the source fault of the 2003 Northern Miyagi earthquake (Mj 6.4), NE Japan: basin inversion of Miocene back-arc rift

The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17–15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.

Underground structure and magmatic activity of Izu-Oshima volcano, Japan as inferred from seismic reflection survey

Abstract A seismic reflection survey using the VIBROSEIS (Trademark of Continental Oil Co.) method was carried out on Izu-Oshima volcano fifteen months after the eruption of November, 1986. The survey line of about 10 km in length traverses the northern part of the volcano from east to west and intersects the fissure eruption craters which were formed by the 1986 eruption. Although the measurement condition was not excellent due to thick layers of unconsolidated volcanic products, we obtained a good profile reaching to a depth of about 3 km below. The main results obtained are summarized as follows: (1) a remarkable reflector exists at a depth of 300–500 m below the surface, which corresponds to the upper boundary of the Neogene system forming the basement of the Izu-Oshima volcano; (2) the reflector is broken beneath the lateral volcanoes of past eruptions, indicating ascent routes of magma; (3) a remarkable fragmentation of reflectors in the basement is observed beneath the central part of the volcano, which indicates repeated intrusion of magma into the basement; (4) a low-velocity zone is found at a depth of 2–4 km in the basement, which most probably indicates the magma reservoir of the 1986 eruption. Although deeper structure was not well resolved, these results demonstrate that the VIBROSEIS method is quite effective for revealing the underground structure of a volcano.