Yukio Fujinawa

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20xa0km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region.The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough. The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations. The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone.The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1xa0cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a ‘tsunami earthquake’ with 5.7xa0MW and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bonin Is. arc on Septemberxa04, 1996. The maximum amplitude of the tsunami signals on the trough-floor was approximately 1xa0cm (P-P), in contrast with approximately 20xa0cm (0-P) at a coastal site on Izu-Oshima, near the trough. Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (7.1xa0MW) at the northern New Guinea, on July 17, 1998.

Microearthquakes and tectonics in an active back-arc basin: the Lau Basin

Abstract An Ocean Bottom Seismograph (OBS) array was deployed for 20–22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back-arc opening in the northern Lau Basin. Using P- and S-wave arrival times from four or more OBSs, the hypocenters of ∼ 300 shallow earthquakes were located with a high confidence level. The magnitudes of most OBS-located earthquakes were estimated to be less than four. In the northern half of the survey area, a narrow, linear zone of microearthquakes, trending NNW-SSE, has been identified. The northern part of the narrow seismic zone is within a central axial depression at the southern end of the Peggy Ridge. Further south, the trend of the seismic zone becomes more N-S. The narrow seismic zone seems to be composed of at least six seismic segments, offset by short aseismic zones. Most of the seismic segments trend NNW-SSE, suggesting a system of left-stepping en echelon spreading ridges, where the spreading ridge segment is seismically inactive and the transform fault is active. The spreading ridges appear to strike N-S or NNW-SSE, but the direction of the back-arc opening is considered to be NW-SE. No hypocenters were located with a high level of precision in the area south of latitude 18°S, except a small isolated zone of shallow earthquakes at the southeastern part of the survey area. We suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the Tonga platelet. This platelet is separated from the major Indo-Australian plate by the back-arc opening system in the Lau Basin.

Microearthquakes along the back-arc spreading system in the eastern Bismarck Sea

OBS’s were deployed for 26 to 29 days in the eastern Bismarck Sea to investigate the back-arc spreading. Hypocenters of 186 shallow earthquakes were determined using P- and S-waves from at least five stations. In the western survey area, a transform fault zone is marked by a linear micro-earthquake activity striking N65°W and less than 5 km wide. The predominant type of their focal mechanisms is strike-slip. In the eastern area, several intermittent zones of micro-earthquakes and their strike-slip type focal mechanisms suggest the location of short-length transform faults separating en-echelon spreading ridges.

New pop-up type Ocean Bottom Seismometer

We designed a new pop-up type Ocean Bottom Seismometer (OBS) in order to study micro-earthquakes in off-shore areas. With a 57 cm O.D. sphere of high tension aluminium alloy, the OBS system, including one vertical and one horizontal geophone, can safely operate on ocean floors of up to 6000 m depth for seismic observations. The amplified seismic data and the time code are directly recorded on the four-channel cassette deck for periods of up to one month. The frequency response curve throughout the recording and play-back system is flat for the range, 1–15 Hz (−3 dB). The anchor release and the geophone clamp are operated by an acoustic command signal.So far, we have deployed our OBSs 42 times in the ocean. All of the OBSs deployed have been recovered safely. Seismic data has provided seismological evidence for a number of processes associated with tectonism along subduction zones and spreading ridges (e.g., Eguchi et al., 1986).

Microearthquakes and tectonics around an outer rise: the Zenisu ridge, Japan

Abstract To investigate the activity and tectonic implications of intraplate microearthquakes occurring within the deflecting oceanic plate along the Nankai-Suruga trough, Japan, we deployed eight ocean bottom seismometers (OBSs) on the Zenisu ridge, for a period of 25 days in 1985. Hypocenters of approximately 40 microearthquakes were identified around the Zenisu ridge, using the OBS data. The OBS-located hypocenters were distributed mainly on the southern half flank of the ridge. There are no earthquakes located by the OBS data within the upper layer of the Philippine Sea plate. The focal depths range from 18 to 35 km, indicating that the earthquakes occurred within the lower layer of the mechanically strong part of the lithosphere. The composite focal plane solution of the microearthquakes that occurred on the Zenisu ridge was a strike-slip or reverse fault type, having P-axis in the WNW. The tectonic origin of the earthquakes located on the ridge was demonstrated to be found in the bending, with a regional compression, of the Philippine Sea plate along the eastern Nankai-Suruga through.

Earthquakes associated with the back-arc opening in the eastern Bismarck Sea: activity, mechanisms, and tectonics

Abstract To make clear the precise locations of earthquakes and their tectonic implications for the active back-arc extension in the eastern Bismarck Sea, we deployed a network of ocean bottom seismometers (OBSs) for a period of 26–29 days in late 1983 in an area that is the intersection of a previously postulated transform fault zone and spreading center. We located the hypocenters of > 200 earthquakes with high resolution, using arrival times for P- and S-waves at five or more OBS stations and using the station corrections. Their focal depths were estimated to be generally In the western survey area (area 1), a linear zone of OBS-located earthquakes, trending N65°W, defines the locus of the major transform fault to within a linear zone In addition, south of the two major seismic zones mentioned above, we identified two isolated spots with microearthquakes shallower than 10 km. The composite focal plane solution of the isolated earthquakes is similar to those teleseismically obtained for shallow events south of the linear seismic segments in the western Bismarck Sea, suggesting a systematic pattern of the intraplate stress regime of the Bismarck plate. We finally suggest that the isolated intraplate events partly reflect the fundamental tectonic origin of the extension in the Bismarck Sea.

Some Results in the Preliminary Data Analyses of the Fixed-Point GPS Baseline Determination Network in Central Japan

We present preliminary results of data analyses of GPS fixed point geodetic measurements of 70–260 km baselines in the Kanto-Tokai district, central of Japan. The system was introduced for the purpose of earthquake prediction researches in the district. Firstly, the analysis employs the determination of initial positions of ten sites by means of the result of the experiment using one VLBI point and one SLR point. Next, one of the station was fixed to determine other receiver locations and the wet term tropospheric delays using the orbital relaxation technique, based on the dual frequency carrier phase and surface meteorological data.

Testing of a towed CTD

The results of laboratory and field experiments on a CTD measuring unit attached to a controllable towed body is described. It was verified that the three-dimensional distribution of salinity and temperature could be observed with sufficient accuracy (to 0.03‰ in salinity and to 0.01°C in temperature if proper correction is adopted) while cruising at up to 4 knots. A double integrated A/D converter is used to compensate for effects of extraneous environmental changes.