Takao Eguchi

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.

Tectonic stress field in East Eurasia

Abstract The overall pattern of the recent tectonic stress field in the eastern part of the Eurasian plate is inferred from the geological features of large transcurrent and reverse faults, graben structures and from focal mechanism solutions of shallow earthquakes. NE-SW striking trajectory lines, of maximum horizontal tectonic stress axis (σ Hmax), are estimated at the north of Tibet and mainland China, but its strike turns to easterly near the eastern margin of the Eurasian plate. This suggests that the main origin of the σ Hmax in East Eurasia is due to a compressive stress regime along, the Himalayan belt, Tibet and the Japan Trench (and also the Sakhalin and Nankai Trough). This tectonic stress field seems to provide some influence on the extensional tectonics in the Shansi region, Baikal, and also in the Okinawa Trough where the N-S opening has been confirmed. The systematic view of the tectonic stress field in East Eurasia, makes it possible to appreciate the fundamental background of a number of complicated intra-plate tectonics.

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.

A simple model of mantle convection including a past history of yielding

A two-dimensional dynamic model of mantle convection, showing plate-like behavior, is constructed by considering the effects of the previously yielded segments. Only within an uppermost thin layer is the material assumed to yield at a constant stress. The advection of the segments affected by yielding is calculated. The area covered by these segments continues to yield until it reaches a certain depth. The resultant flow shows movement of the top layer in the parameter range in which, without yielding, it becomes sluggish. A comparison of the results with and without historical effects shows the enhancement of the plate-like behavior. We consider the horizontal variation of the yield stress, which may be caused by the difference in the amount of H 2 O in the crust. The place of sinking is controlled by the lateral change in the yield stress and the flow behavior becomes different from the case with uniform yield stress.

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.