Sadayuki Koresawa

SEISMIC OBSERVATIONS AT THE YAP ISLANDS AND THE NORTHERN YAP TRENCH

Abstract We conducted two seismic surveys in the Yap region to investigate tectonic activity in this area. One was conducted in the northern Yap Trench for ten days using ocean bottom seismometers, the other was conducted on the Yap Islands for eight months using a small array. From these observations we detected many earthquakes beneath the Yap region and found a characteristic pattern of seismicity. Many earthquakes occurred in the inner trench slope, no seismicity was observed in the axial region of the trench, and a few earthquakes occurred in the outer trench slope. This pattern is similar to the typical pattern in active subduction zones. This result indicates that the Yap Trench is still actively subducting. We did not detect earthquakes deeper than 40 km during the observations.

Depth variation of the crustal structure of the subducting plate along the Nankai Trough, off Kii Channel, Japan

A seismic refraction-reflection experiment using ocean bottom seismometers with artificial sources comprising airguns and explosions was conducted at off Kii Channel, the Nankai Trough, where interplate earthquakes occurred periodically. Using P-wave travel time analyses including non-linear travel time inversion, the depth variation of the crustal structure along the Nankai Trough was revealed. The results show that the subducting depth of the Philippine Sea plate changes at off Kii Channel. The plate is subducting at a shallow location at the west side and at a deep location at the east side. The depth of the plate boundary is about 8 km at the west side and about 9 km at the east side. This result and former studies indicate that the variation of subducting depth exists beneath Kii Channel and off Kii Channel regions. This depth change of the plate boundary might affect the process of interplate earthquake occurrence.

Microseismicity of back-arc rifting in the Middle Okinawa Trough

A one month microearthquake survey was conducted at Iheya Knolls in the middle Okinawa Trough in 1992 to reveal microearthquake characteristics of back-arc rifting. We deployed five ocean bottom seismometers and observed over 1700 events during 30 days. Hypocenters were determined for 48 earthquakes with good resolution. From this survey, we conclude that the survey area with 100km × 100km has high seismicity, many earthquakes occur in the western part of Iheya Knolls, and few occur in the central part. The earthquakes are distributed in an en echelon shape and throughout the entire depth of the crust. There is no evidence for the existence of transform faults, suggesting that the middle Okinawa Trough is in a stage of back-arc rifting.

Sub-Moho seismic profile in the Mariana Basin — Ocean bottom seismograph long-range explosion experiment

Abstract Ocean bottom seismograph (OBS) long-range explosion experiments were carried out in the Mariana Basin in 1973 and 1976. Seven large shots (8.5–1.5 ton) as well as several tens of small shots were fired. The maximum range of observation was about 1900 km. As many as 25 OBS stations were deployed in an array of about 800 km. It is found that the sub-Moho P-wave velocity structure is of stratified nature, being composed of alternating high- and low-velocity layers. High-velocity layers with apparent velocities of 8.1, 8.2, 8.4, 8.6 and 8.7 km/s are identified. Low-velocity layers, sandwiched between the high-velocity layers of 8.4, 8.6 and 8.7 km/s, are very prominent. The sub-Moho high-velocity lid with an apparent velocity of 8.4 km/s is very thin. Thinning of this lid, thickening of the low-velocity layer, and the presence under it of another high-velocity layer (8.6 km/s) appear to characterize the uppermost mantle structure beneath the Mariana Basin.

Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93–3 cruise of the R/V Hakuho-Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well-constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15×5 km2. All of the 13 focal mechanisms showed normal or strike-slip faultings, which means that the extensional stress field characterizes this region.

Seismic observations at a seismic gap in the eastern margin of the Japan Sea using ocean bottom seismometers

Abstract The eastern margin of the Japan Sea is a nascent convergent plate boundary. Previous studies proposed the existence of a seismic gap along this boundary between 39°N and 40°N. The trend of this gap is reported by Ohtake (Island Arc 4, 156–165, 1995) to be north-northwest to south-southeast, but by Ishikawa (Gekkan Kaiyo, Suppl. 7, 102–107, 1994) and Matsuzawa (Prog. Abstr., Seismol. Soc. Jpn. 2, B92, 1995) to be north-northeast to south-southwest. During one month ocean bottom seismic observations were conducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail. The observations revealed that the earthquake epicentral distribution had an echelon shape and could be divided into three groups. These groups have a north-northeast to south-southwest trend. This trend is consistent with the fault system in this area, which was formed by the back-arc spreading in the Early to Middle Miocene. This suggests that previously formed tectonic structures affect the present seismo-tectonics and that this area has weak planes with a north-northeast to south-southwest trend.

Travel-time residuals of teleseismic P-waves at the Rodriguez Triple Junction in the Indian Ocean using ocean-bottom seismometers

We conducted three weeks of seismic observations at the Rodriguez Triple Junction (RTJ) in the Indian Ocean using 18 ocean-bottom seismometers over an area of 90km × 90km. We identified six teleseismic events and obtained significant anomalies in the relative travel-time residuals of the teleseismic P-waves. The residuals are positive at the RTJ, the northern part of the Southeast Indian Ridge (SEIR), and the eastern side of the Central Indian Ridge (CIR), and are negative at and around the Southwest Indian Ridge (SWIR). It is suggested that there is a relatively hotter mantle under the triple junction and the northern part of the SEIR segment, along-axis variations in mantle temperature along the SEIR segment, and cooler mantle under the SWIR segment. The CIR segment has an asymmetrical distribution of relative residuals.

Seismic velocity structure near the extinct spreading center in the Shikoku Basin, North Philippine Sea

Abstract An OBS (ocean-bottom seismograph) airgun refraction survey was conducted near the extinct spreading center in the 18–21 m.y. old Shikoku Basin, North Philippine Sea. The P-wave velocity structure of the ocean crust (layers IIA and IIB) is in accord with that of the average ocean crust. P to S converted waves were remarkably well recorded by the OBS. Utilizing these phases, P-wave velocity, V p , S-wave velocity, V s , velocity ratio V p V s , and Poissons ratio, σ, were determined as follows: V p = 1.8 km/sec, V s = 0.43 km/sec, V p V s = 4.2 , σ = 0.47 for layer I (sediment); V p = 3.7 km/sec, V s = 1.8 km/sec, V p V s = 2.0 , σ = 0.33 for layer IIA; and V p = 4.8 km/sec for layer IIB. The refraction of P to S converted wave, V s = 3.2 km/sec, might be identified as arising from layer IIC, and that of V s = 3.8 km/sec as arising from layer III.

Large Poisson's ratio and low S-wave velocity within the Japan trench inner wall toe

Abstract P to S and S to P converted waves arising from the interface between inner wall rock mass and the subducting oceanic plate were observed by an ocean bottom seismograph (OBS). Using these data, mean S-wave velocity, V s , and Poissons ratio, σ, were determined within the Japan trench inner wall toe. The Poissons ratio is as large as σ = 0.41 , and the average S-wave velocity is as low as V s = 0.91 km / sec . Occurrence of efficient P to S and S to P conversion at this interface implies the existence of a high acoustic impedance. The low shear velocity of the inner trench wall material implies a low rigidity and hence this zone can be easily deformed.