Kazuya Kitada

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free-air gravity anomaly data from eight Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along-axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume-like mantle upwelling under the spreading axis, and the other is a magma-starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet-like mantle upwelling under the spreading axis. Different features in the MBA for off-axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

Development and Performance Tests of a Sensor Suite for a Long-Term Borehole Monitoring System in Seafloor Settings in the Nankai Trough, Japan

In the Integrated Ocean Drilling Program (IODP), the long-term borehole monitoring system (LTBMS) has been planned for installation into boreholes in seafloor settings in the Nankai Trough, Japan. The LTBMS sensors are extremely sensitive instruments for collecting broadband dynamics to elucidate the mechanisms of megathrust earthquakes, which occur repeatedly in plate subduction zones. However, during IODP Expedition 319, it became apparent that the strong ocean current “Kuroshio” causes vortex-induced vibration (VIV) that damages sensors during installation. Consequently, the LTBMS sensors must be not only highly sensitive but also robust to prevail against VIV. Therefore, sensors with antivibration mechanisms were developed by a Japan Agency for Marine-Earth Science and Technology (JAMSTEC, Kanagawa, Japan) project team. After development was completed, noise evaluation tests and vibration and shock tests simulating vibration and shock in the installation scheme were conducted to confirm that the antivibration mechanism was functional. Power spectral density analysis was conducted using background noise recorded in a low-noise location before and after the vibration and shock tests. Results show that the sensor response was not changed by the vibration or shock tests. Finally, all sensors were loaded onto D/V Chikyu for installation at the C0002 site during IODP Expedition 332.

Two-dimensional resistivity structure of the fault associated with the 2000 Western Tottori earthquake

Two-dimensional resistivity surveys were carried out along two profiles that were laid across earthquake faults initiated by the 2000 Western Tottori earthquake. One profile was located 7 m from a trenching pit, thereby enabling a direct comparison of resistivity cross-section with the geological cross-section and, subsequently, a precise interpretation of the resistivity structure. Features of the resistivity cross-section were found to correspond fairly well to the geological cross-section. A clear resistivity boundary between the resistive and conductive zones matches the earthquake fault that was found by the trenching survey. Variations in resistivity depend primarily on the development of fractures. Two types of conductive zones were found: (1) a clear and deep-rooted conductor that corresponds to an earthquake fault and (2) an indistinct and spatially localized conductor that corresponds to a fracture attributed by landslides and collapse. A few weak conductive zones that match with discrete earthquake faults characterize our resistivity model. This feature is different from the resistivity cross-sections found at the Nojima and Ogura Faults that appeared at the time of the 1995 Hyogo-ken Nanbu earthquake; these two latter faults are characterized by distinct single conductive zones. Based on geomorphological, geological, and seismological evidence, the earthquake fault of the 2000 Western Tottori earthquake can be classified as an immature fault. In contrast, the Nojima and Ogura Faults have been active for at least the entire Quaternary period. We conclude that the difference in the fault development stages is reflected in their different resistivity structures.

Long-term monitoring at C0002 seafloor borehole in Nankai Trough seismogenic zone

The C0002 long-term borehole observatory installed during IODP Expedition 332 in December 2010 have been successfully connected to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) this January 2013 during the KY13-02 cruise by the R/V Kaiyo. We confirmed from the DONET landing station that all the borehole instruments was properly functioning and finally started long-term borehole monitoring at Site C0002 in Nankai Trough seismogenic zone.

Plan and technological difficulties on NanTroSEIZE long term borehole monitoring system

IODP (Integrated Ocean Drilling Program) scientific drilling proposal 603 (NanTroSEIZE: Nankai Trough Seismogenic Zone Experiment) not only proposes drilling, coring, geological analyzing, and geophysical logging, but also mandates that several long term borehole monitoring systems (LTBMS) should be installed at Nankai trough, where we expect to encounter the mega splay fault and the locked region of mega thrust fault, respectively, in order to understand on the dynamics of seismogenic zone. Borehole is not mere relic after core sampled, but should be sufficiently utilized as the scientific legacy hole to monitor the interior of the Earth. The advantage of LTBMS is to simultaneously manage precise monitoring on various parameters at multiple layers in the same borehole. Its technological difficulties are hiding in establishing the reliable and robust system against deploying long and narrow structure into tiny borehole through the ocean current from the heaving surface vessel. Also, for deeper penetration, higher temperature and pressure cause many problems on the system. This paper describes the plan of NanTroSEIZE LTBMS and its technological difficulties including some results on our development.

Field Experimental Study on Vortex-Induced Vibration Behavior of the Drill Pipe for the Ocean Borehole Observatory Installation

We conducted two field tests aboard the Drilling Vessel Chikyu (D/V Chikyu) in areas of strong ocean currents (i.e., Kuroshio, Japan) during the March 2010 CK10-01 cruise. The relationship between the amplitude of the vortex-induced vibration (VIV) and the ocean current and ship drift speed were investigated to establish a VIV control method for the ocean borehole observatory installation in the Nankai Trough. These tests demonstrate that the ocean current and ship drift speed mainly control the drill pipe VIV. We find that to install sensitive sensors in a strong current area and avoid damage, the key factors are: 1) lowering the sensor assembly in a low current area; and 2) managing the drift speed of the drilling vessel.

Vortex induced vibration suppression of the drill pipe for the long-term borehole monitoring system installation

This study proposes a Vortex Induced Vibration (VIV) suppression method for the Long-Term Borehole Monitoring System (LTBMS) installation in areas of strong ocean currents such as Kuroshio. One of the primary challenges in realizing LTBMS was to install high-precision, sensitive sensors into the borehole without damaging them. Two field tests were performed using accelerometers attached on instrument carrier and/or drill pipes to investigate the characteristics and causes of drill pipe VIV. This test demonstrates that the reduction of the drag on circular drill pipes and tubings and the vortex suppression can be achieved by the suppression ropes and the drill collars. They were suggested to be attached on the drill pipe above the sensor assembly for the actual LTBMS installation. From the VIV monitoring during the installation over a period of several days, the following three points can be drawn for the further VIV suppression: 1) The bottom hole assembly should be lowered in the low current area, with the relative current speed being as low as possible, 2) the drifting speed should be kept well below 1 knot, and 3) the drifting angle between drifting direction and sea current should be kept as small as possible (definitely less than 45°). The results show the VIV amplitude was further reduced to less than 0.5G, which led to the success of the first LTBMS installation.

The development and evaluation of sensors for long-term borehole monitoring system

In Integrated Ocean Drilling Program (IODP), the Long Term Borehole Monitoring System (LTBMS) is planned to be installed to sea bottom boreholes in the Nankai Trough area. LTBMS sensors are very sensitive instruments for collecting broadband dynamics with wide dynamic range for understanding earthquake mechanism. However, in IODP Exp.319, it was apparent that the strong ocean current “Kuroshio” cause vortex induced vibration (VIV) that gives damage to sensors in installation scheme. Thus, the LTBMS sensors have to be not only high sensitive but also rugged to survive against VIV. For this purpose, sensors with anti-vibration mechanism were developed by JAMSTEC project team. After development, noise evaluation test and vibration and shock tests that simulate vibration and shock in installation scheme were conducted for confirming the mechanism was working well. Furthermore, before and after the vibration and shock tests, power Spectral Density analysis was carried out using background noise recorded in low noise location, Matsushiro Seismological Observatory. As a result of this analysis, it was observed that the responses of sensors were not changed by the vibration and shock tests. Finally, all of these sensors were loaded to D/V Chikyu for installation to C0002 observatory in Exp.332.

Shock and vibration tests on sensors for long-term borehole monitoring systems

Shock and vibration tests on electrical devices have been carried out. Here the devices mean sensors combined with telemetry units and these are the same ones that have been installed in C0002 riserless hole in Nankai area, Japan in December 2010. Acceptable upper limit of shocks and vibrations on the devices have been confirmed through these tests.