Nankai Trough Seismogenic Zone Research Articles

IODP Expedition 322 Drills Two Sites to Document Inputs to The Nankai Trough Subduction Zone

The primary goals during Expedition 322 of the Integrated Ocean Drilling Program were to sample and log the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan). Characterization of these subduction inputs is one piece of the overall science plan for the Nankai Trough Seismogenic Zone Experiment. Before we can assess how various material properties evolve down the dip of the plate interface, and potentially change the fault’s behavior from stable sliding to seismogenic slip, we must determine the initial pre-subduction conditions. Two sites were drilled seaward of the trench to demonstrate how facies character and sedimentation rates responded to bathymetric architecture. Site C0011 is located on the northwest flank of a prominent basement high (Kashinosaki Knoll), and Site C0012 is located near the crest of the seamount. Even though significant gaps remain in the coring record, and attempts to recover wireline logs at Site C0012 failed, correlations can be made between stratigraphic units at the two sites. Sedimentation rates slowed down throughout the condensed section above the basement high, but the seafloor relief was never high enough during the basin’s evolution to prevent the accumulation of sandy turbidites near the crest of the seamount. We discovered a new stratigraphic unit, the middle Shikoku Basin facies, which is typified by late Miocene volcaniclastic turbidites. The sediment-basalt contact was recovered intact at Site C0012, giving a minimum basement age of 18.9 Ma. Samples of interstitial water show a familiar freshening trend with depth at Site C0011, but chlorinity values at Site C0012 increase above the values for seawater toward the basement contact. The geochemical trends at Site C0012 are probably a response to hydration reactions in the volcaniclastic sediment and diffusional exchange with seawater-like fluid in the upper igneous basement. These data are important because they finally establish an authentic geochemical reference site for Nankai Trough, unaffected by dehydration reactions, and they provide evidence for active fluid flow within the upper igneous crust. Having two sets of geochemical profiles also shows a lack of hydrogeological connectivity between the flank and the crest of the Kashinosaki Knoll. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.10.02.2010" target="_blank">10.2204/iodp.sd.10.02.2010</a>

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system – which may slip during plate boundary earthquakes – and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and <i>in situ</i> measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first <i>in situ</i> hydrological measurements of the plate boundary hanging wall, and (c) integration of <i>in situ</i> stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (< 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.10.01.2010" target="_blank">10.2204/iodp.sd.10.01.2010</a>

NanTroSEIZE Stage 1 Expeditions 314, 315, and 316: First Drilling Proram of the Nankai Trough Seismogenic Zone Experiment

Integrated Ocean Drilling Program (IODP) Expeditions 314, 315, and 316 were carried out as a unified program of drilling collectively known as Stage 1 of the Nankai Trough Seismogenic Zone Experiment, or NanTroSEIZE, the first multi-stage complex drilling project in IODP and the most ambitious effort in scientific ocean drilling to date (Tobin and Kinoshita, 2006b; Kinoshita et al., 2006). The first of four planned operational stages, the Stage 1 expeditions were conducted aboard the new riser-capable drilling vessel Chikyu from September 2007 to February 2008, and were Chikyu’s inaugural IODP expeditions. Seventy-one scientists from twelve countries took part in the three expeditions as shipboard participants, and thirty-three holes in total were drilled at eight sites, to as much as 1400 meters below the sea floor (Table 1, Fig. 1).

Petrophysically determined lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314

Abstract: Characterizing the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. Integrated Ocean Drilling Program (IODP) Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging-while-drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at Sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by 3D seismic profiles and initial interpretation of log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterize formations within the accretionary prism. Boundaries that correlate to the seismic-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a megasplay fault previously unresolved by log analysis and borehole images, are identified. Our study demonstrates that INCA analysis of LWD data can accurately define boundaries and characterize sediments in environments where core recovery may be incomplete.

Outline of the Nankai Trough Seismogenic Zone Experiment

This report introduces the scientific drilling project “NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment)” proposed for the IODP (Integrated Ocean Drilling Program). NanTroSEIZE is a large, international, multidisciplinary scientific project that required long-term planning and implementation. The project involved almost two decades of planning and may continue for another decade while carrying out drilling, coring, and long-term borehole monitoring. This large scientific project has now officially started, and we expect that the resulting scientific findings will be of international significance.

Chikyu: The First Three Years of Operation

Three years have passed since the completion of the Chikyu, a state-of-the-art scientific drilling vessel. As the main platform of the Integrated Ocean Drilling Program (IODP), the Chikyu explores the mysteries of planet Earth. The Chikyu aims to achieve scientific breakthroughs in the mechanism of great earthquakes, the origins of life, the past and future of the Earth's global climate, and drilling into the mantle. The Chikyu is the first science research vessel to be equipped with the Riser Drilling System, and she also has shipboard research laboratories on four dedicated decks with a total area of 2,300 m2. These laboratories have an extensive array of cutting-edge equipment used to conduct physical, chemical and biological analyses of recovered cores, pore water and boreholes. After the first dry-docking from March to May 2006, the Chikyu successfully performed shakedown drilling operations off Japan's Shimokita Peninsula, and then again off Kenya and the Northwest Australian Continental Shelf. The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a complex multiyear ocean drilling project implemented as part of the Integrated Ocean Drilling Program, began the Chikyu's involvement with IODP drilling. NanTroSEIZE is a multiple-expedition project involving teams of some of the top scientists from all around the world. NanTroSEIZE attempts for the first time to drill, sample, and instrument the earthquake-causing, or seismogenic, portion of a plate subduction zone. The Chikyu successfully completed the first three Stage 1 NanTroSEIZE expeditions in 2007 and 2008. This paper describes the Chikyu's operations over the past three years.

Estimation of Minimum Principal Stress from an Extended Leak-off Test Onboard the Chikyu Drilling Vessel and Suggestions for Future Test Procedures

To understand the physics of faulting and rupture propagation for the great M8-class Nankai earthquakes that recur approximately every 100 years, a comprehensive drilling project is underway: the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE�� Tobin and Kinoshita, 2007), which is part of the Integrated Ocean Drilling Program (IODP). Stress levels along seismogenic faults must be known in order to understand processes controlling the timing, energetics, and extent of earthquake ruptures. For scientific drilling projects such as NanTroSEIZE, it is very important to determine the in situ stress state at the decollement and the mega splay fault in the Nankai Trough.

Joint IODP-ICDP Workshop Examines Challenges of Fault Zone Drilling

No abstract available. doi:10.2204/iodp.sd.s01.80.2007

The use of VSP techniques for fault zone characterization: An example from the San Andreas Fault

Vertical seismic profiling (VSP) technology is increasingly being used in the fields of earthquake seismology and tectonics. This is motivated in part by the growing number of oil field microseismic monitoring surveys, but more so by projects that involve drilling deep wells for monitoring crustal activity at depth. Examples of these projects are the San Andreas Fault Observatory at Depth (SAFOD), the Nankai Trough Seismogenic Zone Experiment, the Gulf of Corinth Rift Laboratory, and the Taiwan Chelungpu Fault Drilling Project, and other projects by the International Continental and Ocean Drilling Programs (ICDP and IODP). These projects require instrumentation and surveying in deep and possibly hot borehole environments. With higher resolution than surface seismic data, images from 2D and 3D VSP data contribute to better characterization and interpretation of complex reservoirs at smaller scales. The location of receivers in the low-noise borehole environment yields higher signal-to-noise ratios, higher ...

NanTroSEIZE: The IODP Nankai Trough Seismogenic Zone Experiment

No abstract available. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.2.06.2006" target="_blank">10.2204/iodp.sd.2.06.2006</a>

震度インバージョン解析による南海トラフ巨大地震の短周期地震波発生域

An inversion analysis has been developed to evaluate short-period seismic wave radiation zones on an earthquake fault plane using seismic intensity data. It is a useful method for historical earthquakes, for which neither strong motion nor tsunami data have been observed by instruments. Since the accuracy of the present method is verified by using ground motion waveforms synthesized by the stochastic Green's function method, it is concluded that the effects of directivity of the fault rupture process and the radiation pattern on seismic intensity distribution can be neglected in the frequency range that is effective for seismic intensity. The present method is applied to great earthquakes that have occurred since 300 years ago in the Nankai Trough seismogenic zone in southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The reliability of their solutions is discussed based on a sensitivity analysis. It is noted that short-period seismic wave radiation zones are often adjacent to large slip areas, termed asperities, but don't always overlap them. Furthermore, they are sometimes located where fault rupture has stopped. Short-period seismic waves are not always radiated from the same zone. For example, fault zones in the Enshu-Nada Sea, the Kumano-Nada Sea, the Kii Channel and the Kochi seacoast have radiated short-period seismic waves at every event, but whether or not short-period seismic waves were radiated in the interior of Suruga Bay, off the Shiono Cape and off the Muroto Cape varied with the events. It is also concluded that crustal structures such as subducted seamounts and subducted ridges take an important role of radiation of short-period seismic waves.

Microseismicity at the seaward updip limit of the western Nankai Trough seismogenic zone

The seismogenic zone at subduction zones does not normally extend to the trench axis. The shallowest part of the plate interface is considered to be seismically decoupled. The location of the seaward updip limit of the seismogenic zone and its relation to the crustal structure are important for understanding the transition process from aseismic slip to the seismic rupture at the plate interface. The Nankai Trough seismogenic zone, southwestern Japan, is one of the most well‐studied subduction seismogenic zones in the world. However, the offshore seismicity around the Nankai Trough is very low, and hypocenters are not determined accurately by the on‐land seismic network. We performed microseismicity observations using ocean bottom seismographs off Cape Muroto. Hypocenters were determined by using a three‐dimensional Vp and Vs structure based on seismic survey results. The observed microseismicity seems to be classifiable into two types. The first is earthquakes that occur around the plate interface; the second is a group of earthquakes that occur in the uppermost mantle of the subducting oceanic plate. The seismicity around the plate interface forms several clusters. The seaward limit of the clusters is characterized by earthquakes with a similar waveform. These similar waveform earthquakes are considered to occur at small asperities in the aseismic‐seismogenic transition zone of the plate interface. The seismicity in the uppermost mantle may be related to dehydration embrittlement of subducting serpentinized mantle.

Correction to “Crustal structure across the coseismic rupture zone of the 1944 Tonankai earthquake, the central Nankai Trough seismogenic zone”

Correction to “Crustal structure across the coseismic rupture zone of the 1944 Tonankai earthquake, the central Nankai Trough seismogenic zone”

Seismic structure of western end of the Nankai trough seismogenic zone

In 1998 a seismic experiment along a 320‐km‐long profile using an air gun array and ocean bottom seismographs (OBSs) was collected in the on shore‐offshore, forearc region of the western Nankai trough seismogenic zone, southwestern Japan. This area is outside the coseismic rupture area of the 1946 Nankaido earthquake (Ms = 8.2). The main objectives of this paper are to clarify the P wave and the Poisson's ratio profile around the forearc region of southwestern Japan, and estimate differences between this area and the Nankaido coseismic rupture area. The P wave velocity model estimated by two‐dimensional (2‐D) ray tracing and travel time inversion indicates three units with distinctive velocity patterns. The lens‐shaped unit has a maximum thickness of 5 km and velocity of 3.4–4.0 km/s. The underlying unit has velocity of 5.4–5.6 km/s, and its P wave velocity increases on land (Vp = 6.2–6.4 km/s). The relative homogeneous unit with P wave velocity of 6.6–6.9 km/s exists just beneath secondary unit with P velocity of 5.4–5.6 km/s. The angle of subduction is ∼7°. We also obtained a Poisson's ratio of the crustal structure of the forearc region. The Poisson's ratio of the southern part of secondary unit is larger than that of the northern part. We conclude that these data indicate clay‐rich metamorphic rocks plus oceanic basaltic rocks, rather than high‐porosity materials, comprise the majority of the layer. We suggest that the region of high Poisson's ratio probably corresponds to underplating and the metamorphism of the clay‐rich sediments and possibly the oceanic basalt just below the decollement.

Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene–Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.

Structural factors controlling the rupture process of a megathrust earthquake at the Nankai trough seismogenic zone

SUMMARY The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100‐200 yr. The 1946 Nankaido earthquake is well known as an unusual event among these earthquakes, because the rupture zone estimated from long-period geodetic data is more than twice as large as that derived from seismic wave data. In the summer of 1999, an onshore‐offshore deep seismic survey was performed along a 355 km long profile in the western Nankai Trough seismogenic zone. Seismic signals both from an airgun array (207 l) and land explosions (maximum of 500 kg) were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations. Conventional 2-D seismic reflection data were also acquired along part of the offshore profile. From the wide-angle seismic data, we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth. The seismic velocity image also shows that the seamount is now colliding with the Japanese island arc crust. From this significant structure, this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake, i.e. the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount, and then the rupture was deflected around the subducted seamount at the down-dip end of the locked zone between Cape Muroto and Cape Ashizuri. Another significant structure, a highly reflective layer, is obtained beneath Shikoku Island. A very slow P-wave velocity (3 km s −1 ) is necessary in a thin layer at the base of the island arc crust in order to explain the observed high-amplitude reflection phases. An area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer. This suggests a possible water layer at the base of the island arc crust. The water may be generated by dehydration of the downgoing probably partially serpentinized mantle, which is implied by a low P-wave velocity (7.5 km s −1 ) beneath the subducted seamount. A locally observed non-slip region during the 1946 earthquake at the eastern part of Shikoku Island is interpreted as a result of weak coupling at the possible water layer.

Crustal structure across the coseismic rupture zone of the 1944 Tonankai earthquake, the central Nankai Trough seismogenic zone

Differences in the coseismic rupture process between the 1944 Tonankai and the 1946 Nankaido earthquakes have been studied by many fault models. To understand what factors control coseismic rupture zones, it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes. The previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the Neogene‐Quaternary accretionary prism. However, little is known about the structure of the rupture zone of the 1944 earthquake. To obtain a complete image of the seismogenic zone of the 1944 earthquake, a wide‐angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land. Our model for the crustal structure is based on ocean bottom seismographic data. The crustal structure appears characteristic for subducting oceanic crust and a Neogene‐Quaternary accretionary prism bounded by an island arc crust. The Neogene‐Quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front. The subducting oceanic crust can be traced down to 35 km. The subduction angle becomes steeper landward, reaching up to 11° beneath the island arc crust. The depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km, while the updip limit is located beneath the island arc upper crust. Similar structures of the updip and downdip limits are also published for several other subduction zones.

Western Nankai Trough seismogenic zone: Results from a wide‐angle ocean bottom seismic survey

The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well‐studied historic earthquakes. This paper presents results of a wide‐angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms = 8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake. The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (Vp = 5.0–5.6 km/s and 6.6–6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with Vp = 3.4–4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2°and 7.2° at 0–50 km and 50–100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust‐island arc crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.