T31A-1965
A low velocity zone revealed by 3D prestack depth imaging of the Nankai subduction zone off Kii Peninsula, southeast Japan
In order to figure out detailed crustal structure and physical property of the Nankai subduction zone off southeast Kii Peninsula, we have acquired three dimensional (3D) multi-channel seismic (MCS) data. The 3D MCS area (12 x 62 km) was designed to fully cover most of the drilling sites of the project NanTroSEIZE. For the 3D bin gathers after pre-conditioning, we have constructed and updated interval velocity volume model for 3D prestack depth migration (PSDM). After 3D horizon-based tomographic updating, we have applied 3D grid-based tomography for final tuning of the 3D PSDM velocity model. The 3D PSDM result exhibits a low seismic velocity zone with weak reflectivity character along the Nankai subduction zone. This low velocity zone (LVZ) is intercalated between upper, fold-and-thrust offscraping layer and lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the low velocity zone. The LVZ is estimated to be a maximum of ~2 km thick, ~15 km wide, and ~120 km long. Our observations highlight that the low velocity zone should be in an overpressured condition, leading to rigidity reduction of the whole outer accretionary wedge. A numerical simulation by finite element method suggests that the rigidity-lowered outer wedge, containing the low velocity zone, may be responsible for a significant additional uplift of the outer wedge and thus foster tsunami generation during the Nankai megathrust earthquake. Subducting basement highs such as ridge or seamount may cause tectonic erosion of the lower part of the overlying accretionary wedge, producing a fracturing or shear zone with the reduced seismic velocity.
T31A-1966
Subduction Structure beneath the Eastern Part of the Kii Peninsula,SW Japan,from Wide- Angle Reflection Experiment
A region off the Kii Peninsula, SW Japan, is known as one of mega-thrust earthquake areas associated with the subduction of the Philippine Sea (PHS) plate. In 2006, onshore-offshore seismic refraction/wide-angle reflection survey was conducted in the eastern part of the Kii Peninsula and its offshore area. This experiment, supported by JAMSTEC, was aimed to elucidate the geometry of the PHS plate and inhomogeneous structure of in and around the seismogenic zone of the 1944 Tonankai earthquake of M7.9. The onshore seismic line, 87.8 km in length, is a northwest extension of an offshore profile line by JAMSTEC, crossing Jurassic to Cretaceous accretionary complexes and the Median Tectonic Line (MTL), which is one of the major tectonic lines in SW Japan. On this profile, 519 receivers were set to record 5 dynamite shots. The obtained records were of good quality, and clear wide-angle reflections from the PHS plate are recognized in almost the entire part of the profile. The uppermost crust is determined both from inversion (refraction tomography) and forward modelling (ray- tracing method). The uppermost structure north of the MTL is relatively simple and characterized by a 1-2 km thick surface layer with Vp=5.0-5.4 km/s overlying a crystalline basement with a velocity of 5.6-5.8 km/s. The uppermost crust south of the MTL within the Jurassic to Cretaceous accretionary complexes is rather complicated, showing lateral velocity variation with a characteristic wave-length of 5-10 km. In the vicinity of the MTL including the Sanbagawa metamorphic belt, the velocity is 0.1-0.2 km lower than that in the surrounding part. Several wide-angle reflections observed south of the MTL are modeled by relatively flat midcrustal interfaces situated at a depth range of 10-20 km. The MTL is recognized as a steeply northward dipping reflector to the middle crustal level. The PHS plate in the southern part is traced as a northward dipping strong reflector, whose depth is 22 km at the southern edge of the profile. The strong amplitude from the PHS indicates the existence of a thin (~ 0.5 km) layer with a relatively low velocity of 5 km/s at the top of the PHS. Low frequency events are distributed beneath this reflector, suggesting dehydrated fluid plays an important role for generation of strong reflection as well as seismic activity in this area. The northern extension of this reflector becomes flat as compared with the plate geometry roughly estimated from natural earthquakes. This difference may indicate the existence of secondary deformation structure such as a duplex zone associated with the plate subduction as is seen beneath the Shikoku Island.
T31A-1967
2D and 3D Prestack Depth Migration Seismic Imaging around NanTroSEIZE LWD sites as a mission for the IODP Implementing Organization
Since 1997, 2D seismic sections were acquired across the Nankai Trough by IFREE/JAMSTEC. Based on
these crustal structure studies for site survey purposes, the NanTroSEIZE drilling plan has been solidified. In
2003, CDEX acquired an additional commercial 2D MCS survey of closely spaced, high resolution. We used
a single 4240 in3 source and a 6000 m streamer.
The purpose of these site surveys is to reveal a higher resolution image and to define the seismic
characteristics of the accretionary prism in order to operate safety and smoothly during NanTroSEIZE drilling.
We performed 2D seismic processing as follows: apply minimum phase anti-alias filter, resample to 4 ms,
velocity analysis, adjacent trace sum with differential normal moveout, compensation for recording delay,
static sea level correction, spherical divergence correction, and surface consistent deconvolution.
We also refined our velocity model at the drilling sites by comparing the 2D seismic sections and Ocean
Bottom Seismograph (OBS) data acquired along the same line by IFREE/JAMSTEC.
A 3D seismic survey by M/V Nordic Explorer was carried out in May, 2006. This is the first 3D site survey
using a commercial seismic vessel. We designed track lines to cover a box region around the proposed
drilling sites and around 10km deep penetration. The system configuration was as follows: four 4500m
streamers and double array sources of G-guns with 3090 in3., alternate firing at 37.5m interval, resulting in a
792 km2 survey.
Commercial processing consisted of noise and multiple removal and 3D pre-stack time migration. At
IFREE/JAMSTEC we conducted full 3D depth imaging, including a velocity model that proved to be excellent
at the NanTroSEIZE drill sites. We were able to calibrate the depth/velocity model with well markers and to
make judgement a true depth of fracture and fault zone of the accretionary prism.
In August, 2008, we obtained an additional 2D MCS survey northeast of the current transect forpotential riser
drilling sites. These data will be pre-stack depth migrated to obtain a better understanding of the complex
structures in this area.
http://www.jamstec.go.jp/
T31A-1968
Finite Element Modeling of a Weak Fault Within a Coulomb Wedge
In an active margin, the material overlying the subduction plane is often assumed to behave as a Coulomb material. A splay fault may hypothetically be modeled as a weak plane having an effective friction coefficient lower than the internal friction coefficient of the Coulomb material. Accreting margins often display an outer thrust and fold belt, which may be approximated as a critical state Coulomb wedge, and an inner stable forearc domain. Critical Coulomb wedge theory can account for the transition from wedge to forearc, which may for example result from an increase of subduction plane slope, a decrease of basal friction coefficient, or a combination of both. However, this theory cannot be used to determine the state of stress in the transition zone, nor the consequences of a discontinuity within the margin. We use the ADELI finite element code (Chery and Hassani, 2000) to model the quasi-static stress and strain of a decollement and splay fault system, within an elasto-plastic wedge with Von Mises rheology. The subduction plane, the basal decollement of the accretionary wedge and the splay fault are modeled with contact elements. Two initial cases are considered: A simple constant taper Coulomb wedge with constant basal friction and a schematic margin comprising inner and outer domains with distinct tapers and basal friction coefficients. The purpose of this model is (1) to evaluate the friction coefficient threshold for splay fault activation as a function of the basal friction coefficients, (2) examine the consequences of motion along the splay fault on stress and strain within the wedge and (3) on the surface slope at equilibrium. Friction coefficients can be varied in time to mimic the consequence of the seismic cycle on the static stress state and strain distribution. Preliminary results will be presented.
T31A-1969
Fold Thrust Belt Kinematics from 3D Seismic Imaging along the NanTroSEIZE Transect, Nankai Accretionary Prism, Japan.
The accretionary prism of the Nankai Trough, Japan provides an excellent location to study the kinematics of a fold and thrust belt developed primarily in low permeability units. Recently acquired 3D reflection seismic data covering a 12 x 56 km area from the Kumano basin seaward to the deformation front reveals three structural domains within the frontal accretionary prism. The farthest seaward domain of the prism consists closely-spaced, apparently in-sequence thrust sheets forming a steep critical taper angle. The primary decollement beneath these thrusts ramps upsection above a topographic high within the oceanic basement. The farthest landward domain within the study area consists of the hanging wall of an apparently out of sequence megasplay thrust fault that dips landward to the top of the oceanic crust. The central structural domain within the prism consists of thrust sheets formed above a decollement approximately 2 km above the top of the oceanic crust, above the decollement associated with the more seaward thrusts. These thrusts are more widely spaced than those nearer the deformation front and are blanketed with syn and post kinematic sediments. Synkinematic sediments indicate that thrusts in the most seaward and most landward structural domains within the prism are the most active, though some reactivation of structures in the middle domain has occurred. Additionally, gas hydrate bottom simulating reflectors (BSRs) in the lowermost portions of the prism are present as discrete bands striking parallel to thrusts. These BSRs occur primarily along the most active thrusts, and are only present as small patches within the central structural domain of the prism. As these are expected to be the result of fluid migration along faults and permeable structures, they provide insight into the hydrologic controls on thrusting within the accretionary prism.
T31A-1970
Intraoceanic thrusts in the Nankai Trough off the Kii peninsula
We discovered intraoceanic thrusts developed as imbricate structure within the subducting Philippine Sea plate off the Kii Peninsula in central Japan manifesting as strong-amplitude reflections observed in an industry-standard three-dimensional (3D) seismic reflection survey. Although similar intraoceanic thrusts at plate convergent margins have been imaged, their 3D geometries have not been clearly defined, because of limited high-quality 3D seismic reflection data. Here we report on the 3D geometry of these imbricate intraoceanic thrusts, each of which cuts through the oceanic crust as a discontinuous thrust plane striking approximately parallel to trend of the Nankai Trough. In our survey area, furthermore, large reverse fault earthquakes with moment magnitudes over 7 occurred within the oceanic crust on 5 September 2004, causing strong ground motions on the islands of Japan and tsunami waves. The locations of many of the intraoceanic thrusts recognized in the 3D seismic data are within the error limits of the estimated hypocentres of the mainshocks and aftershocks of the 2004 earthquakes. Furthermore, their 3D geometry could generate the kind of complex rupture pattern observed during the 2004 events.
T31A-1971
3D Anisotropic Velocity Structure beneath the Kii Peninsula from P-wave Traveltime Tomography: Diagnostics of Seismic Anisotropy in a Subduction Zone
Seismic anisotropy is one of key elements to understand geodynamics such as mantle convection, plate tectonics, and evolutional process of the crust. Thus, it is crucial to investigate seismic anisotropy in the subduction zone where various phenomena are attributed to dynamic processes caused by interaction among adjacent plates. Actually, recent studies of seismic anisotropy show that the determination of a 3D seismic anisotropy structure can be potential diagnostics of a geological lineament structure inside the crust, and probe earthquake rupture areas and rupture nucleation points. In this study, we have evaluated the three-dimensional (3D) P-wave anisotropic velocity structure in the Kii Peninsula, southwest Japan, as well as the isotropic velocity structure by P-wave travel times tomography. The study area lies on the Eurasian (Amulian) plate above the subducting Philippine Sea Plate. This belongs to an accretionary prism, which is being developed at the margin of the Asian Continent, and is characterized by E-W trending metamorphic belts including a segment of the active faults zone called the Median Tectonic Line (MTL). Additionally, the Kii Peninsula region is presumed to be source regions of megathrust earthquakes along the Nankai trough. The resultant images of both the isotropic and anisotropic tomography show that the upper crust is characterized by E-W trending structure similar to that of the geological structure over the peninsula region. Because deformation of the crust such as preferred mineral alignment and recrystallization associated with planar structures produces significant seismic anisotropy, the plausible factor of the crustal feature is interpreted as E-W orientation of the regional metamorphic belt. Furthermore, in the resultant tomographic image, the E-W trending pattern is found within the deeper crust. This fact indicates that the lineament structure is sustained in the deeper crust. Since our tomography has good resolution in the vertical direction, it has potential for the diagnostics of a geological structure within the deep crust. In the mantle and slab region, tomographic images revealed that the Philippine Sea slab is segmented beneath the Kii Peninsula. We also found a region characterized by N-S anisotropy with low-velocity anomaly in the high-velocity slab with E-W anisotropy acquired during seafloor spreading. This region lies along the segment boundary of the rupture zones of the previous two megathrust events, and can be explained by the existence of the fault fractures with the N-S orientation revealed by wide-angle seismic profiles (Kodaira et al., 2006, year). The anomaly distributes beyond the northern leading edge of the previous rupture. If the anomaly controls the ruptures, a forthcoming event can be more terrific than the previous outbreaks.
T31A-1972
New Research Project For The Next Nankai Trough Mega Thrust Earthquakes -Towards To Understanding The Next Mega Thrust Earthquakes. And Mitigation Damages-
In the Nankai trough, mega thrust earthquakes are occurring with an interval of 100-200 years. So, many researches are focusing on the Nankai trough to elucidate the recurrence system of mega thrust earthquakes. The recent result of recurrence cycle simulation indicates the difference patterns and intervals of mega- thrust earthquake recurrences in each cycle. However, these results are not satisfied for the evaluation and estimation of next mega thrust earthquakes, so, we have to improve the structure model and the recurrence cycle simulation model with higher reliabilities. Especially, the estimation of recurrence cycle between the Tonankai and Nankai earthquake is very important for disaster preventions. Furthermore, the estimation of in coupled mega thrust earthquake around the Nankai trough such as Sumatra earthquake 2004 will be analyzed. To understand and estimate the next mega thrust earthquakes with detail structures and improved simulation model. We will propose research plan as follows, 1) Construct the detailed crustal medium around the Nankai trough using controlled sources and seismic tomography using dense seismic lines and OBS network arrays. 2) Observations of crustal activities around the Nankai trough and north eastern Japan. 3) Construct the database of long term plate coupling dynamics. And study the diversity of recurrence pattern and scale of next mega thrust earthquakes. 4) Develop the advanced simulation methods. 5) Improve the large scale recurrence cycle simulation model based on theoretical and experimental analyses. 6) Evaluate the precise strong motions and tsunamis for the disaster prevention. 7) Develop the reliable risk management system for next mega thrust earthquake. 8) Develop and contract the real time monitoring system around Kii peninsula. 9) Apply scientific results of Nankai seismogenic zone drilling to recurrence cycle simulation. The simulation research consists of the following five research groups. 1) Construction of database for spatiotemporal evolution of slip on the plate interface in the last 120 years. 2) Construction of database for diversity in the historical Nankai trough great earthquake sequences during several thousand years. 3) Development of advanced simulation methods. 4) Simulation research on factors controlling rupture mode of fault segments. 5) Construction of improved physical simulation model reproducing and predicting large diversity in long-term earthquake cycles. In order to understand the strong ground motions and tsunami expecting for future Nankai trough earthquake in detail for mitigation of possible disasters, we need study ground motions and tsunami from the former events occurred in 1944 and 1946 in detail. We will therefore conduct a computer simulation of strong ground motion and tsunami from the 1944 and 1946 events to reproduce strong ground motion and tsunami from the earthquake. We will modify structural model of elastic properties especially for the shear wave speed (Vs) and quality parameters (Q) that are rather difficult to investigate from geophysical explorations. The results of computer simulation for future Nankai trough earthquake are then used for 1) developing a realistic tsunami hazard map, 2) detail understanding of the damage of tall buildings and other important facilities, and 3) the planning of evacuation and recovery from the strong motion and tsunami disasters.
T31A-1973
Segmentation of the Subducting Philippine Sea Slab Beneath the Southwestern Part of Japan: Relationship With the Local Seismicity and Slip Distribution of Megathrust Earthquakes
In southwestern part of Japan, the Philippine Sea plate (PHS) is subducting beneath the Eurasia (or Amurian) plate at the Nankai Trough. This subduction causes recurrent megathrust earthquakes with M=8 or greater in the Nankai region. One clear rupture-segment boundary exists at the southern tip of the Kii Peninsula, where the source regions of the so-called "Tonankai" and "Nankai" earthquakes are divided. Structural features of the subducting plate (slab) beneath southwestern Japan may relate to rupture segmentation of the megathrusts. We applied an improved receiver function (RF) stacking analysis method to teleseismic data observed at 64 high-sensitivity (NIED Hi-net) and 9 broad-band (NIED F-net) seismometer stations in the Kii Peninsula and Shikoku Island, which span both historical rupture segments. Stacking both radial and transverse RFs with the assumption of a dipping slab, we found that PHS megathrust segmentation correlates with the location of intraslab seismicity. In western Shikoku Island and the eastern Kii Peninsula, down-dip direction of the slab Moho is aligned to the northwest, and is almost parallel to the direction of recent plate motion. The intraslab seismicity distributes just above the slab Moho beneath both regions. This means that seismic activity locates within the oceanic crust beneath these regions. The down-dip azimuth changes behavior beneath the southern Kii Peninsula and the northeastern part of Shikoku. Within these regions, the apparent slab-Moho dip azimuths deviate greatly from underlying plate motion and intraslab earthquakes are located below the slab Moho. The slab Moho appears to dip to the west or west-northwest in the central part of Shikoku. In this region, intraslab seismicity is relatively sparse and the slab Moho lies within the seismicity. This feature is very similar to the central Kii Peninsula [Shiomi and Park, 2008; JGR], i.e. the relationship between seismicity and PHS geometry in the Nankai region shows a mirror-reflection axis at the Kii Channel, which divides Shikoku Island and the Kii Peninsula. In the southwestern Japan, non-volcanic low-frequency tremor activity occurs along the subducting PHS, however, its activity is very low around the Kii Channel. Similarly, the slip distribution of the 1944 and 1946 megathrust earthquakes, inferred from old seismograms, geodetic data, tsunami waveforms and other datasets, imply that fault slip was concentrated at the eastern tip of the Kii Peninsula for the 1944 earthquake and the southern and southeastern tip of Shikoku for the 1946 event but rather small beneath the eastern mouth of the Kii Channel. Although slip estimation is uncertain due to the low data resolution, the regions of the greatest reconstructed slip for the 1944 and 1946 events correspond to the regions where the down-dip azimuth of the slab Moho is parallel to the recent plate convergence direction and to the intraslab earthquakes distribute above the slab Moho. At the southeastern tip of Shikoku Island, a fossil ridge called "Kinan Seamount Chain" aligns parallel to the direction of former plate motion before 3 Ma. The seamounts now may be subducting obliquely beneath the Kii Channel and the eastern tip of Shikoku, where the reconstructed slip of the 1944 and 1946 events is rather small. We conclude that spatial change of the relationship among the intraslab seismicity, the slab Moho geometry and rupture segmentation of megathrusts are strongly influenced by the structural anomaly caused by the subduction of the Kinan Seamount Chain.
T31A-1974
Deep seismic transect across the Tonankai earthquake area obtained from the onshore- offshore wide-angle seismic study
In the Nankai Trough subduction seismogenic zone, M8-class great earthquake area can be divided into three segments; they are source regions of the Nankai, Tonankai and presumed Tokai earthquakes. The Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. Hypocenters of these great earthquakes were usually located off the cape Shiono, Kii Peninsula, and the rupture propagated westwards and eastwards, respectively. To obtain the deep structure of the down-dip limit of around the Nankai Trough seismogenic zone, the segment boundary and first break area off the Kii Peninsula, the onshore-offshore wide-angle seismic studies was conducted in the western and eastern part of the Kii Peninsula and their offshore area in 2004 and 2006, respectively. The result of the seismic study in 2004 is mainly shown here. Structural images along the onshore and offshore profiles have already been separately obtained. In this study, an onshore-offshore integrated image of the western part of the Kii Peninsula, ~400km in a total length, is obtained from first arrival tomography and traveltime mapping of reflection phases by combining dataset of 13 land explosions, 2269 land stations, 36 OBSs and 1806 offshore airgun shots. The subduction angle of the Philippine Sea plate (PSP) gradually increases landward up to ~20-25 degree. Beneath the onshore part, the subducting PSP is estimated at ~5km shallower than that previously derived from seismicity. Low frequency earthquakes (identified and picked by Japan Meteorological Agency) are relocated around the plate interface of the subducting PSP by using the deep seismic transect obtained in this study. The offshore research is part of 'Structure research on plate dynamics of the presumed rupture zone of the Tonankai-Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology (MEXT). The onshore research carried by the Kyoto University is part of 'Special Project for Earthquake Disaster Mitigation in Urban Areas (RR2002)' funded by MEXT.
T31A-1975
Low-frequency Events in the Nankai Trough Observed by Ocean Bottom Seismographs
Anomalous very-low-frequency (VLF) earthquakes have been observed near the trough axis along the Nankai trough by land seismic networks (Ishihara, 2003, Obara and Ito, 2005). The focal mechanisms of the VLF earthquakes indicate an activity of thrust faults in the accretionary prism (Ito and Obara, 2006). After the 2004 Off Kii Peninsula earthquake (Mw=7.5), which occurred in the subducting Philippine Sea plate, many VLF earthquakes were observed in the aftershock area by the land seismic network (Obara and Ito, 2005). In addition to this, low-frequency tremors were observed by ocean bottom seismographs (OBS) deployed for the aftershock observation (Sakai et al., 2007). The low-frequency tremors were characterized by their long duration for tens of seconds to several minutes. The dominant frequency of low-frequency tremor observed by a 4.5 Hz seismometer is about 1-5 Hz. The low-frequency tremors might have some relation to the VLF earthquakes observed by the land seismic network. For further discussion in relations among the tremors, the VLF earthquakes, and crustal structures, it is necessary to know low-frequency tremor activities using the OBS data, which was obtained near the source region, including the period before the 2004 off Kii Peninsula earthquake. In this study, data recorded by the OBS experiments for seismicity observations were used to find anomalous low-frequency events along the Nankai trough. The OBS experiments were conducted off Kii Peninsula and Shikoku Island from 2001 to 2005 (Obana et al, 2004, 2005, 2006, 2008). In most cases, the OBSs were equipped with a three-component 4.5 Hz seismometer. The observation period for each experiment was up to 3 months. Several low-frequency events were found in the seismograms recorded by the OBS experiments in 2003 and 2005 off Kii Peninsula. The duration time of these low-frequency events reached tens of seconds to a few minutes. The dominant frequency of the low-frequency event is 2-8 Hz, which is lower than regular earthquakes. The epicenters of these events are estimated using differential times from envelope cross-correlation of vertical component seismograms. The obtained epicenter distribution suggests the low- frequency events observed in 2003 might relate to the thrust faults in the accretionary prism, such as a splay fault.
T31A-1976
Heat flow distribution and thermal structure of the Nankai subduction zone off the Ki-i Peninsula
Knowledge of the thermal structure along the subducting plate interface is critical for studies of large thrust earthquakes because the physical and chemical properties of the material are strongly temperature dependent. We carried out detailed heat flow surveys in the central part of the Nankai Trough off the eastern Ki-i Peninsula, the target area of IODP NanTroSEIZE drilling. Most of the measurements were made with conventional deep-sea heat flow probes. In the Kumano Trough (fore-arc basin) with water depths of about 2000 m, heat flow cannot be measured with the conventional probes because the bottom water temperature variation (BTV) is large, disturbing the temperature distribution in surface sediment. To solve this problem, we conducted long-term monitoring of the temperature distribution in surface sediments using pop-up type instruments we recently developed. Analyzing the temperature records for 300 to 400 days, we could successfully remove the effect of BTV and determine heat flow values at seven stations. The heat flow data measured at the surface were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone. Heat flow is 110-120 mW/m2 on the floor of the Nankai Trough, decreases landward on the accretionary prism slope to about 60 mW/m2 at around 30 km from the deformation front, and is quite uniform in the fore-arc basin, 50-60 mW/m2. The heat flow on the trough floor appears to be consistent with the value estimated from the age of the subducting Shikoku Basin, about 20 m.y. Taking account of the effect of recent rapid deposition of sediments, however, the observed heat flow must have been reduced by about 15%. It suggests the temperature structure of the Shikoku Basin may be warmer than that of normal oceanic lithosphere. The heat flow profile across the prism and the fore-arc basin was used as constraints on thermal modeling of the subduction zone. We calculated the thermal structure using a two-dimensional, steady-state finite element model, in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters. Comparison of the calculated surface heat flow profile with the observed data indicates that the frictional heating should be almost negligible and the shear stress in the seismogenic zone is very low.
T31A-1977
Rate and recurrence interval of short-term slow slip on plate interface detected by non- volcanic deep low frequency tremors
Non-volcanic deep low-frequency (DLF) tremors and slow slip events (SSE) occur in the transition zone between unstable and stable slip zones on the plate interface in subduction zones. In southwest Japan, DLF tremors show a spatial and temporal correlation with a short-term SSE [Obara et al., 2004] and the source area of a long-term SSE is adjacent to those of short-term SSEs [Hirose et al., 1999]. Monitoring of the slip in the transition zone on the plate interface is important to understand the subduction process of oceanic plate, because the slip such as SSEs occurred at deeper extension of the seismogenic zone of large earthquakes on the interface. Hiramatsu et al. [2008] proposed that DLF tremors are useful tool for real-time monitoring of the slip on the plate interface. In this study, we show the rate and the recurrence interval of short-term slow slip on the plate interface in the Shikoku region estimated by DLF tremors. In this study, we analyze DLF tremors in the Shikoku region, southwest Japan, from January 2001 to May 2007 recorded by Hi-net [Obara et al., 2005]. We use the hypocenters of the tremors determined by the envelope correlation method [Obara, 2002; Obara and Hirose, 2006] and analyze the tremors whose duration is longer than two minutes. We use reduced displacements [Aki and Koyanagi, 1981] as the amplitudes of a tremor. Following Hiramatsu et al. [2008], we assume that the envelope of the reduced displacement provides an apparent moment rate function and that a total size of DLF tremors is proportional to the size of corresponding SSE. We estimate a conversion factor from the apparent moment to the seismic moment of tremors and obtain a temporal variation in the cumulative seismic moment due to the slip on the plate interface in the analyzed region [Hiramatsu et al., 2008]. We can find step-like changes in the temporal variation in the cumulative seismic moment. Some changes correspond to the occurrence of short-term SSEs observed geodetically (e.g. Hirose and Obara, 2005) but the others not, indicating that our approach can detect geodetically missing SSEs. The short-term SSEs, larger than 2.5×1017 Nm, detected by DLF tremors repeat at intervals of approximately six months in the western Shikoku except the period from August to November 2003 when a long-term SSE occurred [Hirose and Obara, 2005]. The recurrence interval is approximately three months in the eastern Shikoku, but it is not clear in the central Shikoku. The recurrence interval of the short-term SSEs seems to be proportional to the size of the last SSE. We also find a steady increase in the cumulative seismic moment with time except the period of a long-term SSE. In the western Shikoku, the moment release rate due to short-term SSEs on the plate interface is estimated to be 5.47×1018 Nm/yr for the analyzed period. Assuming that the fault area is equal to the active area of the DLF tremors (3.4×109 m2) and the rigidity is 40 GPa, we obtain the slip rate of 4.0 cm/yr. The convergence rate of the Philippine Sea plate in this region is estimated to be 6.3-6.8 cm/yr from GPS observations [Miyazaki and Heki, 2001]. The average slip rate at the transition zone on the plate interface is, thus, slower than the convergence rate of the plate in the western Sikoku for the analyzed period.
T31A-1978
Effect of Seafloor Current on low Frequency Seismic Noise Observed in the Seafloor
Seismometer installed in the seafloor is affected by water flow in the seafloor. Such effect is significant especially for low frequencies below 0.1 Hz. Burial of seismometer, or installation using deep borehole, is suggested to reduce effect of seafloor current. There are a number of examples showing burial in the seafloor significantly reduce low frequency seismic noise. The reason for such improvement is explained by improved seafloor coupling of seismometer and reduction of tilting effect due to force from water flow by minimizing area of seismometer exposed in the water flow. Another mechanism is that turbulence in the seafloor current impose load on the seafloor that deforms and tilt the seafloor. There are few example, though, to demonstrate a model which explain relationship between speed of seafloor water flow and expected low frequency seismic noise quantitatively. We are currently planning to build seafloor broadband seismic network off Kii Peninsula, Honshu, Japan, where recurrence of magnitude 8 class earthquake is expected in the future. In such location, a network of broadband seismometer will be deployed in the seafloor. In order to design for optimum low frequency performance of seismometers, we conducted test installation of three broadband seismometers (Guralp CMG3T) closely, within 150m distances from each other. The test observation was conducted in the Kumano Basin, from late December, 2007 to early February, 2008. The one cylindrical seismometer was buried in the seafloor, while the other two sit on the seafloor. The seafloor seismometers were different in that the one had grid type anchor and the other had wide planar anchor. Difference in these arrangements was intended to find optimum design of seismometer package and installation method. Differential pressure gauges, a seafloor current meter, thermometers were installed to monitor environmental change during the test observation. Observation data from deployed broadband seismometers suggested that buried seismometer exhibits significantly lower noise especially for horizontal component, more than 20 dB in 13-50 second period. For vertical component, buried seismometer was not so quieter than the seafloor. Comparison result of two seafloor seismometers suggests better coupled seismometer has superior noise performance. We investigated coherence of low frequency noise between the closely spaced broadband seismometers. In lower end frequencies below 0.01Hz where the infragravity noise dominates, good coherence was found not only for seafloor pressure, but also for vertical seismic motion. On the other hand, noise between 13-50 second period was not coherent for both vertical and pressure data, showing inherently very much localized nature of low frequency noise source in the frequencies. Although we see no coherency between seismic data, amplitude of noise in the low frequencies does correlate not only between seismic records but also with seafloor current speed. This relationship between seafloor current speed and seismic noise amplitude is useful to construct quantitative model of mechanisms creating low frequency seismic noise in the seafloor.
T31A-1979
Long-term temperature monitoring at the biological community site on the Nankai accretionary prism off Kii Peninsula
Nankai subduction zone off Kii Peninsula is one of the most intensively surveyed areas for studies on the seismogenic zone. Multichannel seismic reflection surveys carried out in this area revealed the existence of splay faults that branched from the subduction zone plate boundary [Park et al., 2002]. Along the splay faults, reversal of reflection polarity was observed, indicating elevated pore fluid pressure along the faults. Cold seepages with biological communities were discovered along a seafloor outcrop of one of the splay faults through submersible observations. Long-term temperature monitoring at a biological community site along the outcrop revealed high heat flow carried by upward fluid flow (>180 mW/m2) [Goto et al., 2003]. Toki et al. [2004] estimated upward fluid flow rates of 40-200 cm/yr from chloride distribution of interstitial water extracted from sediments in and around biological community sites along the outcrop. These observation results suggest upward fluid flow along the splay fault. In order to investigate hydrological nature of the splay fault, we conducted long-term temperature monitoring again in the same cold seepage site where Goto et al. [2003] carried out long-term temperature monitoring. In this presentation, we present results of the temperature monitoring and estimate heat flow carried by upward fluid flow from the temperature records. In this long-term temperature monitoring, we used stand-alone heat flow meter (SAHF), a probe-type sediment temperature recorder. Two SAHFs (SAHF-3 and SAHF-4) were used in this study. SAHF-4 was inserted into a bacterial mat, within several meters of which the previous long-term temperature monitoring was conducted. SAHF-3 was penetrated into ordinary sediment near the bacterial mat. The sub-bottom temperature records were obtained for 8 months. The subsurface temperatures oscillated reflecting bottom- water temperature variation (BTV). For sub-bottom temperatures measured with SAHF-3 (outside of the bacterial mat), we found that the effects of the BTV propagated into sediment by conduction only. By correcting the effect of the BTV, conductive heat flow estimated is higher than 100 mW/m2. Sub-bottom temperatures measured within bacterial mat (SAHF-4) except for the topmost sensor could be explained by a conduction model. The heat flow estimated based on the conduction model is similar to that measured with SAHF-3. The temperature of the topmost sensor is slightly higher than that expected from the conduction model. To explain the high temperature, upward fluid flow at a rate of 10-7 m/s order is needed. Heat flow carried by the upward fluid flow is higher than that estimated by Goto et al. [2003]. Heat flow value expected from the distribution of heat flow around this area is 70-80 mW/m2. The high heat flow values inside and outside the bacterial mat estimated in the present and previous studies may reflect upward fluid flow along the splay fault.
T31A-1980
High-Velocity Frictional Behavior of Clay-Rich Sediments from IODP Expedition 316, Nankai Trough, Offshore Japan
Subduction zone related earthquakes involve a wide range of slip velocities ranging from low strain-rate aseismic creep to high strain-rate dynamic rupture. Understanding the frictional behavior of accreted sediments, especially at seismic slip velocities, is essential in helping to understand rupture propagation processes within the accretional prism during subduction great earthquakes. In order to investigate the coseismic frictional properties of these sediments, high velocity friction experiments were performed using a rotary-shear friction apparatus at slip velocities, V, of 0.02-1.3 m/s, normal stresses of 0.6-1.8 MPa and displacements of over 3 m under dry and wet (water-saturated) conditions. Samples used in this study were collected from IODP Expedition 316, site C0007D, 437.4 mbsf: the basal part of the accretionary prism above the frontal thrust (Kimura et al. 2008). It consists mainly of clay minerals (smectite and illite) with quartz, plagioclase and calcite. Samples were disaggregated to less than 0.1 mm in grain size, and then sheared between cylindrical sandstones with a porosity of ~9%. A Teflon sleeve was used to keep the disaggregated sediment between the sandstones. Our preliminary results can be summarized as follows: (1) At V >0.17 m/s, the frictional coefficient increased rapidly to 0.7-0.8 at the initiation of slip and then decreased gradually with displacement to steady- state values of 0.2-0.6 and 0.05-0.1 for dry and wet conditions, respectively. In contrast, as V decreased below 0.06 m/s, no marked slip-weakening behavior appeared. Steady-state friction coefficient indicated 0.7- 0.8 for dry and 0.3-0.4 for wet condition. (2) On the experiments at V = 1.3 m/s under the wet conditions, the steady-state shear stress became independent of normal stress (slope of the shear- versus normal- stress curve was nearly zero). (3) Localized zones with tens of microns in thickness were developed within the artificial fault zone, with an initial thickness of ~1.2 mm. Clay minerals in the localized slip zone were dehydrated due to frictional heat generated by the high slip velocity and large displacement, and formed well- developed planar fabrics. The formation and evolution of such a localized slip zone and the effect of frictional heating (i.e. pressurization of fluid) may lead to very low shear strength of clay-rich sediments at coseismic slip.
T31A-1981
Ultrasonic P-wave velocity measurements with variable effective pressure at the boundary between slope basin sediments and the accretionary prism: IODP Expedition 315 Site C0001
IODP Expedition 315 Site C0001 is located on the hanging wall of the midslope megasplay fault in the Nankai subduction zone off Kii peninsula (SW Japan), and penetrated an unconformity between ~200 m thick slope basin sediments and the accretionary prism. While a down-section porosity increase was clearly observed at the boundary from ~50% to ~60%, logging velocity does not appear to decrease at the boundary, which suggests that different diagenetic processes might exist above and below the boundary. In this study, we conducted ultrasonic P-wave velocity measurements with pore pressure control. We also conducted observations of sediment and chemical analysis. We examined the relationships between the acoustic properties, sediment textures, logging data from IODP Expedition 314 Site C0001 and data from shipboard core analysis. The ultrasonic P-wave velocity measurements were conducted under constant pore pressure (500 kPa) and varying confining pressure to control effective pressure. The confining pressure ranges from 550 kPa to a maximum calculated from the density of overlying sediments (lithostatic pressure – hydrostatic pressure). 8 samples were analyzed, located from ~70 m to ~450 m below the sea floor. P-wave velocity ranges from ~1620 m/s to ~1990 m/s under the hydrostatic pressure condition. These velocities are in good agreement with the logging data. Porosity-velocity relationship in the analyzed data also coincide with that observed in the logging data. Samples shallower than ~300 m fall within previously-defined empirical relationships for normal- and high- consolidation. The deeper samples (at ~370 m and ~450 m below sea floor) show much higher velocity than that predicted by the empirical relationship, suggesting that significant cementation is present in those samples. The textural observations of sediments indicate a decrease in pore space with depth. Quartz and feldspar grains are surrounded by clay mineral matrices. Grain size seems to be almost constant through the site. Quartz or carbonate cements are not observed. Instead, authigenic pyrites are abundant in deeper samples.
T31A-1982
Detailed 3D Architecture of a Thrust Fault System and Associated Folding: Nankai Trough Accretionary Wedge
3-D seismic data recently acquired in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) region off the Kii peninsula of Honshu Island, Japan, images a modern accretionary prism in unprecedented detail. Although this large (12 x 56 km), deep (>12 km) survey was shot to image the entire outer forearc region, the excellent pre-stack depth migrated imaging also permits detailed interpretation of individual structures to a fine scale. Due to the high frequency content and horizontal resolution in the upper few kilometers sub-seafloor, even small-scale faults and variation in stratigraphic horizons can be easily discerned. We performed detailed interpretation of thrust faulting and anticlinal folding in a single prominent fault-propagation fold structure in the midslope region, using Landmark Geoprobe software. At an inline- crossline spacing of 12-20 meters, detailed interpretations showing layer geometry and second-order subsidiary faulting can be used to develop a better understanding of how the thrust faults of this area have developed and evolved over time. The anticline exhibits a pronounced central culmination, with lower- amplitude fold relief in both the northeast and southwest directions. Analysis of the geometry of the large population of subsidiary thrusts in the anticline permits inversion for principal axes of strain. These will be compared to the footwall subsidiary faults to shed light on deformation processes in forming this structure.
T31A-1983
Preliminary heat flow results from NanTroSeize Expeditions 314/315/316 Nankai Trough, Japan
Knowledge of the temperature distribution of the frontal thrust and shallow portion of the megasplay system off shore the Kii Peninsula is important for understanding the geodynamics of active plate boundaries and processes influencing the nature of the seismic/aseismic transition. During Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 temperature measurements were made at seven sites using both the APCT-3 and DVTP temperature tools. These data are of high quality and are internally consistent. The deepest measurement at each site varied from ~80 to ~243 mbsf. In addition, extensive thermal conductivity measurements were made on the recovered cores allowing thermal gradients to be reliably converted to heat flow. Thermal conductivities are combined with temperature data to form thermal resistance plots. In general temperature increases linearly with thermal resistance suggesting that a constant heat flow characterizes the data at each site. Heat flow values show relatively large spatial variations consistent with an accretionary prism undergoing active faulting and dewatering. Computed values of heat flow are anomalously low with respect to other values of heat flow along the Nankai Trough and in the vicinity of the Kii transect (Yamano et al., 2003, Phys. Chem. Earth, 28, 487-497). Reconciling these lower values obtained deeper in the section with shallow probe measurements requires a decreasing thermal gradient with depth.
T31A-1984
Geotechnical Characterization of Shallow Sediments from NanTroSEIZE Sites C0004, C0006, C0007, and C0008 (IODP Expedition 316)
Constant-rate-of-strain consolidation experiments completed on shallow (less than 100 mbsf) sediments from Sites C0004 and C0006 define the vertical permeability and compressibility of the sediments while providing insights into the stress history. Experiments on three specimens from Site C0004 document an increase in permeability from 2x10-16m2 at 17 mbsf to 4x10-14m2 at 75 mbsf. While increasing permeability is uncommon where porosity decreases, this observation may be controlled by lithologic variation. Overall, lithologic Unit I at Site C0004 is silty clay, but shipboard observations document a minor downhole increase in sand content for the unit. Minor increases in grain size can yield higher permeability. Site C0006 has a near-seafloor (2 mbsf) permeability of 2.4x10-15m2. This specimen was coarser grained (silty sand or sandy silt) than the specimens from C0004, which is consistent with the shipboard lithologic descriptions. Compression indices for sediments from Site C0004 ranged from 0.5-0.9 which is greater than that measured for the specimen from C0006 (0.3). Lower compressibility at Site C0006 is compatible with the coarser grain size; however the difference could also be related to different stress histories. Comparison of the preconsolidation stress (or maximum past effective vertical stress) to the hydrostatic effective vertical stress suggests all specimens are overconsolidated. The overconsolidation ratio (OCR) at Site C0004 ranges from 1.3-2.7. The specimen from Site C0006 has an OCR of 40. These OCRs suggest erosion of shallow material at both sites. Shipboard analyses of the low porosity at shallow depths also resulted in an interpretation of erosion of shallow material at Site C0006. The high OCRs in this region preclude interpretation of in situ fluid pressures from preconsolidation stresses. These preliminary results will be augmented with horizontal permeability measurements and grain size analyses. Additional, ongoing experiments will provide new geotechnical data for Site C0006 (2 additional samples), Site C0007 (3 samples), and C0008 (3 samples). These geotechnical experiments fit in one of the greater NanTroSEIZE objectives to understand linkages between fluid flow and deformation behavior of sediments within the Nankai accretionary complex.
T31A-1985
A Comparison of Compression Behavior of Mudrock Core Samples from the Nankai Margin, SW Japan and the Ursa Basin, Gulf of Mexico
Characterizing the consolidation behavior and permeability of marine mudstones is an essential step toward estimating in situ pore pressure and stress, and in parameterizing forward models of sedimentation, loading, and consolidation at both active and passive continental margins. Here, we report results of mechanical tests on mudrock samples from the Nankai margin, SW Japan (collected at IODP site C0001E), and from the Ursa Basin in the Gulf of Mexico (GOM) (IODP site U1324), to compare the compression behavior of marine sediments from these distinct environments. Samples from Site U1324 were taken from depths of 50-150 mbsf, and are composed of 40% silt and 60% clay, with porosities of 42-55% depending on sample depth. Samples from the same depth range at site C0001E are more brittle and siltier, with porosities of 58-64%. We conducted tests using two experimental configurations: (1) a triaxial vessel, in which the sample is subjected to axial compression and a condition of zero radial strain (K0 condition) is maintained by a closed loop servo-control system with feedback on sample diameter; and (2) a high-pressure oedometer (uniaxial consolidation) vessel in which axial strain is imposed and the K0 condition is ensured by a fixed steel ring. The triaxial tests also yield a measurement of the K0 value, describing the ratio of horizontal and vertical effective stresses. After consolidation, some specimens were subjected to undrained shearing in the triaxial system, in order to define relationships between mean effective stress, differential stress, and porosity. The consolidation coefficient Cv of samples from the Nankai margin (2-4× 10-7 m2/s) is significantly higher than that of samples from the GOM (2.2±0.2 × 10-8 m2/s), which we attribute to their higher porosity and silt content. The compression index Cc of the samples from Site C0001E (Nankai) is typically >0.70; values of Cc for the samples from site U1324 (GOM) range from 0.2-0.5 and depend strongly on the initial porosity and thus on depth. The difference between the two locations is likely an effect of the higher initial porosity for the samples from the Nankai margin. The values of K0 also differ markedly: samples from Site C0001E exhibit values of K0 = 0.4-0.6, with most values <0.50, whereas K0 = 0.56-0.60 for samples from Site U1324. This suggests that the effective horizontal stress is about 50% of the vertical effective stress in the uppermost sediments at Site C0001E, whereas it is 56-60% of the effective vertical stress at Site U1324. Undrained shear tests define a residual friction angle of 22-26° for the samples from the Gulf of Mexico, and reveal that the mudstone consolidation exhibits a sensitivity to both mean effective stress and differential stress, as predicted by cam-clay models. These results highlight a potential method for predicting in situ stress and pore pressure in actively deforming thrust belts, in which the mean effective stress and effective stress ratio can be inferred from porosity and the frictional angle, respectively.
T31A-1986
Anisotropy of Electrical Resistivity and P-wave Velocity in Discrete Samples From Nantroseize Expeditions 315 and 316
We present results of electrical conductivity and P-wave velocity measurements in discrete samples processed onboard Chikyu during Nantroseize expeditions 315 and 316 in the Mega Splay fault and Frontal Thrust of the Nankai accretionary prism. Quasi cubes of 20 mm thickness nominally saturated with seawater were measured across the three parallel sets of faces, first for electrical conductivity, then for P-wave velocity. Average properties and their anisotropies appeared to show some sensitivity to both lithological and fault related features. Overall, strong transverse anisotropy due to sedimentary compaction was observed for both properties with minimum electrical conductivity and P-wave velocity along the vertical core axis direction. Within the horizontal plane perpendicular to the core axis, slight anisotropies were also measured, which are likely related to tectonically driven horizontal shortening, affecting noticeably the original compaction fabric. In order to get some structural insight, samples were reoriented in our laboratories using alternative field demagnetization technique and the in-plane (i.e. perpendicular to the core axis) data rotated accordingly. For the P-wave anisotropy, initial cubes were shaped into polyhedrons in order to get even more accurate estimates. These additional measurements allowed for retrieving for each sample the 3 principal values and vectors of the best fitting ellipsoidal function. Resulting stereoplots were then compared with electrical resistivity, magnetic susceptibility (AMS) and structural data.
T31A-1987
Update on Advanced CORK Pressure Data from the Muroto Transect of the Nankai Accretionary Prism
In 2001, ODP Leg 196 installed Advanced CORK hydrologic observatories in two holes on the Muroto Transect of the Nankai Trough: the reference Hole 1173B about 12 km seaward of the deformation front, and Hole 808I, which penetrates through the frontal thrust and nearly to the decollement. Unfortunately, the installation at Hole 808I was compromised when poor drilling conditions precluded advancing the 964-m-long ACORK for the final 37 m. This left the ACORK head draped onto the seafloor, fortuitously with pressure- monitoring instrumentation undamaged and still functioning well. However, installation of a seal in the deepest monitoring interval was not possible, leaving it open to interaction with ocean bottom water via the ACORK casing, such that the primary goal of monitoring pressures in the decollement was compromised and there probably were subtle effects on pressures monitored in other intervals as well. In December of 2007, the JAMSTEC Kaiko-7k team succeeded in clearing the mud from the ACORK head and installing a bridge plug in its central bore, which should have finally sealed the deepest monitoring interval. In October 2007, JAMSTEC cruise KR08-13 is scheduled to return to the ACORKs for the first data downloads since the installation of the bridge plug in Hole 808I. We will report the results from these most recent datasets from both holes. Full copies of these and past ACORK data will be made available both as part of the presentation and on the internet at a URL to be reported.