T23A-1988
Crustal deformation in the Kumano Basin along the Nankai Trough inferred from repeated seafloor geodetic observations
At the Nankai Trough (NT), the Philippine Sea plate (PH) subducts beneath the southwest Japan at a rate of about 4-6 cm/yr, where great interplate earthquakes have repeatedly occurred every 100-200 years. A number of researchers have investigated crustal deformation caused by subduction of the PH based on geodetic measurements as represented by GPS observation. However it is difficult to infer the plate coupling strength in offshore areas, due to the poverty of offshore geodetic data. From a viewpoint of disaster mitigation, it is important to know the updip and downdip limit of the plate locking depth. For this issue, we have conducted observations of the seafloor crustal deformations around the NT using a GPS/Acoustic technique since 2004. In this system, we estimate the position of a surveying vessel by Kinematic GPS analysis and measure the distance between the vessel and the benchmark on the sea floor by Acoustic measurements. Next we determine the location of the benchmark. For the repeatability of observation, the location of benchmark is determined within a precision of 2-3 cm at horizontal components (Tadokoro et al., 2006). In the Kumano Basin, we have two seafloor benchmarks, which are located about 60 and 80 km away from the deformation front of the NT. The observations from 2005 to 2008 have illustrated that these benchmarks are moving at rates of about 5-6 cm/yr with velocity uncertainties of 1-3 cm/yr relative to the Amurian plate. In this study, in order to estimate interplate coupling at the NT, we calculated surface deformations accompanied with plate subduction in an elastic half-space and compared them with on- and offshore GPS velocities. Then, we investigated the effect of observation for the seafloor crustal deformations on slip resolution on the plate interface. We conclude that offshore crustal deformation data provide good constraints for the estimation of fault slips at the shallower part of the plate interface, especially at the depths of 10-20 km, where slip resolution is low without using only offshore geodetic data. Future additional crustal deformation observations on the seafloor would lead to the better estimation of sea floor velocities, which help us reveal strain accumulation process in the seismogenic subduction zone during interseismic period.
T23A-1989
Can Interseismic Geodetic Observations Resolve Persistent Rupture Asperities? A study of the Japan trench off Tohoku.
In the last century, several large (M > 7) earthquakes have occured on the megathrust interface along the Japan Trench, offshore of Japan's Tohoku region. Published earthquake source inversions based on seismological data suggest that some of these earthquakes have repeatedly ruptured the same region of the fault (i.e., asperities), while others have ruptured closely clustered asperities (e.g., Yamanaka and Kikuchi, 2004). For instance, the 1978, M 7.4 and the 2005, M 7.2 Miyagi-oki events are inferred to have ruptured the same asperity, while the 1968, M 7.9 Tokachi-oki event, and the 1994, M 7.5 Sanriku-oki event ruptured distinct asperities that are close to each other. In contrast, inversions of geodetic data from interseismic periods produce models that are locked over more spatially extensive regions (e.g., Suwa et al, 2003). These broad and smooth regions are in contrast to the smaller discrete asperities indicated by earthquake source studies, and may be a consequence of lack of model resolution and a resulting need for regularization that is inherent to the use of onshore geodetic data. Alternatively, the differences may imply the potential for a large earthquake in the future. Thus, the different levels of apparent coupling implied by these two classes of models have very different implications for regional seismic hazard. Here, we test the hypothesis that mechanical coupling on inferred asperities alone is sufficient to explain available geodetic observations or alternatively, that these data require additional regions on the megathrust to be coupled. To address this question, we use a 3-D mechanical model of stress-dependent interseismic creep along the megathrust, that is consistent with a given frictional rheology and the known spatio-temporal distribution of large earthquakes. These mechanical models predict that asperities are surrounded by a "halo" of very low creep-rates (a "stress-shadow" effect) late in the seismic cycle, which also results in a relatively smooth and long wavelength surface velocity field (see poster by Hetland et al. in this session). We test if this "physical" smoothing preserves any signature of the original asperities, in comparison to the artificial smoothing produced by model regularization in inversions of interseismic geodetic data. Underlying this analysis is the assumption that known asperities persist across multiple earthquake cycles.
T23A-1990
Coseismic and early postseismic deformation of the 14 Mw=7.7 Tocopilla earthquake: Results from space-geodetic and seismological data
The November 14, 2007 Mw=7.7 earthquake nucleated at 15:40:53 (UTC time) on the west coast of northern Chile. Centred at 22.64°S and 70.61°W, about 40 km south-southeast of the town of Tocopilla, it destroyed about 1200 homes and left about 15% of the inhabitants homeless. The mainshock was felt strongly through much of northern Chile, as far north as La Paz and as far south as Santiago. The 2007 event took place in the central part of a large seismic gap that last ruptured in 1877 with an earthquake of magnitude 9. Here, we examine space-geodetic (InSAR & Ws-InSAR) and seismological data to study coseismic and early postseimic deformation of the 14 Mw=7.7 2008 Tocopilla earthquake, Chile. We propose a slip model that is consistent with both, geodetic and seismic dataset, and demonstrate that joint data analysis may significantly improve interpretability and physical understanding of the rupture pattern and earthquake dynamics.
T23A-1991
3-D P-wave velocity structure and seismicity in Central Costa Rica from Local Earthquake Tomography using an amphibic network
The Central Pacific sector of the erosional margin in Costa Rica shows a high seismicity rate, coincident with the subduction of rough-relief ocean floor, and generates earthquakes up to Mw 7. Precise earthquake locations and detailed knowledge of the 3-D velocity structure provide key insights into the dynamics of subduction zones. To this end, we performed a 3-D Local Earthquake Tomography using P-wave traveltimes from 595 selected events recorded by a seismological network of off- and onshore stations, deployed for 6 months in the area. The results reflect the complexity associated to subduction of bathymetric highs and the transition from normal to thickened oceanic crust (Cocos Ridge). The slab is imaged as a high-velocity anomaly with a band of low velocities (LVB) on top enclosing the intraslab events deeper than ~30 km. Below the margin slope, the LVB is locally thickened by at least two seamounts. We observe an abrupt, eastward widening of the LVB, preceded by a low-velocity anomaly under the continental shelf, which we interpret as a big seamount. The thickening coincides with an inverted basin at the inner forearc and a low-velocity anomaly under it. The latter appears in a sector where blocks of inner forearc are uplifted, possibly by underplating of eroded material against the base of the crust. The anomaly promotes seismicity by high-friction with the upper plate, and could be linked to a Mw 6.4 earthquake in 2004. In the west part of the area, the interplate seismicity forms a cluster beneath the continental shelf. Its updip limit coincides with the 150° C isotherm and an increase in Vp along the plate boundary. This further supports a proposed model in which the seismicity onset along the plate interface is mainly due to a decrease in the abundance of the fluids released by subducted sediments. Higher seismicity rates locally concur with seamounts present at the seismogenic zone, while seamounts under the margin slope may shallow the onset of interplate seismicity. To the east, a sharp decrease of interplate seismicity could be caused by a reduced mechanical competence of the upper plate, coincident with a big embayment in the margin slope. The highest rates of intraslab seismicity coincide with subduction of thickened crust. Since hydration at the outer rise is much diminished due to impaired bending ability, then hydration at other tectonic scenarios seems to play an important role for seismogenesis.
T23A-1992
Stress Changes and Aftershock Distribution of the Java Tsunami Earthquakes
Tsunami earthquakes are earthquakes that generate tsunami waves that are large with respect to their moment magnitude. The long rupture duration and low radiated energy (with respect to their moment magnitude) at long periods of these events imply that they are the result of the slow rupturing velocity and shallow rupture. Two examples, the 1994 and the 2006 Java tsunami earthquakes, occurred at the same subduction zone with similar moment magnitudes (Mw=7.8 and Mw=7.7, respectively) and thrust faulting mechanisms. Unusually, a large percentage of the aftershocks for these events have normal, rather than reverse focal mechanisms. One model proposed for these unusual mainshock-aftershock sequences in Java describes rupture of a locked subducted seamount on an otherwise decoupled zone, resulting in extension of the outer-rise causing the normal faulting aftershocks. Other models suggest that these tsunami earthquakes occurred in a part of the subduction zone where usually earthquakes cannot propagate (or initiate) leading to occurrence of normal faulting aftershocks in the outer-rise, overriding and subducting plates. Here we investigate the stress conditions in the Java subduction zone within the region of these earthquake sequences by calculating the Coulomb stress changes (CFS) imparted by the two large subduction events. Thus we can test whether the normal faulting aftershocks following each of the earthquakes are simply occurring on appropriately oriented planes that experience stress increases due to the interface slip during the tsunami earthquakes. Preliminary results for the 1994 tsunami event show that the dominant stress changes in the system are produced by the mainshock with Coulomb stress increases in the region north of the 1994 event and stress decreases south of the mainshock. Stresses calculated on fault planes matching the normal fault orientations range from positive to negative, thus the aftershocks do not appear to be simply triggered by the stress changes from the mainshock. However, our simplified model here does not incorporate other physical properties, such as variations in coupling along the fault that may be important for understanding the normal fault aftershocks.
T23A-1993
Imaging The Deep Crust Of The Chilean Continental Margin Using Wide-angle Data
We present the application of Kirchhoff prestack depth migration (KPDM) to two amphibious wide-angle datasets, with the aim to obtain new images of the seismogenic coupling zone in North and South of Chile. Between 19°S and 26°S the CINCA-95 project combined offshore/onshore seismic measurements including wide-angle and near-vertical data to investigate the evolution and structure of the subduction zone. In southern Chile, between 36°S and 40°S, active seismic experiments during the SPOC-2001 project were carried out to compile data of the seismogenic plate interface in the region. The velocity models used for migration were derived from interpretation and inversion of traveltime data, showing the most prominent structural features. The migration results in Northern Chile image the subducting Nazca Plate as a strong dipping reflector (18°-25°) at 20-30 km depth. Fresnel Volume Migration (FVM), a novel extension of Kirchhoff prestack depth migration was applied to the data providing successful results with an improved signal to noise ratio and resolution. Additionally, an offset range analysis was performed by dividing the data into short, intermediate and far offsets and by migrating each range separately. The short and intermediate offset migrated sections show a couple of intracrustal reflections between 15-25 km depth whereas the far offset section shows the Nazca Plate similar to the full-offset section. The migrated sections corresponding to the southern profiles, show the oceanic plate as a sharp reflector at 20-23 km depth. In summary the sections provide a first insight into the structural architecture of this seismogenic coupling zone which has not been imaged before.
T23A-1994
The Chilean Subduction Zone at 38.2° S: Implications for the Seismogenic Coupling Zone and the Subduction Channel
The crustal structure of the subduction zone in south central Chile was revealed with high-resolution reflection seismic imaging within project TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes). The near-vertical incidence reflection seismic profile spans across 140 km from the coast of the Pacific Ocean to the Central Valley. The downdip end of the seismigenic coupling zone and the hypocenter of the great Chilean earthquake of 1960 (Mw = 9.5) lie in the center of the seismic section. Here, we show the structural inventory of the Chilean fore-arc at 38.2°S. The downgoing oceanic plate can be traced from 27-55 km depth. The oceanic crust has a varying reflectivity. A smooth increase in reflectivity below 30 km depth may be caused by the release of fluids because of the porosity collapse in the oceanic basalt. A zone of high Vp/Vs ratio supports this observation. A clear structurally imaged continental Moho could not be found, but it may be inferred together with constraints from gravimetrical modelling. Strong reflectivity above the plate interface may be associated with a subduction channel with a varying thickness of 2-5 km. Local seismicity possibly defines its upper boundary. The segmented crust of the overriding plate has two great seismically transparent zones, devided by the crustal Lanalhue Fault Zone. The eastern transparent zone may be caused by the Coastal Batholith which is covered by sediments in the Central Valley. A nearsurface first-break tomography of the TIPTEQ data set revealed the subsurface continuation of the batholith and a sediment thickness of ca. 1 km in the western part of the Central Valley. The seismic depth section is discussed in an integrated interpretation with magnetotelluric, gravimetrical and seismological findings along the TIPTEQ transect at 38.2°S.
T23A-1995
Seismic Reflection Character of the Hikurangi Subduction Interface, New Zealand, in the Region of Repeated Gisborne Slow Slip Events
Changes in reflection character and geometry of the Hikurangi subduction interface offshore the east coast of North Island, New Zealand, observed in deep (8-12 s twt) 2D prestack time migrated seismic reflection data may relate to changing seismogenic properties and can potentially help define locations of slow slip. Slow slip events have been recorded on the Hikurangi subduction interface in the Gisborne area, every two years since the installation of continuous Global Positioning System (CGPS) instruments in 2002. Preliminary inversion of CGPS displacements suggest the slow slip occurs on the subduction interface with a down-dip limit of 15 km. However, the other dimensions of the slow sliding segment are poorly constrained. The subduction interface offshore Gisborne includes subducting seamount asperities (capable of building up seismic stress) coupled with fluid-rich subduction erosion material (increased pore-pressures promote more stable slip) and may potentially provide an explanation why slow slip is constrained within this area. Margin perpendicular profiles image the subduction interface beneath the minor accretionary wedge and outer arc high as a narrow band of reflectivity overlying a subducting seamount on the down-going plate. Landward of the subducting seamount the subduction interface is defined as the top-most reflector of a 1 s wide band of high amplitude reflectivity which terminates up-dip against the seamount. This seismic character terminates down-dip when the subduction interface steps down to a lower level of 15 km at a distance of ~20 km offshore, coincident with the CGPS constrained down-dip limit of slow slip. The step-up in decollement level and introduction of a wedge of high amplitude reflections on the down-going plate is likely to be related to the accumulation of fluid-rich material that has been eroded from the accretionary wedge and/or overlying plate due to seamount subduction. The mapped region of the subduction interface with this reflection character is interpreted to potentially facilitate slow slip and has been inverted with the CGPS displacements for slow slip events in 2002, 2004 and 2006 to model the amount of slip on this surface during each event.
T23A-1996
Tectonic Variations Along the Hikurangi Subduction Margin, New Zealand, and Relationships to Fluid Flow and Cold Seep Sites
The structure and geomorphology of the Hikurangi subduction margin varies along strike primarily in response to changes in subducting crustal structure, convergence rate and obliquity, and sediment supply. New seismic reflection and multibeam bathymetric data are used to interpret the stratigraphy of the subducting sequence, the upper plate tectonic structures, and the geological framework for cold vent seep sites. The imbricated frontal wedge of the central margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (~4°) accretionary thrust systems associated with a relatively smooth subducting plate, a thick trench-fill sedimentary sequence, weak basal decollement, and moderate convergence rate. This region differs from the northern, Hawkes Bay to East Cape, sector of the margin where subducting seamounts, faster convergence rate, and reduced trench sediment supply have resulted in a dramatically reduced and steeper active frontal wedge, complex deformation and uplift of frontal ridges above subducting asperities, and a tectonic regime dominated by non-accretion and tectonic erosion. Bottom simulating reflectors (BSRs) are widespread along the length of the margin. Five areas with multiple fluid/methane seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in about 700-1200 m water depth on the crests of thrust faulted ridges along the mid-slope. All of these seep sites lie near the outer edge of a deforming Cretaceous and Paleogene inner foundation, at the rear of the accreted trench fill turbidites. One seep site lies in close proximity to a major strike slip fault. Another occurs directly above a subducting seamount. Beneath the seafloor seeps on ridge crests there is typically a conspicuous break in the BSR, and commonly a seismically-resolvable fault-fracture network through which fluids and gas percolate. There is a clear relationship between the seep sites and major thrust faults, which are conduits for fluid and gas migration sourced from the deeper, inner parts of the thrust wedge, and probably from subducting sediments. We consider that the Cretaceous and Paleogene inner foundation is, on the whole, relatively impermeable and focuses fluid migration preferentially to its outer edge via major low angle thrust faults and the decollement.
T23A-1997
Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data
The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, whereas the data from MANGO 2 likely show thermal activity from the adjacent arc volcanism.
T23A-1998
Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand
We use 2800 line km of seismic-reflection data to map the offshore character and three-dimensional geometry of the Hikurangi subduction thrust and outer forearc wedge to depths of c. 15 km. Several first order subduction characteristics (e.g., convergence rate, apparent plate locking, margin morphology) vary systematically over relatively short along-strike distances on the Hikurangi margin, making it an excellent natural laboratory for studying subduction tectonics assessing the significance of variations in subduction thrust geometry. For 200 km along-strike south of Hawke Bay, the subduction thrust is relatively smooth, dips less than 8 degrees, and the wedge is characterised by accretion of young sediment and topographic slopes of less than 3 degrees. In Hawke Bay and north for 200 km, a kink in the subduction thrust is apparent, with a down-dip increase in dip to angles greater than 8 degrees at depths of 10-15 km; there is a corresponding steepening of the topographic slope to greater than 3 degrees outboard of the kink and the wedge is characterised by lithified sedimentary rock and slope failure. We suggest that the kink in the subduction thrust is caused by a combination of a northward change in subducting lithosphere chemistry and subduction rate that, in turn, controls fluid release rates and intra-slab deformation patterns. The subduction thrust geometry, in combination with a northward increase in subducting plate roughness and decrease in sediment cover, causes the observed spatial change in character of the subduction thrust and forearc wedge.
T23A-1999
Sunda-Banda Arc Transition: Marine Multichannel Seismic Profiling
After the Indian Ocean Mw 9.3 earthquake and tsunami on December 26, 2004, intensive research activities focussed on the Sunda Arc subduction system offshore Sumatra. For this area a broad database is now available interpreted in terms of plate segmentation and outer arc high evolution. In contrast, the highly active easternmost part of this subduction system, as indicated by the south of Java Mw 7.7 earthquake and tsunami on July 17, 2006, has remained almost unexplored until recently. During RV SONNE cruise SO190 from October until December 2006 almost 5000 km of marine geophysical profiles have been acquired at the eastern Sunda Arc and the transition to the Banda Arc. The SINDBAD project (Seismic and Geoacoustic Investigations along the Sunda-Banda Arc Transition) comprises 30-fold multichannel reflection seismics with a 3-km streamer, wide-angle OBH/OBS refraction seismics for deep velocity control (see poster of Shulgin et al. in this session), swath bathymetry, sediment echosounder, gravimetric and geomagnetic measurements. We present data and interpretations of several 250-380 km long, prestack depth-migrated seismic sections, perpendicular to the deformation front, based on velocity models from focussing analysis and inversion of OBH/OBS refraction data. We focus on the variability of the lower plate and the tectonic response of the overriding plate in terms of outer arc high formation and evolution, forearc basin development, accretion and erosion processes at the base of the overriding plate. The subducting Indo-Australian Plate is characterized by three segments: i) the Roo Rise with rough topography offshore eastern Java ii) the Argo Abyssal Plain with smooth oceanic crust offshore Bali, Lombok, and Sumbawa, and iii) the Scott Plateau with continental crust colliding with the Banda island arc. The forearc responds to differences in the incoming oceanic plate with the absence of a pronounced forearc basin offshore eastern Java and with development of the 4000 m deep forearc Lombok Basin offshore Bali, Lombok, and Sumbawa. The eastern termination of the Lombok Basin is formed by Sumba Island, which shows evidence for recent uplift, probably associated with the collision of the island arc with the continental Scott Plateau. The Sumba area represents the transition from subduction to collision. Our seismic profiles image the bending of the oceanic crust seaward of the trench and associated normal faulting. Landward of the trench, they image the subducting slab beneath the outer arc high, where the former bending-related normal faults appear to be reactivated as reverse faults introducing vertical displacements in the subducting slab. The accretionary prism and the outer arc high are characterized by an ocean-verging system of imbricate thrust sheets with major thrust faults connecting seafloor and detachment. Compression results in shortening and steepening of the imbricated thrust sheets building up the outer arc high. Tilted piggy-back basins and downlaps of tilted sediments in the southern Lombok forearc basin indicate ongoing uplift of the entire outer arc high, abrupt displacements, and recent tectonic activity.
T23A-2000
Sunda-Banda Arc Transition: Marine Wide-Angle Seismic Modeling
The Sunda-Banda Arc transition is the region of active convergence and collision of the Indo-Australian and Eurasian Plates. The style of subduction changes from an oceanic-island arc subduction to a continental- island arc collision. The character of the incoming plate varies from the rough topography of the Roo Rise, to the smooth seafloor of the Abyssal Plain off Bali, Sumbawa. Forearc structures include well-developed forearc basins and an accretionary prism/outer forearc high of variable size and shape. To quantify the variability of structure of the lower plate and the effects on the upper plate a refraction seismic survey was carried during cruise SO190-2. A total of 245 ocean bottom seismometers were deployed along 1020 nm of wide-angle seismic profiles in four major north-south oriented corridors. To assess the velocity structure we used a tomographic method which jointly inverts for refracted and reflected phases. The sedimentary layers of the models, obtained by the analysis of high-resolution MCS data (see Lueschen et al), were incorporated into the starting model. The obtained models exhibit strong changes of the incoming oceanic crust for the different portions of the margin: The westernmost profile off eastern Java shows a crustal thickness of more than 15 km, most likely related to the presence of an oceanic plateau. Profiles off Lombok reveal an oceanic crust of 8-9 km average thickness in the Argo Abyssal Plain. Crustal and upper mantle velocities are slightly decreased within an area of about 50-60 km seaward of the trench, indicating fracturing and related serpentinization due to bending of the oceanic crust and associated normal faulting. The outer forearc high is characterized by velocities of 2.5-5.5 km/s. For the Lombok Basin, the profiles show a sedimentary infill of up to 3.5 km thick and typical sediment velocities of 1.75-3.0 km/s. A reflector at 16 km depth and velocity values of 7.4-7.8 km/s beneath it suggest the presence of a shallow forearc mantle and a hydrated mantle wedge in this part of the margin. See in this session Planert et al.
T23A-2001
Anatomy of the Java plate interface from depth-migrated seismic images: Implications for sediment transfer
We present seismic data from the western Java margin off Indonesia. The newly pre-stack depth migrated seismic images resolve the structural details of the western Java forearc and the fate of sediment subducted at the trench. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is transported down a subduction channel. Basal mass transfer occurs by episodic accretion of sediment beneath the submerged forearc as the active detachment stepwise descends to a deeper level below the outer wedge. Fluctuations in subduction channel dimensions are enhanced by deep-reaching thrust faults that are traced from a velocity singularity marking the top of the oceanic basement towards the seafloor. These thrust faults breach the subduction channel and inhibit recycling of material to mantle depth, while serving as an incremental ramp along which the active detachment is transferred to a lower position. The high ratio of accreted/subducted sediment is associated with the evolution of a large bivergent wedge (>100 km) despite the comparatively low sediment input to the trench (<2 km). We used quantitative DEM modeling to gain some insight into the evolution of the distinct tectonic units. In the modelling, initiation of sediment accretion occurs against the arc rock framework, which is imaged in the MCS data. Overthrusting of the wedge onto the forearc basin is also expressed in a prominent retro-thrust imaged in the seismic data. The seismic data document an end-member type of subduction zone where near-complete accretion of the trench sediment fill by frontal and basal accretion is supported by the lack of evidence for subducted sediment in the geochemical signature of Mt. Guntur and Mt. Gallunggung, two volcanoes positioned in the prolongation of our seismic line on Java.
T23A-2002
Seismic evidence for overpressured subducted oceanic crust and sealing of the megathrust: Relations with ETS in Cascadia
Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurence of non-volcanic tremor (NVT) to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly-reflective, dipping, low-velocity zone (LVZ) beneath the forearc crust, however, its continuity along the margin remains unknown and its interpretation is still debated. In this work we have assembled a large teleseismic body-wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well-developed along the entire margin from the coast eastward to the forearc basins (Georgia Strait, Puget Sound, Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal-petrological modelling of subduction zone metamorphism. The location of NVT epicenters along the revised slab contours possibly defines the continental Moho at ~35 km depth. In northern Cascadia we document the evidence for high pore-fluid pressure within the subducted oceanic crust based on measurements of Poisson's ratios and velocity contrasts. High pore pressure at depths <40 km and evidence for the release of water into the mantle wedge points to a transition from a low-permeability barrier that traps fluids within the oceanic crust, to a high-permeability interface at depths corresponding to eclogitization of oceanic crust. This transition occurs in the vicinity of the wedge corner, where ETS occurrence is observed. We suggest that ETS events are related to transient micro-fracturing of the plate interface by volume changes across the boundary due to continuous dehydration reactions (eclogitization) of the oceanic crust, and hydration (serpentinization) of the mantle wedge. The reccurence of ETS at regular intervals are possibly explained by episodic pore-fluid pressure build-up and fluid release across the plate boundary.
T23A-2003
Collision and subduction structure of the Izu-Bonin arc, central Japan, revealed by refraction/wide-angle reflection analysis
Since the middle Miocene, the Izu-Bonin arc has been colliding with the Honshu arc in central Japan associated with subduction of the Philippine Sea (PHS) plate. This process is responsible for forming a complex crustal structure called the Izu Collision Zone. To obtain direct evidences of the deep structure dominated by collision and subduction, an intensive seismic experiment using explosive and vibroseis sources was conducted in the eastern part of the Izu Collision Zone in 2003 (Sato et al., 2005). CMP reflection and refraction/wide-angle reflection data were acquired on a 130-km-long seismic line crossing the collision boundary named Tonoki-Aikawa Tectonic Line (TATL) and the Tanzawa block, the fragment of the former Izu-Bonin arc, with N-S direction. The structure from refraction tomography and forward ray tracing modelling showed remarkable lateral velocity variation across TATL and some clear reflectors in the deep crust. A north dipping reflector beneath the Kanto Mountain was interpreted to be the deeper extension of the TATL. From the geometry of reflectors, we interpret the Tanzawa block is delaminated from the subducting slab due to the collision to form a wedge-like body thrusting between the upper and lower crust of Honshu. The velocity model also indicates that the Tanzawa block corresponds to the upper crust and the upper part of the middle crust of the Izu-Bonin arc, offscrapped from the subducted PHS plate. The relocated hypocenter distribution using our velocity model showed prominent seismic activity concentrated around the collision boundary, which is in a marked contrast of low seismicity within the Tanzawa block. These features of seismicity are strongly dominated by the ongoing collision of the Izu-Bonin arc to the Honshu arc.
T23A-2004
Crustal Erosion and Accretion Processes Leading to Forearc Uplift of Raukumara Basin, Hikurangi-Kermadec Subduction Zone, Northeastern New Zealand
New seismic reflection and refraction data from northern New Zealand allow us to determine crustal thickness, map a forearc basin containing 12 km of sediment, and image the subduction thrust at 30-40 km depth. The Moho lies at 18 km beneath the basin centre, and at 35 km at the southern margin. Raukumara Basin is uplifted along its eastern and southern margins but is only weakly deformed, suggesting uplift by crustal underplating. We infer from the spatial correlation between maximum uplift and the intersection of the Moho with the subduction thrust that lower crustal accretion processes are modulated by crustal thickness: crustal material is accreted from a subduction channel when the density instability becomes large enough. The trench-slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of >1 km of sediment from the down-going plate to the trench-slope; this was previously interpreted as evidence for subduction erosion. We propose cyclical mechanics that involves net accretion: lower crustal accretion from a subduction channel causes uplift of the forearc ridge; the trench-slope becomes steeper; the slope collapses; collapsed material and sediment from the subducted plate is transported towards the lower crust down a subduction channel; more lower crustal material is accreted. We suggest that this process led to net crustal accretion but left no evidence for accretion at the subduction front or on the trench-slope.
T23A-2005
Long-term and short-term vertical deformation rates of the forearc along the NE Japan subduction zone
We estimated the long-term vertical deformation rate of the northeastern (NE) Japan forearc along the Japan Trench by using the height distribution of MIS 5.5 marine terraces as determined from tephra and cryptotephra stratigraphy. The uplift rate at the north Pacific coast of NE Japan was estimated from the relative heights between the MIS 5.5 marine terrace surface and eustatic sea levels to be 0.10-0.31 (mostly 0.19-0.31) m ka-1, which is faster than the uplift rate of the south Pacific coast of NE Japan (0.11-0.19 m ka-1). The short-term vertical velocity profile, obtained from GPS observations, showed that the north Pacific coast of NE Japan is today being uplifted at a maximum rate of 3.7 +/- 0.4 mm yr-1 as a result of after-slip related to the 1994 inter-plate moment release, whereas tidal-gauge records show that it was subsiding over several decades preceding the 1970s. The south Pacific coast of NE Japan has also subsided for those several decades and is currently subsiding at a maximum rate of 1.9 +/- 0.4 mm yr-1. Thus, the current observed short-term (geodetically determined) vertical velocities do not reflect long-term (geological) vertical tectonic movement. Short-term vertical movement is probably driven by elastic deformation caused by interplate coupling. However, long-term uplift is probably the result of crustal thickening rather than mega-thrusting.
T23A-2006
Rapid outer forearc uplift inboard of the Panama Triple Junction: Burica Peninsula, southern Central America
New geomorphic and structural mapping on the Burica Peninsula, an outer forearc peninsula located only 15 km inboard of the Panama Triple Junction, reveals temporal and spatial patterns of rapid Plio-Quaternary uplift that characterize the effect of Panama Fracture Zone subduction on the upper plate. There are two potential impacts: 1) oblique subduction of a bathymetric step due to the juxtaposition of thick Cocos plate west of the Panama Fracture Zone against thinner Nazca plate to the east and 2) changes in basal traction due to the major change in convergence rate and direction at the triple junction. Our analysis of structure and late Quaternary deformation indicates that tilting and thrust faulting associated with the bathymetric step is the more important of the two impacts. Uplift is recorded by a flight of up to five marine terraces that surround the peninsula and reach elevations as high as 100 m above modern sea level. Radiocarbon dating of a suite of seven shell samples (4 conventional and 3 AMS) within the lowest terraces located ~ 1.5 – 4 m above modern sea level yields Holocene ages that range between 510 +/- 40 and 10,650 +/- 50 YBP. These ages combined with known terrace inner edge elevations and facies constraints suggest that the peninsula has experienced uplift within the Holocene at a rate as high as ~3 mm/yr. The uplift and possible east-directed tilting of marine terraces results from slip along a deeply rooted NE- dipping thrust fault located offshore of the western coast of the peninsula. This fault roots within the Cretaceous basement of the Nicoya Fm, and contains a NW-striking thrust splay that bisects the peninsula down the center. This thrust splay is marked by sheared basalts of the Nicoya Fm with a deformation fabric that dips to the west overlying an overturned syncline of mudstones within the Charco Azul Fm. This newly defined structural architecture runs counter to other interpretations that emphasize the importance of right lateral strike slip faulting as an upward continuation of the Panama Fracture Zone.
T23A-2007
Seafloor Uplift Recorded by Pressures in the CORK in Hole 857D, Middle Valley, Northern Juan de Fuca Ridge
Over the last 12 years, in-situ seafloor and basement pressures have been continuously monitored and recorded by an ODP subseafloor hydrogeological observatory (CORK) located in Middle Valley, Juan de Fuca Ridge. Hole 857D was drilled in 1991 in thickly-sedimented crust to a depth of 936 mbsf and instrumented with an original CORK that was replaced in 1996. Previous results from this site have shown a strong formation underpressure that is the consequence of the hydrologic structure of rift valley and the hydrothermal state, as well as hydrologic and geodynamic responses within basement to both local (1991, 2001, and 2004) and distal (1999) earthquake swarms along the Juan de Fuca Ridge. We will present data from the last 3 years of monitoring, obtained via recent Alvin submersible operations, which show a marked change in both seafloor and formation pressures at 857D. After a rapid increase of ~40 kPa within basement during the second half of 2005, formation pressure peaked in early 2006 and has steadily decreased by ~15 kPa over the past 2.5 years. Seafloor pressure was relatively constant during the first 9 years of monitoring and did not vary significantly during the 2005 rapid increase in formation pressure, but it mirrored the subsequent decrease by ~18 kPa, equivalent to nearly 2 m of uplift. Inspection of Alvin depth records confirmed that water depth at 857D has decreased by at least 1 m over the past three years. It is not known if this possible inflation event is associated with deep magma injection or is tectonic in origin. During the same three-year interval, temperatures of hydrothermal fluids venting from chimneys about 800 m NE of 857D have remained around 270°C, with no indication for any effects on the local hydrothermal system.
T23A-2008
Active Uplift Within the Inner Forearc Along the Japan Trench, Northeastern Honshu
Field observations, structural modeling, and geomorphic analyses provide evidence for an active fault system producing shortening and uplift of the inner forearc along the northeastern Japan convergent margin. The outer forearc along this margin exhibits a well-documented history of subsidence throughout the Neogene that is argued to reflect basal erosion of the forearc basement. Underthrusting of the forearc along this margin contributes to trench retreat and calls into question the relative contributions of underthrusting versus basal erosion in driving outer forarc subsidence. The Futaba fault system is the easternmost onshore thrust fault in a system of east-vergent faults and may mark the boundary between the uplifting inner forearc and the subsiding outer forearc. The fault extends along the eastern margin of the Abukuma Massif and places Cretaceous basement in fault contact with a deformed Cenozoic cover sequence. New kinematic modeling of the fault-related folds associated with the southern tip of the Futaba fault are consistent with a steep ~60° west-dipping fault producing a total of 1-2 km shortening. Biostratigraphic ages from growth and pre-growth strata indicate deformation began in the early Pliocene to latest Miocene, and suggest average slip rates over this time of ~0.5 mm/yr. Recent exhumation of the Abukuma massif associated with ongoing fault slip is supported by topographic patterns of hanging wall incision. The highest topography of the Abukuma massif is characterized by low hillslope angles, deeply weathered bedrock, and alluviated channels. However, topography near the mountain front is characterized by high hillslope angles, deeply incised gorges, and bedrock channels. Channel long profiles in the hanging wall of the Futaba fault exhibit knickzones clustered around 400m elevation that are coincident with the transition from low-relief to high-relief topography. These knickzones separate low-gradient upper channel reaches from downstream channel reaches that are significantly steeper. These observations suggest ongoing, transient incision of a relict landscape in the Abukuma Massif resulting from recent uplift along the Futaba fault. Combined, these structural and geomorphic data provide evidence for an active, onshore, basement-involved, thrust fault along the northeastern Japan margin and imply that inner forearc shortening is contemporaneous with, and potentially linked to, outer forearc subsidence along erosive margins.
T23A-2009
Different Causes of Local and Regional Wedge Instability Along Accretive and Erosive Convergent Margins: Case Studies From the Offshore Hikurangi and Peru Fore- Arcs
The mechanics of a forearc, a wedge-shaped part of the overriding plate between the trench and the volcanic arc, are elegantly described in terms of the critical taper (CT) concept. Based on the Mohr-Coulomb failure criterion and applying an elasto-plastic rheology, CT describes the state (sub-critical, stable, super- critical) of any point within the wedge as a function of its geometry (slope and dip), basal and internal friction as well as basal and internal fluid pressure. Subduction erosion and the subduction of seamounts and other lower plate topographic features such as basement ridges may temporarily increase surface slope and therefore facilitate local to regional mechanical instability. Here we study the causes of local and regional failure at the central Hikurangi wedge offshore New Zealand's North Island and the Peruvian margin. The geometry of both margins is well known from both seismic studies and swath bathymetry coverage and allows quantification of local slope gradients and other curvature attributes. The Rock Garden area at the central Hikurangi margin is characterized by the presence of an accretionary wedge with a relatively low overall taper of 7°, which would be critical or stable if basal friction is 5° or larger. However, compared to other accretive margins, the Rock Garden ridges have steep flanks with the landward flanks being as steep as the seaward ones. Local slope gradients of more than 10° are widely found. In terms of CT this would be unstable even if the wedge and its base would be largely at hydrostatic pressure. However, there is evidence for at least moderate overpressuring. Here, the wedge locally is unstable and numerous, but local slumps are found along the flanks of the accretionary ridges, predominantly at their crests and close to their base. A subducting seamount and a close-by ridge shaped lower-plate feature, which causes a pronounced re-entrant in the outermost accretionary ridge, lead to regional slope gradients of more than 15°. Here, slumping of an area as large as 100 km2 could occur in future. In contrast, the Peruvian margin, which undergoes fast subduction erosion, is characterized by a large taper of 13° to 16°, and a narrow 15 km margin wedge. Here, removal of upper plate material through the subduction channel makes the lower slope mostly super-critical and leads to gravitational failure. The strength of the plate interface and the amount of overpressuring play a crucial role for the mechanical stability of both margins. Fluid pressure fluctuations within the seismic cycle are well capable of pushing large parts of the lower and middle slopes outside the taper stability field. Our comparison highlights that, while the causes of individual slumps are different at both types of margins, seismic behaviour and local as well as regional mechanical stability may well be intimately linked.
T23A-2010
Coseismic Stress Transfer From the Seismogenic Zone to the Shallow Portion of the Megathrust and its Cumulative Effect on Wedge Taper
One of the important characteristics of the submarine wedge at subduction zones is a morphological and structural contrast between the actively deforming frontal part (outer wedge) and the much less actively deforming landward part (inner wedge). The surface slope of the outer wedge is generally steeper than that of the inner wedge. This contrast may contain important information on the frictional properties of the subduction fault. The dynamic Coulomb wedge model postulates that the outer wedge overlies the coseismic- strengthening shallowest part of the subduction fault, and the inner wedge overlies the coseismic-weakening seismogenic zone. The coseismic strengthening (stress increase) of the shallow zone may be responsible for much of the permanent deformation of the outer wedge. In this work, we use a numerical model of stress transfer to investigate how the stress is coseismically transferred from the seismogenic zone to the updip zone to cause wedge deformation. Using this static model, we show that the stress increase in the updip zone depends on the force drop of the seismogenic zone, defined as the product of the average shear stress drop and the area of the seismogenic zone. We define a "critical strengthening" (CS), which is the degree of strengthening required to prevent the rupture from breaking the trench, as a reference measure of the coseismic strengthening. In a simple model of uniform material properties with a few MPa average stress drop over a seismogenic zone of 120 km downdip width, the CS for a 30 km wide updip zone is an increase in the effective friction coefficient by about 0.05, producing a few MPa average stress increase. Using the Coulomb wedge theory, we demonstrate that this level of stress increase can readily cause permanent deformation to the overlying outer wedge. If the stress increase is greater than CS, the rupture is able to extend into the updip zone only slightly, causing localized wedge compression in the area of slip termination. We examined wedge geometry of twenty-three subduction zones and found that the surface slope of these wedges can be explained using the dynamic Coulomb wedge model including coseismic strengthening of the shallow portion of the megathrust.