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At the southernmost part of the Ryukyu subduction zone, data recorded by 28 ocean-bottom seismometers with active sources during the TAiwan Integrated GEodynamics Research (TAIGER) experiment in 2009 were re-analyzed. Additionally, six long-offset multi-channel seismic profiles across three forearc basins and the Yaeyama accretionary prism north of the southernmost Ryukyu Trench, obtained by the TAIGER experiment in 2009 and the TAICRUST project in 1995, were processed to generate pre-stack depth migration (PSDM) sections. Due to the NW convergence of the Philippine Sea Plate (PSP), the Gagua Ridge may have been obliquely subducting northwestward beneath the Yaeyama accretionary prism and below the Nanao Basin. The PSDM sections and the velocity-interface models show that the Yaeyama accretionary prism near the trench, the sedimentary basement and the lower crustal structure below the Nanao Basin were uplifted by the tip of the subducted Guagua Ridge. Furthermore, the sedimentary and crustal structures near ~ 122.5°E beneath the Nanao Basin were shown a north-trending uplift where shallow earthquakes had been occurred by the oblique subduction of the Gagua Ridge depicted in the isopach map of the crust. Moreover, a recent earthquake (Mw ~ 6.0) near the NE coast of Taiwan in 2018 at a depth of approximately 12 km below the Hsincheng Ridge was caused due to a thrust fault near the décollement that might have been formed by subduction of the PSP.

The northern South China Sea (SCS) plate between Taiwan and the Philippines experienced complex underplating and overriding. Ocean-bottom seismometer (OBS) data along the southern profile (20.5 °N) with 18 OBSs, the NE-SW profile with 11 OBSs and the northern profile (21.5 °N) with 12 OBSs were collected from a large-offset active-source seismic survey during the 2009 Taiwan Integrated Geodynamics Research. Here we re-analyzed the OBS data to image the transitional continental crust (TCC) and the subducted slab across the northern SCS, the Manila Trench, the deformation front (DF) and the accretionary wedge off southwestern Taiwan. By picking both refracted and reflected arrivals from OBS data and adopting irregular grids during modeling, we have derived P-wave velocities and interfaces of the crustal structures to identify the characteristics of the crustal structures and the collision-subduction transitional zone in the northern SCS. Based on the lateral velocity variation (2.5–5 km/s) of the accretionary wedge along three OBS profiles, the geometry of the splay faults on top of a decollement was imaged. Brittle deformation due to the hardened sediment with a relatively high velocity (3.5–5.5 km/s) from the collision was also found between a mid-crust detachment and the decollement along the NE-SW profile. Our study indicates the deeper Moho in the south (15–18 km deep) but shallower in the north (14–17 km deep), and the Moho is generally shallower than that derived from the previous studies. West of the trench or the DF, the TCC in the northern SCS has been identified in shallow crustal layers while underplated by the thick oceanic crust or the oceanic crust from the SCS in the middle and lower crustal layers. East of the trench or the DF, the TCC in the upper and middle crust has subducted beneath the accretionary wedge. We found a failed rift zone west of the DF along the northern profile. One seismically active region with large lateral velocity change (2.5–4.5 km/s) in the accretionary wedge was imaged. Another seismically active region is near the southern end of the Kaoping Canyon where the crustal thickness from the basement to the Moho is extremely thin (8 km).

Ocean-bottom seismometers (OBSs) were deployed at a shallow water depth (

In this study, P- and S-wave velocity models were built based on two pre-stack depth migration (PSDM) profiles of long-offset reflection data and 25 four-component ocean-bottom seismometers (OBS) data at Jiulong Methane Reef off SW Taiwan in the passive margin of the northern South China Sea (SCS). According to the velocity models, the average P-wave velocity and Vp/Vs ratio of the free gas beneath the bottom-simulating reflector (BSR) are 1.52–1.58 km/s and 4.5–5.10, respectively. The depth of the BSR is found at 80–300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50–100 m and 70–100 m, respectively. P-wave velocity of about 1.75 km/s in the hydrate-bearing sediment is southeastward increased to about 1.85 km/s above the BSR. Similarly, Vp/Vs ratio of about 3.06 in the hydrate-bearing sediment is increased southeastward to about 3.48 above the BSR. Based on the pseudo-3D map of gas-hydrate saturation estimated from the PSDM and OBS models, the average saturations of hydrate and free gas at Jiulong Methane Reef are about 7% and 0.9–2.4%, respectively. The highest hydrate saturation (11%) is located at 5–20 m above the BSR in the SE portion of the Jiulong Methane Reef. On the other hand, the highest gas saturation of about 2% is observed at 10–70 m below the BSR in the NW portion of the Jiulong Methane Reef. We suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at Jiulong Methane Reef. Therefore, the highest gas saturation is observed below the anticline in the NW portion of the Jiulong Methane Reef and the highest hydrate saturation (high P-wave velocity and high Vp/Vs ratio) is identified above the BSR in the SE portion of the Jiulong Methane Reef.

We present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern South China Sea. These data reveal moderately stretched crust ~25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin. Along the continental slope, crust rapidly thins to ~4 km in a basin characterized by tilted fault blocks that sole into a low-angle detachment. Strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading. Crust in the distal margin is ~12–15 km thick. Few extensional structures are apparent in the distal margin, but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust. The magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata, sills intruded into the postrift sedimentary section, and a high-velocity (~6.9–7.5 km/s) lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions. These features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading. The postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the South China Sea.

New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc–continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10–14 km thick outboard of the trench, but thins to 6 km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10 km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0 km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc–continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin.

During TAiwan Integrated GEodynamics Research of 2009, we investigated data from thirty-seven ocean-bottom seismometers (OBS) and three multi-channel seismic (MCS) profiles across the deformation front in the northernmost South China Sea (SCS) off SW Taiwan. Initial velocity-interface models were built from horizon velocity analysis and pre-stack depth migration of MCS data. Subsequently, we used refracted, head-wave and reflected arrivals from OBS data to forward model and then invert the velocity-interface structures layer-by-layer. Based on OBS velocity models west of the deformation front, possible Mesozoic sedimentary rocks, revealed by large variation of the lateral velocity (3.1–4.8 km/s) and the thickness (5.0–10.0 km), below the rift-onset unconformity and above the continental crust extended southward to the NW limit of the continent–ocean boundary (COB). The interpreted Mesozoic sedimentary rocks NW of the COB and the oceanic layer 2 SE of the COB imaged from OBS and gravity data were incorporated into the overriding wedge below the deformation front because the transitional crust subducted beneath the overriding wedge of the southern Taiwan. East of the deformation front, the thickness of the overriding wedge (1.7–5.0 km/s) from the sea floor to the decollement decreases toward the WSW direction from 20.0 km off SW Taiwan to 8.0 km at the deformation front. In particular, near a turn in the orientation of the deformation front, the crustal thickness (7.0–12.0 km) is abruptly thinner and the free-air (−20 to 10 mGal) and Bouguer (30–50 mGal) gravity anomalies are relatively low due to plate warping from an ongoing transition from subduction to collision. West of the deformation front, intra-crustal interfaces dipping landward were observed owing to subduction of the extended continent toward the deformation front. However, the intra-crustal interface near the turn in the orientation of the deformation front dipping seaward caused by the transition from subduction to collision. SE of the COB, the oceanic crust, with a crustal thickness of about 10.0–17.0 km, was thickened due to late magmatic underplating or partially serpentinized mantle after SCS seafloor spreading. The thick oceanic crust may have subducted beneath the overriding wedge observed from the low anomalies of the free-air (−50 to −20 mGal) and Bouguer (40–80 mGal) gravities across the deformation front.

Horizon velocity analysis and pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 across the accretionary prism off SW Taiwan and along the continental slope of the northernmost South China Sea were implemented for identifying gas hydrates. Similarly, a survey of 32 ocean-bottom seismometers (OBS), with a spacing of about 500 m, was conducted for exploring gas hydrates on the accretionary prism off SW Taiwan in April 2006. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of these OBS data were applied for imaging P-wave velocity and Poisson’s ratio of hydrate-bearing sediments. In the accretionary prism off SW Taiwan, we found hydrate-bearing sediment, with a thickness of about 100–200 m, a relatively high P-wave velocity of 1.87–2.04 km/s and a relatively low Poisson’s ratio of 0.445–0.455, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free-gas layer, with a thickness of about 30–120 m, a relatively low P-wave velocity of 1.4–1.8 km/s and a relatively high Poisson’s ratio (0.47–0.48), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 12–30% and the free-gas saturation of about 1–4%. The highest saturation (30% and 4%) of gas hydrates is found below anticlines due to N–S trending thrust-bounded folds and NE-SW thrusting and strike-slip ramps in the lower slope of the accretionary prism. We suggest that fluid may have migrated through the relay-fault array due to decollement folding and gas hydrates have been trapped in anticlines formed by the basement rises along the thrust faults. In contrast, in the rifted continental margin of the northernmost South China Sea, P-wave velocities of 1.9–2.2 km/s and 1.3–1.6 km/s, and thicknesses of about 50–200 m and 100–200 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosional ridges in the upper continental slope. High P-wave velocity of hydrate-bearing sediment below erosional ridges may also indicate high saturation of hydrates there. Normal faults due to rifting in the South China continental crust may have provided conduits for gas migration below the erosional ridges where P-wave velocity of hydrate-bearing sediment in the passive continental margin of the northernmost South China Sea is greater than that in the active accretionary prism off SW Taiwan.

This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.

A nearly 500-km-long seismic profile with reflective and refractive wide-angle Ocean Bottom Seismometer (OBS) data and Multi-Channel Seismic (MCS) data was acquired across the northeastern continental margin of the South China Sea (SCS). The S-wave crustal structure and Vp/Vs ratios have been obtained based on a previously published P-wave model using the software Raylnvr. Modeling of vertical- and horizontal-component OBS data yields information on the seismic crustal velocities, lithology, and geophysical properties along the OBS-2001 seismic profile. S-wave velocities in the model increase generally with depth but exhibit high spatial variability, particularly from the shelf to the upper slope of the northeastern SCS margin. Vp/Vs ratios also reveal significant lithological heterogeneity. Dongsha-Penghu Uplift (DPU) is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly. With a Vp/Vs of 1.74 and a P-wave velocity of 5.0-5.5 km/s, the DPU primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of Zhejiang-Fujian volcanic provinces, which perhaps is associated with a Mesozoic volcanic arc. The ocean-continent transition (OCT) in the northeastern SCS is characterized by a thinning continental crust, volcanoes in the upper crust, and a high velocity layer (HVL) in the lower crust. The S-wave velocity and Vp/Vs ratio suggest that the HVL has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading. (c) 2009 Elsevier B.V. All rights reserved.

Crustal structure across the passive continental margin of the northeastern South China Sea (SCS) is presented based on a deep seismic survey cooperated between Taiwan and China in August 2001. Reflection data collected from a 48-hydrophone streamer and the vertical component of refraction/reflection data recorded at 11 ocean-bottom seismometers along a NW–SE profile are integrated to image the upper (1.6–2.4 km/s), lower (2.5–2.9 km/s), and compacted (3–4.5 km/s) sediment, the upper (4.5–5.5 km/s), middle (5.5–6.5 km/s) and lower (6.5–7.5 km/s) crystalline crust successively. The velocity model shows that the thickness (0.5–3 km) and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the SCS. Furthermore, several volcanoes and igneous rocks in the upper/middle crust (7–10 km thick) and a high velocity layer (0–5 km thick) in the lower crust of the model are identified as the ocean–continent transition (OCT) below the lower slope in the northeastern margin of the SCS. A thin continent NW of the OCT and a thick oceanic crust SE of the OCT in the continental margin of the northeastern SCS are also imaged, but these transitional crusts cannot be classified as the OCT due to their crustal thickness and the limited amount of the volcano, the magma and the high velocity layer. The extended continent, next to the gravity low and a sag zone extended from the SW Taiwan Basin, may have resulted from subduction of the Eurasian Plate beneath the Manila Trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the SCS.

SUMMARY Imaging of subsurfaces based on 3-D inverse rays shooting from coincident sources and receivers along oblique profiles is presented. Two take-off angles of a 3-D inverse ray, one along the seismic profile and another orthogonal to the profile, are determined based on the common direction of inverse rays shooting from three non-collinear sources/receivers. Inverse-ray tracing from the triangulation of sources/receivers to the reflection points provides more objective imaging than does map migration, for which traveltimes along the oblique and/or sparse profiles must be interpolated into rectangular grids. Error analysis of 3-D inverse-ray imaging over a 2-D subduction model, with variation in spacing and orientation of profiles, indicates that the variation of the structural interface can be well resolved when spacing of the traveltime grids is less than the lateral scale of structural variation. All possible triangulations of reflection times along two neighbouring profiles are also implemented in inverse-ray imaging of a SEG/EAEG over-thrust model. The result shows that the coverage of inverse rays from triangulation of all reflection times is much denser than that from triangulation based on the three closest-spaced stations. Furthermore, imaging errors of the interface depth from most of the SEG/EAEG over-thrust model, where the imaging density is higher than 2000 points km−2, are less than 0.1 km even when the profile spacing is relatively large. In this study, migration velocity analysis of 2-D multi-channel seismic data and all possible triangulations of picked reflection times along oblique seismic profiles in the southernmost Ryukyu subduction zone are conducted. Application of 3-D inverse rays to a forearc basin in this subduction zone shows that the sediment thickness near the western end of the Ryukyu forearc region varies strongly with a maximum thickness greater than 6 km. Furthermore, a sediment subsidence greater than 1 km is found in the SE portion of the basin. Slab buckling and basement rise due to the oblique subduction of the Philippine Sea Plate may have resulted in this subsidence and induced strong and shallow earthquakes below the forearc basin.

We have used combined onshore and offshore wide-angle seismic data sets to model the velocity structure of the Taiwan arc–continent collision along three cross-island transects. Although Taiwan is well known as a collisional orogen, relatively few data have been collected that reveal the deeper structure resulting from this lithospheric-scale process. Our southern transect crosses the Hengchun Peninsula of southernmost Taiwan and demonstrates characteristics of incipient collision. Here, 11-km-thick, transitional crust of the Eurasian plate (EUP) subducts beneath a large, rapidly growing accretionary prism. This prism also overrides the N. Luzon forearc to the east as it grows. Just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure. Because this break is accompanied by intense seismicity, we interpret that the forearc block is being detached from the N. Luzon arc and Philippine Sea plate (PSP) at this point. Our middle transect illustrates the structure of the developing collision. Steep and overturned velocity contours indicate probable large-scale thrust boundaries across the orogen. The leading edge of the coherent PSP appears to extend to beneath the east coast of Taiwan. Deformation of the PSP is largely limited to the remnant N. Luzon arc with no evidence of crustal thickening to the east in the Huatung basin. Our northern transect illustrates slab–continent collision—the continuing collision of the PSP and EUP as the PSP subducts. The collisional contact is below ∼20 km depths along this transect NE of Hualien. This transect shows elements of the transition from arc–continent collision to Ryukyu arc subduction. Both of our models across the Central Range suggest that the Paleozoic to Mesozoic basement rocks there may have been emplaced as thick, coherent thrust sheets. This suggests a process of partial continental subduction followed by intra-crustal detachment and buoyancy-aided exhumation. Although our models provide previously unknown structural information about the Taiwan orogen, our data do not define the deepest orogenic structure nor the structure of western Taiwan. Additional seismic (active and passive), geologic, and geodynamic modeling work must be done to fully define the structure, the active deformation zones, and the key geodynamic process of the Taiwan arc–continent collision.

SUMMARY In 1995, a combined ocean-bottom-seismometric (OBS) and multichannel seismic (MCS) survey with strong air-gun shots was carried out in the southernmost Ryukyu subduction zone. A crustal velocity structure constructed from the layer-stripping Monte Carlo inversion of three OBS/MCS profiles and the associated density models inverted from gravity data in the SW end of the Ryukyu arc-trench system are presented. Parallel to the arc in the southernmost Ryukyu subduction system, the OBS/MCS profiles show sedimentary layers of the Hoping, Nanao and East-Nanao forearc basins from west to east, warping of the arc basement and buckling of the subducted slab beneath the Hoping basement rise. The arc-parallel variation of the crustal structure may result from increasing lateral compression westward due to oblique subduction of the Philippine Sea plate and collision with the Luzon arc near the northwestern edge of the forearc region. Northward subduction and arc-parallel compression of the slab also have generated thrust faulting along the subduction interface and strike-slip faulting within the subducted slab, respectively. On 2002 March 31, an earthquake with a moment magnitude of 6.84 was induced by buckling of the subducted slab and strongly affected cities within an epicentral distance of 100 km. The velocity-interface models, the density models and the focal mechanisms presented in this paper therefore suggest that earthquakes induced by slab buckling or arc-parallel compression have been stronger but less frequent than those generated by northward subduction in the Ryukyu seismogenic zone off Taiwan.

In this study, we present the results of the combined analyses of ocean bottom seismometer and multi-channel seismic reflection data collection offshore southwestern Taiwan, with respect to the presence of gas hydrates and free gas within the accretionary wedge sediments. Estimates of the compressional velocities along EW9509-33 seismic reflection profile are obtained by a series of pre-stack depth migrations in a layer stripping streamlined Deregowski loop. Strong BSR is imaged over most of the reflection profile while low velocity zones are imaged below BSR at several locations. Amplitude versus angle analysis that are performed within the pre-stack depth migration processes reveal strong negative P-impedance near the bottom of the hydrate stability zone, commonly underlain by sharp positive P impedance layers associated with negative pseudo-Poisson attribute areas, indicating the presence of free gas below the BSR. Ray tracing of the acoustic arrivals with a model derived from the migration velocities generally fits the vertical and hydrophone records of the four ocean-bottom seismographs (OBS). In order to estimate the Poisson’s ratios in the shallow sediments at the vicinity of the OBSs, we analyze the mode-converted arrivals in the wide-angle horizontal component. P-S mode converted reflections are dominant, while upward P-S transmissions are observed at large offsets. We observe significant compressional velocity and Poisson’s ratio pull-down in the sediment below the BSR likely to bear free gas. When compared to Poisson’s ratio predicted by mechanical models, the values proposed for the OBSs yield rough estimates of gas hydrate saturation in the range of 0–10% in the layers above the BSR and of free gas saturation in the range of 0–2% just below the BSR.

Shooting angle of an inverse ray for imaging 2D multi-layered structures from reflected travel-times is derived in a closed form. By considering the normal incidence of two neighboring rays reflected at interfaces when sources are at the same locations as receivers, the traveling distance and direction of two inverse-rays are determined successively from the lowermost layer to the uppermost layer. This approach is similar to and also confirmed with the Huygens' principle that the equal travel-time along a wave front (perpendicular to the rays) is conserved. The closed-form solution of the inverse rays is further applied to image a complex structure of a ramp-flat fault with eleven layers. The results demonstrate that the inverse-ray imaging from travel-time picks of all layers is superior to that picked by a layer-stripping approach.

A wide-angle seismic survey, combining ocean-bottom seismometers (OBS) and multi-channel seismic (MCS) profiling, was implemented in the southwestern Ryukyu subduction zone during August and September 1995. In this paper, we present the data analysis of eight OBSs and the corresponding MCS line along profile EW9509-1 from this experiment. Seismic data modeling includes identification of refracted and reflected arrivals, initial model building from velocity analysis of the MCS data, and simultaneous and layer-stripping inversions of the OBS and MCS arrivals. The velocity-interface structure constructed along profile EW9509-1 shows that the northward subduction of the Philippine Sea Plate has resulted in a northward thickening of the sediments of the Ryukyu Trench and the Yaeyama accretionary wedge north of the trench. The boundary between the subducting oceanic crust and the overriding continental crust (represented by a velocity contour of 6.75 km/s) and a sudden increase of the subducting angle (from 5 degrees to 25 degrees) are well imaged below the Nanao Basin. Furthermore, velocity undulation and interface variation are found within the upper crust of the Ryukyu Arc. Therefore, the strongest compression due to subduction and a break-off of the slab may have occurred and induced the high seismicity in the forearc region.

The Taiwan arc–continent collision involves four geodynamic processes: intra-oceanic subduction; initial arc–continent collision; advanced arc–continent collision; and arc collapse/subduction. These processes now occur simultaneously in 19–24°30′N but have operated sequentially southward since the Late Miocene. Although the geological and geophysical features appear to change progressively from north to south across the island, they are distinct within individual tectonic regimes. Using the present scenario as a basis for comparison, it is suggested that Timor is at the initial arc–continent collision stage, while Papua New Guinea and Urals proceeded to the advanced arc–continent collision stage. However, Corsica represents an even more advanced stage than in Taiwan, where a previously accreted arc has collapsed and been totally removed.

SUMMARY The summation of Gaussian beams in homogeneous layered media has been implemented for 3-D seismic forward modelling in this paper. The Gaussian beam method, which considers a bundle of rays in the vicinity of a central ray with a bell-shaped amplitude, is known as an extended ray method for handling caustics and diffractions. However, owing to the restriction of the paraxial assumption and the lack of a universal beam phase, this method has not been accurately applied to 3-D problems. These difficulties are overcome in this paper by providing (i) the optimal determination of the beam orientation and the beam shape by minimizing their beam widths; (ii) the ranges of validity, such as the beam conditions and the paraxial condition; and (iii) the diffractions of Gaussian beams from edges and tips. Comparisons with other solutions for caustics and diffractions demonstrate the accuracy of this extended method even in singular regions.

Imaging of sub-surfaces based on 3-D inverse rays shooting from three non-collinear sources/receivers is presented. Two take-off angles of a 3-D inverse ray, one along the seismic profile and another orthogonal to the profile, are determined when the inverse rays from three non-collinear sources/receivers possess the common direction of shooting rays. In this paper, migration velocity analysis (MVA) of 2D MCS data and triangulation of picked reflection times along oblique seismic profiles are conducted. Application of 3-D inverse rays to a forearc basin in the southernmost Ryukyu subduction zone show that the thickness of sediment near the western end of the Ryukyu forearc region varies strongly with a maximum thickness greater than 6 km. Furthermore, a major fault with a subsidence of about 1 km is found at the SE portion of the basin. The slab buckling and the basement rise due to the oblique subduction of the Philippine Sea Plate may have generated this fault and induce strong, shallow earthquakes.

As a part of the U.S.-R.O.C.collaborative deep seismic imaging experiment,six ocean-bottom seismographs(OBSs)were deployed along the12°12'N parallel west of Hengchun Peninsula and wide-offset seismic signalsfrom a large air-gun array shot over the line were recorded.A preliminaryanalysis of these data together with those from another OBS tothe east of the peninsula reveals that a 11-km thick crustal layer,interpretedto be an extended continental crust of the southern margin of theChinese mainland,lies under most of the line and dips toward the east atlow angle.A complex margin wedge is observed at depth immediate westand underneath the peninsula,which may have resulted from the thicksedimentary layer,quite likely including the basement complex,pushedagainst the strong backstop provided by the Philippine Sea plate convergingfrom the east.

A velocity model constructed using ocean bottom seismometer(OBS)data collected along TAICRUST Line 14 indicates that the Ryukyu Arcbasement(velocity＞～5 km/s)extends beneath the Nanao forearc basin to adepth of 18 to 20 km.Furthermore,our velocity model indicates that theRyukyu Arc basement extends west to at least 122°E,near the west end ofline 14,within about 50 km of the Coastal Range of eastern Taiwan.Thewestward extent of arc basement to 24°N,122°E eliminates the possibilityof a large northward transform offset of the trench as commonly shown.Beneath the Nanao Basin the depth to the base of this forearc crustal layeris consistent with the position of the thrust boundary between the subductingPhilippine Sea Plate(PSP)and overlying Ryukyu Arc as determined byearthquake studies.In the western portion of the line the probable base ofPSP crust appears to be at depths greater than 30 km.The velocity modelis based on data recorded by seven,4-component OBSs deployed at nominal25 km spacing.The seismic energy source for the OBS data was a 20airgun array operated from the R/V Maurice Ewing,which simultaneouslycollected deep-penetration multi-channel seismic reflection data.We constructedthe velocity model using forward and inverse travel time modelingof reflected and refracted arrivals recorded at offsets of 0 to 95 km.Thecoincident MCS data were used to develop the starting model and the finalmodel remains consistent with the sedimentary layering and basement structureevident in the MCS data set.