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On September 5, 2019, a moderate earthquake of Mw5.4 unexpectedly occurred in the apparently quiescent central basin of the South China Sea. We immediately carried out a seismicity monitoring experiment around the epicenter by using broadband ocean bottom seismometers (OBS) for the following three scientific targets. The first is knowing the earthquake seismogenic mechanism, fault structure and further development. The second is finding the role of the residual spreading ridge playing in earthquake processes and further revealing the deep structures of the ridge directional turning area. The third is confirming the existence and significance of the so called "Zhongnan fault". This paper reports the preliminary results of the first phase experiment. Five OBSs were deployed for seismicity monitoring with a duration of 288 days, but only three were recovered. Micro-earthquakes were firstly detected by an automatic seismic phase picking algorithm and then were verified by analyzing their seismic phases and time-frequency characteristics in detail. A total of 21, 68 and 89 micro-earthquakes were picked out from the three OBSs respectively within the distance of 30 km. The dominant frequency of these micro-earthquakes is 12–15 Hz, indicating tectonic fracturing. During the first two months after the mainshock the seismicity was relatively stronger, and micro-earthquakes were still occurring occasionally till the end of observation, indicating the epicenter area is active. We used Match&Locate method to locate 57 micro-earthquakes preliminarily. Their spatial distribution shows that the seismicity is developed mainly along the NE direction roughly parallel to the residual ridge with depth variations between 10–20 km. [ABSTRACT FROM AUTHOR]

Based on the optimum P-wave model, the S-wave velocity structure of a wide angle seismic profile (OBS2006-1), across the northwestern sub-basin (NWSB) and the Macclesfield, is simulated by a 2-D ray-tracing method. The results indicate the S-wave velocities in the upper and lower crust of the NWSB are 3.2-3.6 km/s and 3.6-4.0 km/s, with Vp/ Vs ratios of 1.82-1.88 and 1.74-1.82, respectively, which reflect typical oceanic crust characteristics. The S-wave velocity in the upper crust of the NWSB is a little higher in the NNW segment than that in the SSE segment, while the lateral variation of Vp/ Vs ratio is in the opposite. We suggest that the NWSB might have experienced asymmetrical magma flows during sea floor spreading, which may have blurred the magnetic anomaly lineation. The comparison of S-wave velocities along the northern margin of the SCS shows that the west section is different from the east section, and the northwestern margin has a non-volcanic crust structure. The S-wave structures and P-wave velocity models along the northern margin, Macclesfield and Reed Bank show that the Macclesfield might have a conjugate relationship with the Reed Bank. [ABSTRACT FROM AUTHOR]

As a supplementary study, we used passive seismic data recorded by one ocean bottom seismometer (OBS) station (49°41.8′E) close to a hydrothermal vent (49°39′E) at the Southwest Indian Ridge to invert the crustal structure and mantle transition zone (MTZ) thickness by P-to-S receiver functions to investigate previous active seismic tomographic crustal models and determine the influence of the deep mantle thermal anomaly on seafloor hydrothermal venting at an ultra-slow spreading ridge. The new passive seismic S-wave model shows that the crust has a low velocity layer (2.6 km/s) from 4.0 to 6.0 km below the sea floor, which is interpreted as partial melting. We suggest that the Moho discontinuity at ~9.0 km is the bottom of a layer (2-3 km thick); the Moho (at depth of ~6-7 km), defined by active seismic P-wave models, is interpreted as a serpentinized front. The velocity spectrum stacking plot made from passive seismic data shows that the 410 discontinuity is depressed by ~15 km, the 660 discontinuity is elevated by ~18 km, and a positive thermal anomaly between 182 and 237 K is inferred. [ABSTRACT FROM AUTHOR]

We present two E-W trending wide-angle seismic profiles (OBS2013-ZN, OBS2014-ZN), which cross the boundary (Zhongnan fault zone) between the east sub-basin and the southwest sub-basin of the South China Sea (SCS). We processed the data and used 2D ray-tracing to determine the oceanic crust thickness, velocity structures and Moho depth variations related to the fault zone. The simulated velocity models show that the oceanic basin of the SCS has a typical oceanic crust covered by a 1–2 km thick sediment layer with a velocity of 2.0–3.5 km/s. The crust has a thickness of 5–8 km, of which the oceanic layer 2 is 1.8–3 km thick, with velocity increasing downward from 4.3 km/s to 6.4 km/s, and the oceanic layer 3 is 3–5 km thick, with velocity increasing downward from 6.4 km/s–7.0 km/s. The Moho depth in the oceanic basin is approximately 6–7 km below seabed. The Moho discontinuity has a prominent upheaval zone with a low velocity of 7.6 km/s, whose location corresponds to the low velocity zone in oceanic layer 2. Our results suggest the presence of a NW-SE-trending fracture zone (40–60 km wide) rather than a major “Zhongnan fault” oriented N-S by connecting the upheaval portions of the Moho in the two profiles. The NW-SE orientation Zhongnan transform fault zone in our study area is consistent with the direction of opening of the South China Sea in the last stage of its evolution. This large transform fault zone connected and offset the spreading centers of the east and southwest sub-basin. [ABSTRACT FROM AUTHOR]

The shear wave velocity structure of a wide angle seismic profile (OBS973-2) across Reed Bank in the southern continental margin of the South China Sea (SCS) is simulated by 2-D ray-tracing method, based on its previous P-wave model. This profile is 369-km-long and consists of fifteen three-component ocean bottom seismometers (OBS). The main results are as follows.(1) The model consists of seven layers and the shear wave velocity increases from 0.7 km/s at the top of sediment layer to 4.0 km/s in the lower crust. (2) The Moho depth decreases from 20-22 km at the Reed Bank to 9-11 km at the deep oceanic basin with the shear wave velocity of 4.2 km/s below the Moho. (3) The V p/ V s ratio decreases with depth through the sedimentary layers, attributed to increased compaction and consolidation of the rocks. (4) In the continental upper crust (at model distance 90-170 km), S-wave velocity (2.5–3.2 km/s) is relatively low and V p/ V s ratio (1.75–1.82) is relatively high compared with the other parts of the crust, corresponding to the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating that a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (5) The S-wave structures indicate that Reed Bank crust has different rock compositions from that in the east section of the northern margin, denying the presence of conjugate relationship of Reed Bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same continental crust during the rifting and break-up process. [ABSTRACT FROM AUTHOR]

The seismic experiment of 3D array of OBS in the southwestern sub-basin of the South China Sea (SCS) is briefly introduced in this paper. The data analysis of broadband OBS shows that totally 93 earthquakes with magnitude of Ms 6.0-6.9 and 10 earthquakes with magnitude above Ms 7.0 were recorded in high quality during this experiment, especially the catastrophic earthquake Ms 9.0 occurred in the east sea area of Japan on March 11, 2011. The anisotropy parameters inversion of ScS wave of four events above Ms 7.0 indicates that the fast direction of shear wave is N58°E parallel to the ceased spreading ridge of the southwestern sub-basin of SCS (the slow direction is S35°E perpendicular to the spreading ridge), which means the spreading ridge is under compressing stress at present and the cessation of seafloor spreading is related to such stress field as well. [ABSTRACT FROM AUTHOR]

A hyper-stretched continental crust with a thickness of 10–18 km and high/low velocity anomaly is imaged in the southwest continental margin of the South China Sea. • A hyper-stretched continental crust with a thickness of 10–18 km is imaged in the SWCM of the SCS. • Depth-dependent extension of the crust occurred in the SWCM. • High velocity bodies in the lower crust are related to the remnant Mesozoic arc. • Low velocity anomaly with the maximum thinning crust is due to a frontal thrust. A 680-km long wide-angle seismic profile was acquired in the southwest continental margin (SWCM) of the South China Sea to better understand its crustal variation and continental extension. By using two-dimensional forward ray-tracing, and joint refraction and reflection travel time inversion methods, velocity structures of the SWCM are yielded. The model results show a hyper-stretched continental crust with a thickness of 10–18 km, which is variable under sedimentary basins and reef areas. Compared with βwc, βuc, and βlc derived from the crustal thickness, the degree of the whole crust extension appears to be larger than that of the upper crust but lower than that of the lower crust, except for the model of 560–610 km. Thus, we infer that depth-dependent extension of the crust has occurred in the SWCM. High velocity bodies with velocities larger than 7.0 km/s are sporadically distributed in the lower crust, which might be related to the remnant Mesozoic arc that can be attributed to the subduction of the paleo-Pacific plate. The obvious low velocity anomaly with the maximum thinning crust below the boundaries of the Nansha Trough and Palawan-Borneo thrust wedge (PBTW) is interpreted as the frontal thrust of the PBTW. [ABSTRACT FROM AUTHOR]

• A thickened oceanic crust rather than a mature island arc crust along the central and southern KPR. • Lower crust thickness is well-correlated with the quantitatively calculated volume of seamounts. • Central and southern KPR is at the juvenile stage of arc evolution. As an intra-oceanic arc, the Kyushu–Palau Ridge (KPR) is an exceptional place for studying intra-oceanic arc evolution and related magmatism using its interior velocity structure. Two wide-angle seismic profiles along the KPR provide an opportunity to unveil the crustal structure of its central and southern parts. Strong along-ridge variations are found either in P-wave velocity or crustal thickness of the central and southern KPR. The crustal thickness is between 6 and 12 km, with the velocity increasing from 4.0 km/s to 7.0 km/s from top to bottom. The upper crust has a large velocity gradient of ∼1 s−1 and a thickness between 2 and 4 km, while the lower crust has a lower velocity gradient of <0.2 s−1 and is characterized by inhomogeneous velocity anomalies and a substantial lateral variation in thickness. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, we suggest the central and southern KPR is a thickened oceanic crust rather than a mature arc crust. Partly thick crust, velocity anomalies in the lower crust and upper mantle imply the crust is in a transition stage from oceanic crust to mature arc one. Compared with the quantitatively calculated seamount volumes, the lower crustal thickness is generally correlated well with the distribution of the seamount volumes along the central and southern KPR. These observations imply that the central and southern parts of the KPR are at the juvenile stage of arc evolution, with crustal growth primarily attributed to the lower crust thickening. [ABSTRACT FROM AUTHOR]

Postspreading seamounts are unique seamounts built by volcanism after seafloor spreading had ceased. Limited understanding of their formation processes is mainly due to the lack of investigation. Therefore, we conducted a three‐dimensional wide‐angle seismic experiment in the Southwest Subbasin of the South China Sea and four profiles were acquired within the inactive spreading center over a postspreading seamount. Based on ocean bottom seismometer data, we report two‐dimensional velocity structures of the Longnan seamount. Our results show that a low P wave velocity body (2.9–5.0 km/s, 4 km thick) is observed within the seamount summit, suggesting that material has extensively erupted as volcaniclastic rocks and high‐porosity basalts. The extrusive/intrusive boundary is depressed beneath the seamount, indicating the absence of an intrusive core. Our results further show that the ratio of extrusive to intrusive volume is as high as 3.0 and that the accretion to the oceanic crust occurs in layer 2. Combined with an analysis of velocity structures of seamounts worldwide including seamounts in the South China Sea, we propose that the extrusive/intrusive ratio and type of crustal thickening are related to the magma supply mechanisms, and intrusive cores are more likely to be associated with a hot spot. The above analysis also suggests that the Longnan Seamount is a nonplume origin seamount and structurally different from other seamounts in the South China Sea. Ridge jumps in the South China Sea increase the duration or amount of volcanism, which could explain seamount structural differences. Key Points: Studied seamount is extrusion dominated with an extrusive/intrusive ratio as high as 3.0 with nonplume originSeamount structures are mainly controlled by magma supply mechanismsAnomalous mantle due to southward ridge jumps could explain the difference of volcanism (and its structures) in the South China Sea [ABSTRACT FROM AUTHOR]

It has long been established that plastic flow in the asthenosphere interacts constantly with the overlying lithosphere and plays a pivotal role in controlling the occurrence of geohazards such as earthquakes and volcanic eruptions. Unfortunately, accurately characterizing the direction and lateral extents of the mantle flow field is notoriously difficult, especially in oceanic areas where deployment of ocean bottom seismometers (OBSs) is expensive and thus rare. In this study, by applying shear wave splitting analyses to a dataset recorded by an OBS array that we deployed between mid-2019 and mid-2020 in the South China Sea (SCS), we show that the dominant mantle flow field has a NNW–SSE orientation, which can be attributed to mantle flow extruded from the Tibetan Plateau by the ongoing Indian–Eurasian collision. In addition, the results suggest that E–W oriented flow fields observed in South China and the Indochina Peninsula do not extend to the central SCS. [ABSTRACT FROM AUTHOR]

Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P -wave velocity (Vp), S- wave velocity (Vs), Vp / Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp / Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp / Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD. [ABSTRACT FROM AUTHOR]

We present results from an ocean bottom seismometer experiment surveying the fossil spreading centre in the Southwest Sub-basin of the South China Sea. The detailed velocity model shows that oceanic layer 2 exhibits across-axis variations in thickness and velocity, whereas oceanic layer 3 displays a variation in crustal thickness. A low-angle (24°) SE-dipping oceanic detachment fault is proposed to explain the anomalous structure on the NW side of the spreading centre, which exhibits uplifting of the upper mantle beneath a thinned oceanic crust. The inferred oceanic detachment fault was at its initial stage, localized within the basaltic crust and did not exhume lower crust. We suggest that the low-velocity (7.6-7.9 km/s) body located within the upper mantle beneath the footwall of the detachment fault is caused by both mantle serpentinization and partial melting. The difference in crustal thickness in the Southwest Sub-basin indicates that the magma supply varied in time and space during or even after the seafloor spreading. Compared with other fossil spreading ridges, the fossil spreading centre of the northeast Southwest Sub-basin studied here represents a third type of fossil spreading ridge, characterized by a reduced melt supply at the waning stage of spreading and a strong post-spreading magmatism. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The crustal structure of the northwestern sub-basin area of the South China Sea was modeled by inverting a wide-angle seismic survey line across the entire region and on both sides of its bounding continental margins. The survey line extended over 484 km. A total of 14 Ocean Bottom Seismometers (OBS) were deployed at intervals of 30 km to record air-gun array sources with a combined volume of 5160 in. The crustal velocity structure of the northwestern sub-basin area was acquired through the integration of multi-channel seismic data. OBS data were processed and modeled initially using ray tracing inversion techniques. Results indicate that crustal thickness under the continental slope decreases from 21 to 11 km, crustal thickness of the northwestern sub-basin is 7.7 km, and the depth to the Moho ascends from 21 km under the upper continental slope to 11 km under the middle basin. The crust of the northwestern sub-basin is similar to that of the eastern sub-basin in its oceanic crustal structure. This structure has a thicker layer 1 (sedimentary layer) and a thinner layer 2. These characteristics are especially clear in the eastern sub-basin, which differs somewhat from typical oceanic crust. The tectonic geometry and velocity structure of the northwestern sub-basin and its margins comprise a symmetrical conjugate and indicate a pure shear mode with regard to the continental margin rifting mechanism. We did not find clear seismic signals from high velocity layers under the lower crust of the continental margin in the northern part of the northwestern sub-basin, which provides new evidence for the idea that the western part of the northern continental margin of the South China Sea constitutes non-volcanic crust. Because the seafloor spreading period of the northwestern sub-basin was short, layer 2 might have experienced asymmetrical basalt magma flows, which may have blurred the magnetic anomaly lineations of the northwestern sub-basin. [ABSTRACT FROM AUTHOR]

Based on the multibeam morpho-tectonic analysis of the Manila Trench accretionary wedge and its indentation tectonics and the contrasting researches with other geological and geophysical data, three tectonic zones of the wedge are established, faulting features, tectonic distribution and stress mechanism for the indentation tectonics are analyzed, oblique subduction along Manila Trench with convergent stress of NWSS° is presented, and the relation- ship of the ceasing of Eastern Subbasin spreading of South China Sea Basin to the formation of subduction zone of Manila Trench is discussed. By the model analysis and regional research, it is found that the seamount subduction along Manila Trench does not lead to the erosion of the accretionary wedge and the oblique subduction actually is a NWW- trending obduction of Luzon micro-plate that results from the NWW-trending displacement of the Philippine Sea plate. [ABSTRACT FROM AUTHOR]

Deep crustal structure of the Nansha Trough (NT) plays a crucial role in understanding its crustal nature and magmatic activity. This study used the data from a wide-angle seismic profile (NDL1), which passes through the Jinghong seamount (JHSM) and Yinqing seamount (YQSM) along the trough. By using 2D forward ray-tracing and joint refraction and reflection travel-time inversion methods, we obtained the velocity structure of the NT. The model results show a thinned continental crust with a thickness of ∼9–18 km in the east of the trough, which is variable drastically below the seamounts. The upper crust slightly varies at a thickness of ∼5 km except at the seamounts, while the lower crust shows lateral variations in thickness and velocity, accompanied by high velocities below the JHSM due to the magmatic addition. Velocity models of the NDL1 also reveal crustal differences between the JHSM and YQSM. Combined with the magnetic anomaly and multichannel seismic data, we infer that the seamounts formed from multiple stages of magmatism after the spreading of the South China Sea and the NT are still active. High velocities in the lower crust result from the mantle upwelling and decompression melting of flexural responses of the NT, which may provide a genesis clue for the formation of the submarine seamounts. • A thinned continental crust in the east and a probable oceanic crust in the west of the Nansha Trough. • Both differences in crustal structure and magnetic anomalies indicate seamounts have different origins. • The seamounts experience multiple stages of magmatism and the Nansha Trough is still active. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is a typical marginal sea basin formed by the seafloor spreading under the tectonic background of plate convergence. Many crustal-scale studies indicate that the SCS basin has undergone asymmetric spreading, multi-phase ridge jumps, and intense post-spreading volcanic activity. Due to the lack of seismic data in the oceanic basin of the SCS, it remains unclear about the scale and basin control of the Zhongnan fault, the magma source depth of the SCS basin, and the transport channel after the cessation of seafloor spreading. Phase velocity derived from ambient noise surface wave tomography may provide useful information to shed light on the mechanisms of the aforementioned problems. From October 2019 to July 2020, a passive source seismic experiment was carried out with many Ocean Bottom Seismometers (OBS) deployed in a broad area of the SCS. Based on the seismic ambient noise data recorded by 16 OBSs in the SCS basin, we inverted the phase velocity images over a period range of 10–20 s using ambient noise surface wave tomography. Our results indicate that the Zhongnan fault zone is a lithospheric-scale fault, which played a regulating role in the expansion of the SCS basin from the East Subbasin to the Southwest Subbasin. In addition, the low-velocity body in the north flank of the Southwest Subbasin extends from the post-spreading seamounts on the ocean crust to the uppermost mantle (i.e., about 10–30 km), which indicates an oblique magma migration during the post-spreading volcanism. • OBS ambient noise data reveal phase velocity images of the South China Sea basin. • The Zhongnan fault zone is a lithospheric-scale fault. • There is an oblique magma migration in the Southwest Subbasin on the post-spreading. [ABSTRACT FROM AUTHOR]

Seamounts comprise up to 25% of the seafloor (Gilbert, 2004) and most of them are volcanic origin (e.g., Smith & Cann, 1992). Submarine volcanism has resulted in the large vertical and horizontal displacements (e.g., Staudigel & Clague, 2010) and the emplacement of seamounts(e.g. Contreras-Reyes et al., 2010). However, the internal structure and evolution of the seamounts remains unclear, which limits the understanding of post-spreading magmatism in the South China Sea. We conducted a seismic refraction survey over an axial seamount (Longnan Seamount) in the Southwest Sub-basin, South China Sea in 2010. Results show that the post-spreading axial seamount was mainly formed by an extrusive process. The upper-crustal velocities (2.9–4.7 km/s) observed at the seamount are significantly lower than those of normal oceanic crust (~4.7 km/s), suggesting explosive materials and increased porosities in the basalts. The lack of clear Moho reflections and a low-velocity upper mantle (7.7–7.8 km/s) may indicate mantle serpentinization beneath the seamount(e.g. Delescluse et al., 2015). Compared with the Huangyan and Zhenbei seamounts in the East Sub-basin, the Longnan seamount in the Southwest Sub-basin is characterized by the crustal accretion occurring in layer 2 only, which may be result from the difference tectonic characteristics. The proposed model for post-spreading seamount formation is marked by an episodic process and controlled both by tectonics and by magmatism.This research was granted by the National Program on Global Change and Air–Sea Interaction, SOA (GASI-GEOGE-01), the National Natural Science Foundation of China (41706044, 91628301, 91028006, 91228205) [ABSTRACT FROM AUTHOR]

The Kyushu–Palau Ridge (KPR) represents a natural laboratory for understanding tectonic evolution and magmatism from initial subduction to initial back-arc spreading, thus demonstrating along−/across-arc variations in structures. However, how magma emplaces and causes diversities in arc structures remains unclear, and the southern KPR structure has not been well studied. Herein we studied the crustal magmatism at the southern KPR based on the detailed velocity structure imaged by two two-dimensional wide-angle seismic profiles. Results show that the KPR's crust to the west is 6.0–7.6-km-thick with one summit at the basement and a flat Moho boundary at a 12-km depth; to the east, the crust has a thickness of 7.0–11.1 km, and three summits are observed at the basement sharing one depressed Moho boundary (12–15 km depth). These structural diversities indicate the presence of two independent seamounts (west and east), with different periods of magmatism. By calculating the duration of volcanism, we hypothesized that the formation of the western seamount of the KPR requireed a magmatism duration of ∼4 Myr and one stable volcanic source. In contrast, the eastern seamount needed two volcanic sources and a total magma loading time of ∼10 Myr with ∼19-Myr period gap in magmatism. By analyzing the KPR evolution, we conclude that 1) multiple volcanic sources and episodic magmatism caused the diversities in arc structures at the KPR; 2) the initiation of back-arc spreading at ∼26 Ma might have inhibited the magmatism related to the subduction; however, the back-arc-related partial remelting of the lower island arc would have led to a new magma source and reconstructed the preexisting island arc, thus leaving multiple conduits inside it. • Revealing seismic feature of the southern Kyushu–Palau Ridge (KPR) • The KPR comprises two independent seamounts (west and east) with different periods of magmatism. • The magmatism at the KPR is characterized by multiple volcanic sources with obvious time gaps. [ABSTRACT FROM AUTHOR]

Abstract: Beneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation. [ABSTRACT FROM AUTHOR]

Abstract: The generating mechanism and process of slow earthquakes can help us to better understand the seismogenic process and the petrological evolution of the subduction system, but they are still not very clear. In this work we present robust P and S wave tomography and Poisson's ratio images of the subducting Philippine Sea Plate beneath the Kii peninsula in Southwest Japan. Our results clearly reveal the spatial extent and variation of a low‐velocity and high Poisson's ratio layer which is interpreted as the remnant of the subducted oceanic crust. The low‐velocity layer disappears at depths >50 km, which is attributed to crustal eclogitization and consumption of fluids. The crustal eclogitization and destruction of the impermeable seal play a key role in the generation of slow earthquakes. The Moho depth of the overlying plate is an important factor affecting the depth range of slow earthquakes in warm subduction zones due to the transition of interface permeability from low to high there. The possible mechanism of the deep slow earthquakes is the dehydrated oceanic crustal rupture and shear slip at the transition zone in response to the crustal eclogitization and the temporal stress/strain field. A potential cause of the slow event gap existing beneath easternmost Shikoku and the Kii channel is the premature rupture of the subducted oceanic crust due to the large tensional force. [ABSTRACT FROM AUTHOR]

The Southwest Subbasin (SWSB) is an abyssal subbasin in the South China Sea (SCS), with many debates on its neotectonic process and crustal structure. Using two-dimensional seismic tomography in the SWSB, we derived a detailed P-wave velocity model of the basin area and the northern margin. The entire profile is approximately 311-km-long and consists of twelve oceanic bottom seismometers (OBSs). The average thickness of the crust beneath the basin is 5.3 km, and the Moho interface is relatively flat (10-12 km). No high velocity bodies are observed, and only two thin high-velocity structures (~7.3 km/s) in the layer 3 are identified beneath the northern continent-ocean transition (COT) and the extinct spreading center. By analyzing the P-wave velocity model, we believe that the crust of the basin is a typical oceanic crust. Combined with the high resolution multi-channel seismic profile (MCS), we conclude that the profile shows asymmetric structural characteristics in the basin area. The continental margin also shows asymmetric crust between the north and south sides, which may be related to the large scale detachment fault that has developed in the southern margin. The magma supply decreased as the expansion of the SWSB from the east to the west. [ABSTRACT FROM AUTHOR]

The Cenozoic tectonic evolution of the South China Sea is reflected in diverse tectonic processes including continental rifting, seafloor spreading, subduction, terrane collision and strike–slip fault movement. A continent–ocean transition zone in the northern South China Sea caused by the lithospheric extension when the continent underwent break-up, rifting and later seafloor spreading, is clearly imaged in the multi-channel seismic data presented in this study. The morphological units of the continent–ocean transition zone are the rift-depression, the volcanic zone and tilted fault blocks. The volcanic zone represents a highest extension zone within the continent–ocean transition zone and is mainly distributed in the southern slope uplift zone along the northern passive margin of the South China Sea. The large listric-normal faults bounding the Dongsha Rise and Baiyun Sag are evidenced in the seismic image. The passive margin in the northern South China Sea underwent the wide-rift to narrow-rift process in the transition zone as inferred from the relation between the surface heat flow and initial rifted crustal thickness. The continent–ocean transition zone in the continental margin of the South China Sea is consistent with high heat flow zone (average 90 mW.m − 2 ) observed in the previous heat flow measurements and at ODP Site 1148, and is manifested in a sharp change of the P-wave velocity. The rifted margin of the South China Sea is a non-typical magma poor passive margin or an intermediary form between the Iberian-type non-volcanic and the Greenland-type volcanic margin compared to the world's typical passive margins. [ABSTRACT FROM AUTHOR]

Although the East China Sea (ECS) is widely accepted as the offshore extension of the South China Block (SCB), an alternate hypothesis is that the ECS basement is a buoyant allochthon that collided with Eurasia in the Cretaceous. In order to constrain the Mesozoic collision position, or the Mesozoic suture zone, wide-angle reflection/refraction seismic profiles (ECS2017 and ECS2019) were carried out in 2017 and 2019 to map the crustal structure of the ECS and clarify the possible Mesozoic suture zone. Crustal structures are generated by both a ray-tracing forward method (RayInvr) and a travel time tomographic inversion (Tomo2D). The resulting velocity models show significant lateral variations along these two seismic profiles. An extended continental crust gradually decreases in thickness from 30 km in the Zhemin Volcanic Belt (ZMVB) to 15 km in the Continental Shelf Basin (CSB) with lateral velocity variations of 4.40–7.15 km/s and 4.30–6.90 km/s, respectively, indicating differences in crustal structure between the ZMVB and the CSB. A 50 km-wide high-velocity anomaly with high density and uplifted Moho is imaged between the ZMVB and the CSB. Combined with the Mesozoic subduction and collision zone of the exotic ECS basement by previous studies, we infer that the high-velocity anomaly marks the position of the Mesozoic suture zone, which is associated with magma intrusion due to upwelling molten through the detached slab with the rollback of the Paleo-Pacific subduction slab. • Velocity models of two wide-angle profiles in the East China Sea are constructed to help constrain the position of a Mesozoic suture zone • A 50 km-wide high-velocity anomaly between the Zhemin Volcanic Belt and the Continental Shelf Basin is identified • The anomaly is interpreted as a Mesozoic suture zone associated with magma intrusion related to subduction of the Paleo-Pacific slab [ABSTRACT FROM AUTHOR]

An electrical resistivity monitoring experiment using a fixed Schlumberger sounding array was conducted at Pixian in Sichuan Province, China, from January 1984 to June 1989, for the purposes of better understanding some geoelectrical phenomena and detecting earthquake-associated changes. Ancillary data of well water fluctuations and rainfall changes, along with seismicity pattern, fault map, atmospheric humidity, and temperature, have been collected. A very peculiar resistivity behavior was observed: the direction of the change in apparent resistivity for large electrode expanders was opposite to that of the resistivity change for the overburden. The geoelectrical structure beneath the monitoring array could be inferred from resistivity sounding data and available geological information. A method of data analysis applying singular value decomposition to the first-order sensitivity matrix yields the estimated change in true resistivity for the constituent medium in the sense of generalized least-squares. The sensitivity analysis shows that for the Pixian station, the abnormal annual variations in resistivity monitoring with large electrode spacings are related to the multilayered resistivity section with conductive substratum and are caused by the resistivity changes of overlying layer because the sensitivity coefficient for the topmost layer for such a section is negative. The resistivity changes of the overlying layer could be attributed mainly to seasonal rainfall changes. During the experiment, no significant resistivity changes are correlated with the earthquakes of ML 5.4 or less in the Longmen Shan and adjacent regions. However, a detailed interpretation of the observed phenomenon is helpful to improve our understanding of possible resistivity precursors to earthquakes. [Copyright &y& Elsevier]

A thick Infra-Cambrian to Phanerozoic sedimentary sequence in eastern Arabia conceals a poorly known Neoproterozoic basement that once separated the currently exposed Arabian Shield (western Arabia) from NW India in Gondwana. In this study, the Arabian Platform (eastern Arabia) is targeted by relatively high-resolution passive-source seismic tomography for the first time, which aims to illuminate crust and upper mantle structure in order to make inferences on their nature and origin. This is facilitated by a new broadband seismic network of 55 stations in the United Arab Emirates (UAE), which has recorded sufficient teleseismic body wave data to allow for the determination of three-dimensional seismic structure. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival-time residuals from teleseismic earthquakes are mapped as 3-D P-wave velocity perturbations in the upper mantle beneath the seismic network. The resultant tomographic images reveal a marked transition from higher velocities in the Gulf of Oman to lower velocities in the west that we interpret to indicate the lithospheric boundary between Cretaceous Tethyan oceanic lithosphere and the Arabian passive continental margin. The northern extent of Tethyan oceanic lithosphere appears to terminate against a possible offshore continuation of the Dibba fault, until now only identified onshore. Further inland, the tomographic model exhibits a number of low- and high- velocity anomalies with a predominant NW orientation. Significantly, one well-defined high velocity anomaly is situated along strike of the Neoproterozoic exposure of volcanic-arc granodiorites found in northern Oman. We relate this anomaly to an intra-oceanic arc created during the Tonian subduction of the Mozambique Ocean, which strongly support the idea of continental growth in the Arabian Platform via magmatic arc accretion. • New P-wave tomographic model reveals upper mantle structure of northeastern Arabia. • Boundary between Tethys oceanic lithosphere and Arabian continental margin detected. • Velocity anomalies provide evidence for continental growth in eastern Arabia. [ABSTRACT FROM AUTHOR]

The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using a total of 40 ocean bottom seismometers (OBS) array, four air guns as a source with a volume of 6000 in3 and its three-dimensional refraction seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28′E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (0.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30–50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge. [ABSTRACT FROM AUTHOR]

Beneath ultraslow-spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17-days-long microearthquake study on two segments (27 and 28) of the ultraslow-spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5-km-thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation. [ABSTRACT FROM AUTHOR]

Seismic azimuthal anisotropy characterized by shear wave splitting analyses using teleseismic XKS phases (including SKS, SKKS, and PKS) is widely employed to constrain the deformation field in the Earth's crust and mantle. Due to the near‐vertical incidence of the XKS arrivals, the resulting splitting parameters (fast polarization orientations and splitting times) have an excellent horizontal but poor vertical resolution, resulting in considerable ambiguities in the geodynamic interpretation of the measurements. Here we use P‐to‐S converted phases from the Moho and the 410‐ (d410) and 660‐km (d660) discontinuities to investigate anisotropy layering beneath Southern California. Similarities between the resulting splitting parameters from the XKS and P‐to‐S converted phases from the d660 suggest that the lower mantle beneath the study area is azimuthally isotropic. Similarly, significant azimuthal anisotropy is not present in the mantle transition zone on the basis of the consistency between the splitting parameters obtained using P‐to‐S converted phases from the d410 and d660. Crustal anisotropy measurements exhibit a mean splitting time of 0.2 ± 0.1 s and mostly NW‐SE fast orientations, which are significantly different from the dominantly E‐W fast orientations revealed using XKS and P‐to‐S conversions from the d410 and d660. Anisotropy measurements using shear waves with different depths of origin suggest that the Earth's upper mantle is the major anisotropic layer beneath Southern California. Additionally, this study demonstrates the effectiveness of applying a set of azimuthal anisotropy analysis techniques to reduce ambiguities in the depth of the source of the observed anisotropy. Key Points: Azimuthal anisotropy above the core‐mantle boundary, 660‐ and 410‐km discontinuities, and the Moho beneath Southern California are measuredSimilarities among splitting parameters using the three deepest discontinuities suggest insignificant anisotropy below the 410‐km discontinuityThe study demonstrates the effectiveness of investigating anisotropy layering using P‐to‐S conversions from interfaces at various depths [ABSTRACT FROM AUTHOR]