Search

Graphical abstract Highlights • Seismogenic characteristics near the NE rifted margin of the South China Sea are revealed. • NW-SE tensional stress controls the Taiwan collision-Manila subduction transition. • Intraslab pre-existing faults affect the seismogenic behavior of a convergent system. Abstract Based on the distribution of an earthquake swarm determined from an ocean bottom seismometer (OBS) network deployed from 20 August to 5 September 2015 in the northernmost part of the South China Sea (SCS) and data from inland seismic stations in Taiwan, we resolved a M L 4.1 earthquake occurring on 1 September 2015 as a NE-SW-trending left-lateral strike-slip event that ruptured along the pre-existing normal faults generated during the SCS opening phases. The direction of the T-axes derived from the M L 4.1 earthquake and the background seismicity off SW Taiwan present high consistency, indicating a stable dominant NW-SE tensional stress for the subducted Eurasian Plate (EUP). The distinct stress variations on the two sides of these reactivated NE-SW trending features suggest that the presence of pre-existing normal faults and other related processes may lessen the lateral resistance between the Taiwan collision and Manila subduction system, and facilitate the slab-pull process for the subducting portion, which may explain the NW-SE tensional stress environment near the transition boundary in the northernmost part of the SCS. [ABSTRACT FROM AUTHOR]

Located at the arc-continental collision region between the Eurasian (EP) and Philippine Sea Plates (PSP), Taiwan is usually considered to have a complex tectonic environment, particularly along the eastern coast of the island. To gain a better understanding of the geological evolution of the east Taiwan area, the data from 8 Ocean Bottom Seismometers (OBS) acquired during the Across Taiwan Strait Explosion Experiment in 2012 and 14 inland seismic stations were used to determine a more detailed and accurate distribution of marine earthquakes. Based on the 333 relocated earthquakes and available geophysical data, we suggest two main tectonic boundaries for eastern Taiwan. South of 23.25°N, the homogeneous distribution of earthquakes in the crustal portion for both the inland and offshore areas suggests an ongoing collisional process. North of this location, between approximately 23.25°N and 23.8°N, the abrupt increasing of seismicity depth infers that the underthrusted arc/fore-arc material is deforming due to the collisional compression at depth. In this segment, the subsidence of the arc/fore-arc area determines the transition from collision to subduction. North of 23.8°N, the northwestern dipping PSP is well illustrated by the seismicity both onshore and offshore, indicating a dominant subduction process. [ABSTRACT FROM AUTHOR]

Abstract: Detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer (OBS) records requires the analysis and identification of noise due to instrumental problems, deployment sites or sea state conditions. The temporary OBS deployment at the Lesser Antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor (NVT), long-period (LP) or ultra-long period (ULP) events. Analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise. Episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data. In order to reliably detect NVT (1–10Hz) originating from inside the solid Earth, one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea-bottom as well as on land. The semidiurnal modulation of noise amplitude can be shown here not to be due to that of the NVT from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs. Here, the semidiurnal modulation is rather due to the effect of the tides themselves, such as tidal currents, since they do not affect all types and all components of the unique multi-station array of OBS that could be deployed on this submerged forearc. The short period cut-off of the strong noise due to ocean surface infragravity waves increases to longer periods with OBS depth, thereby increasing the observational window with low noise to lower frequencies, and deep OBS sites may be advantageous for detecting LP events. [Copyright &y& Elsevier]

Abstract: 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 10–14km thick outboard of the trench, but thins to 6km 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 10km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0km/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. [Copyright &y& Elsevier]

Abstract: This paper investigates velocity structure of the active plate boundary in southern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. P and S–P arrival time data from 3238 earthquakes were used. In addition to CWBSN permanent networks, seismic data include the Central Weather Bureau permanent networks and a temporary network consisting of 11 ocean bottom seismometers (OBSs) that was deployed to detect the aftershocks of the 2006 (ML7.1) Henchun earthquake occurred beneath southern Taiwan. The total available OBS data set consists of ~700 detected earthquakes, from which around 500 could be well located where about 450 events have been used in simultaneous inversion for hypocenters, three-dimensional Vp and Vp/Vs models for the study area. The main objective of incorporating gravity analysis is used to improve the velocity model for the offshore area, where it is poorly sampled by local earthquakes. This study found a low-velocity zone existing above the subducting South China Sea slab in the mantle wedge. Based on gravity modeling and our resulting velocity and Poisson''s ratio models suggest that the subduction complex, which is characterized with a low P-wave velocity and low Poisson''s ratios beneath the southern Taiwan. This duplex structure is characterized by a zone of low P-wave velocities in the range of 6.2–6.8km/s between 25 and 40km depth. It also shows that earthquake hypocenters do not fall within this low velocity zone. We have also used seismic tomography velocities to estimate the volume percentage of serpentinite and silica concentrations in southern Taiwan. The calculated serpentinite is about 30% and the volume percentage of quartz estimated is about 20% at the base of the forearc lower crust. [Copyright &y& Elsevier]

Abstract: We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (~300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu–Palau ridge at depths greater than 120 km. Along the Izu–Bonin(Ogasawara)–Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of ~500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab. [Copyright &y& Elsevier]

On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap <200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes. Unlabelled Image • 1D velocity model, station corrections and hypocentral locations were obtained based on the aftershocks of the 2016 Pedernales earthquake. • Our minimum 1D velocity model allows us to observe and discuss depth ranges and to first order the velocity structure with depth. • Station correction terms are consistent with areas affected by site effects during the earthquake. • A portion of the relocated seismicity occurs very close to the trench (~5 – 10 km). • Regional moment tensors suggest the presence of a strike-slip fault in the marine forearc. [ABSTRACT FROM AUTHOR]