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The southeastern Korean Peninsula (SeKP) has experienced intense deformation owing to subduction and backarc extension at the eastern continental margin of the Eurasian Plate, leading to the formation of complex tectonic structures. Abnormally high surface heat flux, Cenozoic volcanism, signatures of mantle degassing and hydrothermal alteration, and several active fault systems with extensional sedimentary basins have been identified; however, the major driving forces that promote local seismic events and hydrothermal activities remain enigmatic. Here, we constructed 3-D P -wave velocity of the crust and upper mantle in the SeKP for the first time using a teleseismic traveltime tomography method and an extensive data set obtained from a dense seismic network. Our model revealed three distinct velocity patterns at different depths: (1) in the upper crust (depth ∼0–10 km), a low-velocity anomaly beneath the Cenozoic sedimentary basin exhibiting a prominent lateral velocity contrasts with higher velocities in the Cretaceous sedimentary and plutonic rocks; (2) a N–S trending low-velocity anomaly extending from the lower crust to the uppermost mantle (depth ∼20–35 km) beneath the major active fault systems interpreted as a thermally or mechanically weakened structure that could transfer high surface heat flux and transport mantle-driven gases and (3) a low-velocity anomaly adjacent to the Cenozoic basin in the upper mantle at depths of 35–55 km interpreted as the higher temperature upper mantle. Via a series of geodynamic simulations, we demonstrated that the extensional deformation at the eastern continental margin during the Early to Middle Miocene locally enhanced the temperature of the crust and upper mantle beneath the SeKP. We propose that a hydrothermal system, resulting from the thermally modified lithosphere of the continental margin, has contributed to the enhanced local seismicity and geothermal activities observed in the SeKP region. [ABSTRACT FROM AUTHOR]

We analyze seismic records collected at the oldest (170–180 Ma) Pacific seafloor using broadband dispersion array analysis. Using ambient noise and teleseismic waveforms, we measure Rayleigh‐wave phase velocities in a period range of 5–200 s that are inverted for array‐average one‐dimensional isotropic and azimuthally anisotropic shear‐wave velocity depth profiles from the crust to a depth of 300 km. The high‐velocity Lid and the low‐velocity zone are well‐resolved with velocity difference of ∼4%, whose transition occurs at depths between 80 and 100 km. The profile is compared with that obtained at the 130‐ and 140‐Ma seafloor. Accounting for the cooling effect due to the plate age difference, the low‐velocity zone of the oldest Pacific seafloor is ∼1.3% slower (∼110°C warmer) than that beneath the 140‐Ma seafloor, suggesting the occurrence of some reheating process beneath the oldest lithosphere. The azimuthal anisotropy at shallow depths (<50 km) is significantly different between the western and eastern areas of the array where the peak‐to‐peak amplitudes are estimated to be ∼2.8% and ∼1.6%, respectively. The fast direction is nearly parallel to the past seafloor spreading direction (perpendicular to the magnetic lineation) in the west, while it largely deviates in the east. The observed difference in azimuthal anisotropy may represent complicated evolution dynamics of the infant Pacific plate that involved a ridge‐ridge‐ridge triple junction. Plain Language Summary: One of the classical enigmas of plate tectonics is the cause of seafloor flattening where the seafloor depth becomes nearly constant for old ages (>∼70 Ma) while a simple cooling model of plate evolution predicts the age dependence. To solve this enigma, it is essential to resolve the subsurface structure of the oceanic plate and the asthenosphere below. As a part of an international effort, Pacific Array, that aims to cover the Pacific seafloor by arrays of ocean bottom seismometers, a team of Japanese and Korean scientists deployed an array in the oldest (170–180 Ma) Pacific seafloor. We determine isotropic and anisotropic shear‐wave velocity structures as a first step toward resolving the enigma. The results indicate that the asthenosphere beneath the oldest Pacific lithosphere is significantly slower than the prediction of the simple cooling and that the occurrence of some reheating process may be needed. Furthermore, azimuthal anisotropy (the dependence of the wave velocity on the horizontal propagation direction) in the shallow lithosphere shows that the anisotropy patterns differ between the eastern and western areas of the array, and simulation results indicate that the evolution process of the infant Pacific plate is not merely a simple triangular plate spreading. Key Points: Broadband Rayleigh‐wave dispersion array analysis has been conducted in the oldest (∼170 Ma) Pacific seafloorThe low‐velocity zone is significantly slower to indicate the occurrence of some reheating process beneath the oldest Pacific lithosphereThe spatial variation of the shallow azimuthal anisotropy indicates a complicated evolution of the infant Pacific plate [ABSTRACT FROM AUTHOR]

We provide teleseismic constraints on the continental collision zone along the SW-NE profile from the Tibetan Plateau to the Alxa block, traversing Lenglong Ling, a topographic boundary of the northeastern Tibetan Plateau. Our seismic profile crosses the Qilian Shan thrust belt, a zone of intense earthquake activity that includes a pair of M W 5.9 earthquakes (in 1986 and 2016) with a 30-year time interval occurring at the city of Menyuan near Lenglong Ling. A high degree of similarity between the two waveforms and their similar hypocenters indicate that the two events are repeating earthquakes. Both P - and S -wave receiver functions show contrasting lithospheric structure and crustal seismic properties across the repeating earthquake region, suggesting that Lenglong Ling can be the crustal boundary between the Tibetan Plateau and the Alxa block. High-frequency P receiver functions clearly show a SW-dipping underthrusted upper crust beneath the foreland of the Qilian Shan thrust belt. A strong low-velocity feature above the intracrustal underthrust beneath southwest Lenglong Ling indicates a mechanically weak zone developed coeval with the main compressional deformation during the last major tectonic event. Our observed, localized upper-crustal deformation beneath Lenglong Ling can be direct evidence of the lateral growth of the northeastern Tibetan Plateau, and can control the seismicity of the Qilian Shan thrust belt, including the Menyuan repeating earthquakes. [ABSTRACT FROM AUTHOR]

We review geophysical and geochemical properties in the vicinity of the stagnant Pacific slab beneath northeast China, Korea and Japan to understand the origin and current state of intraplate volcanism in East Asia. East Asia has been susceptible to intensive volcanism since the Cretaceous. In particular, during the Cenozoic, Jeju Island, Korea is the most significant volcanic construct in East Asia. The generation of adakites and A-type granitoids littered throughout East Asia during the Cretaceous has been previously explained by the subduction of the Izanagi–Pacific ridge system. However, a recently revised plate reconstruction model does not comprise such a tectonic episode and consequently fails to explain adakites in arc magmatism during the Cretaceous. Thus, we propose an alternative hypothesis: temporal plume–slab interaction on the source of East Asian magmatism in order to trigger the adakites in arc magmatism without ridge subduction. In addition, we provide an overview on the tectonics and volcanism of Jeju Island during the Cenozoic in the context of the lithospheric structure from seismological constraints and recent geochemical results. The main scientific aims are to understand the consequences of mantle mixing in response to long-term subduction and subsequent changes on the stress state to determine the distribution of chemical heterogeneities, and thus define the origin and development of intraplate volcanism in East Asia. Although geophysical and geochemical data for Jeju Island are not sufficient at present, we strongly state that Jeju Island and the surrounding regions hold a key to unraveling the source of intraplate magmatism since the Cretaceous. [ABSTRACT FROM AUTHOR]

The horizontal Nazca slab, extending over a distance of ∼800 km along the trench is one of enigmatic features in Peruvian subduction zone. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment. We use data from the recent seismic experiment in southern Peru to find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of ∼20–40% near the top plate interface along- and perpendicular-to the trench from ∼40–120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of ∼800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel). [ABSTRACT FROM AUTHOR]

• We present 3D seismic attenuation (Q) model of the Nazca Plate subduction zone. • Our model shows clear high- Q slab and low- Q continental plate structures. • Our model for the Brazilian Shield shows high- Q at 40–80 km depth in the backarc. • Along-arc slab Q variations indicate different slab hydration states. • Relatively high- Q at the mantle wedge corner indicates cold thermal state. Seismic attenuation provides key constraints on understanding the structure and dynamics of subduction-zone system. In this study, we provide the first three-dimensional P and S -wave attenuation models beneath the Nazca-South America subduction zone by inverting t * data from 397 local shallow and intermediate-depth earthquakes. The study area covers the southern part of the Peruvian flat-slab region (11°–15 °S) (where the subducting Nazca Ridge is migrating), and the Central Andean Plateau (∼13°–18 °S) (associated with the subducting Nazca Plate) and the Brazilian mantle lithosphere. We obtain five major features in our attenuation model with seismicity including (1) well-defined along-strike and along-dip Nazca slab structure in terms of low attenuation, (2) high attenuation associated with the passage of Nazca Ridge and present location of the oceanic ridges, (3) high attenuation in the backarc mantle wedge and continental crust, (4) clear image of Brazilian Shield in terms of low attenuation, underthrusting to the west, and (5) high attenuation related to the slab dehydration beneath the volcanic arc at 100 and 200 km depths. In particular, prominent low- Q zones in the continental crust and mantle wedge beneath the active volcanic arc reflect source zones of arc magmatism caused by fluids from the slab dehydration. The observed along-arc differences in slab Q estimates can be attributed to different hydration states between the flat and normal-dip slabs due to the subduction of the Nazca Ridge. [ABSTRACT FROM AUTHOR]