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Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal‐dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal‐dip slab). Anisotropy determined by shear wave splitting observation suggests that the low‐velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low‐velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge. Plain Language Summary: Understanding the subduction process and how the mantle flows is pivotal in understanding the planetary evolution of Earth. Yet, several subduction characteristics remain unsolved, and the angle of plate subduction, which is often categorized into <30° (shallow), ∼30–35° (normal), >35° (steep), is one of those. Subduction of an oceanic ridge on a plate has been proposed as a cause of a shallow subduction angle since the thick crust of the ridge is less dense than the surrounding mantle. However, normal angles have been observed in some cases where oceanic ridges are subducting. In this study, we compare the Nazca Ridge and the Iquique Ridge, on the Nazca Plate subducting beneath South America. The subduction angles of the Nazca and Iquique Ridges are shallow and normal, respectively. When we incorporate directional variations in seismic wave velocities, which are produced by mantle flow, in seismic tomographic imaging, we find that the subducting oceanic ridge may not be a primary factor producing shallow angle subduction. Instead, the warm mantle beneath the Nazca Ridge may provide the buoyancy to support the Nazca Plate. Comparably, since the mantle beneath the Iquique Ridge is not warm, the subduction angle would stay normal. Key Points: Despite having different subduction angles, subslab low‐velocity anomalies are found beneath both Nazca Ridge and Iquique RidgeWe introduced hypothetical seismic anisotropy in tomographic inversion to explore the origin of the subslab low‐velocity anomaliesThe low‐velocity anomalies beneath the Nazca Ridge and Iquique Ridge may come from buoyant warm mantle and anisotropy, respectively [ABSTRACT FROM AUTHOR]

As the mantle plume rises from the deep mantle and reaches the base of a tectonic plate, it changes its traveling direction from vertical to horizontal. The horizontal spread of plume material is often radially asymmetric. An example is a plume found below the Canary Hotspot. Previous studies have suggested that the channeling of the Canary Plume toward the westernmost Mediterranean (Alboran Sea) and consequent lithospheric delamination may have contributed to the low‐velocity anomalies found beneath the Moroccan Atlas Mountains. Regional upwelling and edge‐driven convection have been proposed as other candidates to explain the origin of the low‐velocity anomalies. In this study, we incorporated anisotropy as an a priori constraint in teleseismic P‐wave travel‐time tomography as mantle flow can develop seismic anisotropy. We inverted a new set of travel‐time delays by removing the hypothetical anisotropy‐imposed travel‐time delays from the observations. Our improved results are more consistent with the hypothesis that the low‐velocity anomalies come from the mantle material of the Canary Plume. Plain Language Summary: The propagation velocity of seismic waves is sensitive to the temperature. Low seismic velocities in the mantle found from seismic tomography, a technique used to image the velocity structure of the inner Earth with seismic waves, are commonly interpreted as high‐temperature, hydrous regions, the presence of partial melting, changes in the compositional chemistry, and grain sizes of mantle minerals. Thus, seismic tomography has been the primary tool for revealing mantle structures, such as hot plumes rooted in the deep mantle that play significant roles in mantle dynamics. Previous studies have shown that rising plume material can drag the surrounding mantle and consequently cause directional dependence of the seismic velocities, which can also affect the results of seismic tomography. This study improved the quality of seismic tomography results by considering the directional dependence of wave speeds in the mantle to elucidate the evolution of the mantle plume and its interaction with the upper plate in Morocco. The imaged low‐velocity conduit below the high‐altitude Moroccan Atlas indicates the lateral travel of the mantle plume originating from the Canary Hotspot in northwest Africa, which may have dragged the surrounding mantle beneath the Moroccan Atlas. Key Points: Teleseismic P‐wave tomography is used in combination with anisotropy models to study the origin of the low‐velocity anomalies below the Moroccan AtlasWe incorporate anisotropy as an a priori constraint in tomography and show the reduction in low‐velocity anomalies below the Moroccan AtlasThe lateral travel of plume material from the Canary Hotspot may be the origin of low‐velocity anomalies [ABSTRACT FROM AUTHOR]

Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites. Our coupled fluid-flow and geomechanical model describes the physical processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that prolonged accumulation of poroelastic energy and pore pressure along a fault can nucleate seismic events larger than Mw3 even after terminating well operations. In particular the possibility of large seismic events can be increased by multiple-well operations with alternate injection and extraction that can enhance the degree of pore-pressure diffusion and subsequent stress transfer through a rigid and low-permeability rock to the fault. This study demonstrates that the proper mechanistic model and optimal well operations need to be accounted for to mitigate unexpected seismic hazards in the presence of the site-specific uncertainty such as hidden/undetected faults and stress regime. [ABSTRACT FROM AUTHOR]

Two earthquakes ( M 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the M 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes ( M 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes ( M 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics. [ABSTRACT FROM AUTHOR]

The M 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an M 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised. [ABSTRACT FROM AUTHOR]

More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges. Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using teleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence. [ABSTRACT FROM AUTHOR]

Seismic anisotropy of polycrystalline materials depends on the characteristics of microcracks as well as the crystallographic orientations of minerals. Here, we present a MATLAB-based software, AnisEulerSC (Anis otropy from Euler angles using S elf- C onsistent approximation), for modeling the anisotropic seismic properties of a polycrystalline aggregate with microcracks using the self-consistent approximation. In this program, several commands of MTEX (a MATLAB toolbox for texture analysis) are utilized to analyze constituent components of a rock sample based on experimental measurement data (e.g., electron backscatter diffraction) and to calculate and visualize seismic wave velocities and polarizations. We provide several examples of SC modeling to explore the effects of grain shapes on a polycrystalline aggregate and the effects of crack shapes and orientations on the anisotropic seismic properties of a polycrystalline aggregate. [ABSTRACT FROM AUTHOR]