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This study presents an approach to better characterize the P‐wave and S‐wave velocity structure of the seafloor sediment layer using ocean bottom seismometers. The presence of low‐velocity seafloor sediment layers influences the observed seismic record at the seafloor over a broad frequency range, such that detailed knowledge of this sediment structure is essential to predict its effect on teleseismic records. We use the radial component of teleseismic P waves and autocorrelation functions of the radial, vertical, and pressure components of teleseismic P and S waves to obtain sediment layer models using the Markov chain Monte Carlo approach with parallel tempering. Synthetic tests show that the body waves constrain the P‐ and S‐wave impedances and travel times and the P‐ to S‐wave velocity ratio of the sediment layers. The proposed method resolves thin layers at a high resolution, including the uppermost thin (∼50 m to a few hundred meters) low S‐wave velocity layer. Real data applications at sites across the Pacific Ocean that are coincident with previous in situ studies demonstrate the effectiveness of this method in characterizing the seafloor sediment unit. The sediment models characterized by this new approach will allow us to more accurately predict and correct the effects of sediment layers in generating P‐ and S‐wave reverberations. Plain Language Summary: Ocean bottom seismometers are powerful tools for investigating the detailed structure of the oceanic mantle. However, most ocean bottom seismometers are placed above the low‐velocity water‐saturated sediment layer, such that their seismic records are inevitably affected by this seafloor sediment layer. Detailed constraints on the structure of the seafloor sediment layer allow us to accurately predict its effects on the observed seismic records. The seafloor sediment layer may consist of multiple discontinuities and vary regionally. This study proposes a new approach for characterizing the seismic properties of the seafloor sediment layer using teleseismic body waves recorded at ocean bottom seismometers. Synthetic tests show that the method can resolve thin sublayers within the sediment layer, such as a very thin low shear‐wave velocity layer and sharp seismic velocity jumps. Real data applications show that the sediment models obtained using this approach are in good agreement with those obtained from direct measurements and previous body‐wave analyses. Our proposed method will enable more detailed prediction and correction of the influence of the sediment layer on ocean bottom seismometer records, thereby providing more accurate constraints on deeper Earth structures. Key Points: The seismic properties of the seafloor sediment layer are characterized using ocean bottom seismometer teleseismic body‐wave recordsThe P‐ and S‐wave impedances and travel times and the VP/VS values of each sublayer within the sediment layer are resolvedIn situ seafloor sediment structure constraints will enable accurate correction and prediction of shallow structure effects [ABSTRACT FROM AUTHOR]

This study presents reflectivity images of the northeast (NE) Japan subduction zone continuous across the ocean and land. As nearly half of its forearc region is under the ocean, data from ocean bottom seismometers (OBSs) must be utilized to fully image the region by passive seismic analysis. The use of OBS data has been a challenge due to inherent characters of the ocean bottom observations: high noise level and effects of seafloor sediment. Now, decent imaging is possible in NE Japan overcoming the high level noise due to the accumulated data set of the OBSs. The low‐velocity of seafloor sediment significantly delays and amplifies S waves passing through them, and thus complicates teleseismic waveforms. We identify and correct these effects to produce coherent receiver function images throughout the amphibious subduction zone. Our images provide a potential for discussing new structural features and will help better understanding of the dynamics of the NE Japan subduction zone. Plain Language Summary: In northeast Japan, the Pacific plate is subducting beneath the Japanese arc, and delineating how the actual subduction is taking place at depths has been the target of many seismological studies. This study presents our effort of continuous imaging of subsurface interfaces across the ocean and land employing an amphibious data set. On the land side, there are dense seismic networks that have been used to reveal the seismic interfaces using converted phases of distant earthquakes. On the ocean side, the accumulation of data from temporary ocean bottom seismometers in the last decade and the recently installed ocean bottom cable network has made it possible to use a similar approach as on the land. However, the low‐velocity sediment beneath the seafloor causes systematic slower arrivals and larger amplitudes of the seismic phases of interest that result in highly disturbed images if we analyze them in the same way as the land. Here, we identify and correct the effects of the seafloor sediment to analyze the seismic interfaces across the ocean and land. As a result, we succeed in imaging the interfaces of the subducting oceanic crust throughout the region, as well as previously unidentified features under the ocean that may deserve future investigation. Key Points: The northeast Japan subduction zone is imaged using receiver functions (RFs) of an amphibious data setWe identify significant amplitude and time delay effects on RFs from seafloor sedimentCorrections of the sediment effects significantly improve images across an amphibious region [ABSTRACT FROM AUTHOR]

We performed waveform inversion of the P waveforms recorded by our BroadBand Ocean Bottom Seismometers (BBOBSs) deployed in the Northwestern Pacific. Consequently, the depth profile of the P velocity of the oceanic upper mantle, which has not been well resolved by previous surface wave or receiver function analyses, was revealed. We considered the azimuthal anisotropy in the lithosphere, which significantly improved the variance reduction from 34% to 44%. The resulting P model exhibited higher and lower velocities in the lithosphere and asthenosphere, respectively. The velocity contrast was found to be second/third of that observed in the previous S models; however the obtained model appeared to have some trade‐off with the VS structures in the vicinity of the source. We compared our P model with the previous S model obtained using our BBOBSs and obtained the VP/VS model. The resulting VP/VS model has two notable features. First, the lithosphere is characterized by a rapid increase in the VP/VS values with depth, which implies chemical stratification. Second, the VP/VS values in the vicinity of the lithosphere‐asthenosphere boundary (LAB) are larger than the synthetic values of any major upper mantle mineral predicted by considering the anharmonic effects, which suggests the effects of anelasticity or melt. Key Points: We inverted P waveforms recorded by our BBOBSs to obtain the depth profile of the P velocity beneath the older Pacific plateCombining our P model with a previous S model revealed two notable features in Vp/VS structuresThe obtained Vp/Vs model suggests chemical stratification in the lithosphere and the effects of anelasticity or melt at the LAB [ABSTRACT FROM AUTHOR]

We investigate slab and mantle structure of the NE Japan subduction zone from P wave azimuthal and radial anisotropy using travel time tomography. Trench normal E-W-trending azimuthal anisotropy (AA) and radial anisotropy (RA) with VPV > VPH are found in the mantle wedge, which supports the existence of small-scale convection in the mantle wedge with flow-induced LPO of mantle minerals. In the subducting Pacific slab, trench parallel N-S-trending AA and RA with VPH > VPV are obtained. Considering the effect of dip of the subducting slab on apparent anisotropy, we suggest that both characteristics can be explained by the presence of laminar structure, in addition to AA frozen-in in the subducting plate prior to subduction. [ABSTRACT FROM AUTHOR]

Abstract: We report anomalous core phase PKPbc‐PKPdf differential travel times relative to 1‐D spherically symmetric model with a uniformly anisotropic inner core recorded by stations in Alaska for South Sandwich Islands (SSI) earthquakes. The data sample the inner core for the polar paths, as well as the lowermost mantle beneath Alaska. Our major observations are the following: (1) fractional travel time residuals of PKPbc‐PKPdf increase rapidly within 2 °, (2) a clear shift of the residual pattern could be seen for earthquakes with different locations, and (3) the residuals show systematic lateral variation: at the northern part, no rapid increase of residual can be seen. A structural boundary with a P wave velocity contrast of about 3% at the lowermost mantle beneath East Alaska is invoked to explain the observation, and the required strength of anisotropy in the quasi‐western hemisphere of the inner core might be reduced if those anomalous data are excluded from analysis. [ABSTRACT FROM AUTHOR]

Although plate tectonics started as a theory of the ocean basins nearly 50 years ago, the mechanical details of how it works are still poorly known. Our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere-asthenosphere system (LAS) beneath the ocean. We review the existing observational constraints on the seismic and electrical properties of the LAS, particularly for normal oceanic regions away from mid-oceanic ridges, hot spots, and subduction zones, where plate tectonics is expected to present its simplest form. Whereas a growing volume of seismic data on land has provided remarkable advances in large-scale pictures, seafloor observations have been shedding new light on essential details. By combing through these observational constraints, researchers are unveiling the nature of the enigmatic LAS. Future directions for large-scale seafloor observations are also discussed. [ABSTRACT FROM AUTHOR]

Properties of a new parameter, ηκ, that is recently introduced by Kawakatsu et al. for transverse isotropy are examined. It is illustrated that the parameter nicely characterizes the incidence angle dependence of bodywave phase velocities for vertical transverse isotropy models that share the same P- and S-wave anisotropy. When existing models of upper-mantle radial anisotropy are compared in terms of this new parameter, PREM shows a distinct property. Within the anisotropic layer of PREM (a depth range of 24.4-220 km), ηκ < 1 in the upper half and ηκ > 1 in the lower half. If ηκ > 1, anisotropy cannot be attributed to a layering of homogeneous isotropic layers, and thus requires the presence of intrinsic anisotropy. [ABSTRACT FROM AUTHOR]

We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region. Plain Language Summary: Olivine is the most abundant mineral in the oceanic upper mantle and exhibits preferred crystallographic orientations when it experiences plastic strain‐induced deformation (Nicolas & Christensen, 1987, https://doi.org/10.1029/GD016p0111). Olivine in the oceanic lithosphere is considered to retain the orientations from its formation at the mid‐ocean ridge unless it experiences tectonic perturbations such as reheating by hotspot volcanisms. Seismic anisotropy observations in the oceanic lithosphere therefore provide clues to plate tectonics processes. In this study, we analyze two independent data sets (active‐ and passive‐source data) and reveal the detailed azimuthal dependence of the P velocity traversing the oceanic lithosphere in the northwestern Pacific. We find that the two data sets consistently show a strong and skewed anisotropic signal. We infer that secondary deformation and recrystallization in the Pacific Plate may be responsible for these observations. Key Points: Strong and skewed azimuthal dependence of Pn velocity is robustly observedCompatible direction of the crystallographic axes of olivine is constrainedSecondary deformation and recrystallization in the older Pacific is suggested [ABSTRACT FROM AUTHOR]

Inversion for seismological structures of the oceanic lithosphere–asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties in fitting phases for shorter-period components in the previous global tomography have yielded resultant models that possess poor resolutions above |$\sim \, 50$| km depth. Recent developments of broad-band ocean-bottom seismometer (BBOBS) arrays provide larger amount of seismic data with epicentral distances of <20°. In this study, we develop an appropriate method to fully utilize the information contained in the shorter-period components of BBOBS arrays. We first fit the envelopes without phase information to analyse the shorter-period components (8–60 s) which are generally unavailable in the conventional phase fitting. We then use the resultant model as our initial model for waveform inversion of the longer periods (12.5–200 s) to fit the phase, which allows us to infer a continuous structure model from the crust to the asthenosphere. We demonstrate the validity of this combined envelope-fitting and waveform inversion method by analysing the waveform data from a BBOBS array that was deployed in the Northwestern Pacific and has recorded events in the vicinity of the Japan Trench to obtain the average velocity structure between the event and station arrays. We independently resolve the crustal compressional and shear wave velocities, and thickness by analysing the envelopes, which minimizes biases in the subsequent waveform inversion. We also find that the waveform inversion improves the resolution in the asthenosphere. Our results suggest that further extension of this method should improve our knowledge of the oceanic lithosphere–asthenosphere system. [ABSTRACT FROM AUTHOR]

Abstract: Song and Kawakatsu (2012) have shown that the sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subduction of the oceanic asthenosphere that has strong radial anisotropy. This model not only explains the non-intuitive trench-parallel splitting pattern in most of subduction zones, but also predicts the trench-normal behavior (fast polarization direction sub-parallel to the absolute plate motion of the incoming plate) observed in several shallow subduction zones. The general validity of such a scenario is crucial to fundamental understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern and mass transport near subduction zones. To validate this scenario, we examine SKS splitting patterns observed across the fore-arc in central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views to models of seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important. [Copyright &y& Elsevier]

The article discusses geophysics research by N. Schmerr and colleagues within the issue. The author presents a brief overview of the theory of plate tectonics which posits that horizontal plates below the Pacific Ocean are moving laterally at a rate of approximately 10 centimeters (cm) per year above a weak layer called the asthenosphere. He notes that scientists are puzzled by how the plates, which are as large as 10 thousand kilometers (km), can maintain their rigidity while causing earthquakes at their edges. Topics include the use of high-resolution seismic maps by Schmerr's team to observe the process of plate tectonics, the role of the asthenosphere, which is a viscous layer below the lithosphere, in plate movement, and a diagram illustrating plate tectonics in the ocean.

Abstract: Oceanic plates subducted at trenches penetrate into the deep mantle, and encounter a structural boundary at a depth of 410km where olivine, the dominant element of mantle rocks, transforms into a higher density form wadsleyite. This transformation may be delayed within the coldest core of subducting plates (slabs) due to kinetic effects, and it has been suggested that metastable olivine may persist deeper than 410km. Using high density seismic array data in Japan, we show the direct image of the structure corresponding to this metastable olivine wedge (MOW) beneath southwest Japan. Numerical simulation of a subducting slab, including the kinetic effect of water (H2O) on the olivine–wadsleyite transformation, indicates that the presence of the imaged MOW requires an insignificant amount of water (less than 100wt. ppm) be present in the slab mantle, thus a deep dry cold slab. We infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab, but no significant amount within the slab itself. We also demonstrate that a numerical simulation including the kinetics of 660-km phase transformation can reconcile the observed deep depression of the 660-km discontinuity with a gentle Clapeyron slope. [Copyright &y& Elsevier]

Abstract: The petrological consequences of deep subhorizontal deflection (“stagnation”) of subducting slabs should affect both apparent seismic velocity structures and slab morphology. We construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high-pressure phase transitions in mantle minerals, focusing upon Japan as our study area. We calculate associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics) may contribute to slab deformation. We consider effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, transition triplication due to multiple intersection of geotherms with phase boundaries, and potential persistence of metastable phases due to kinetic hindrance. We also estimate seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by receiver-function analysis. We find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation. Such buoyancy forces vary with the depth at which stagnation occurs, so that slabs may seek an equilibrium slab stagnation depth. Metastable phase bending moment gradients further enhance slab stagnation, but they thermally decay after ∼600–700km of horizontal travel, potentially allowing stagnant slabs to descend into the lower mantle. Stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660-km seismic discontinuity, affecting the seismological visibility of such features. Seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique (travel-time tomography vs. boundary-interaction phases). While seismic tomography appears to yield images of stagnant slabs, discontinuity topography beneath Japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone. However, model slabs which bottom around ∼780–810km and then bend upwards by a few degrees can match both the tomographic and receiver-function images. [Copyright &y& Elsevier]

Abstract: We present evidence of two layers of anisotropy in Earth''s upper mantle beneath the central orogenic belt of the North China Craton from teleseismic shear-wave splitting analysis. A one-layer model is in disagreement with the well-constrained azimuth-dependent splitting parameters for stations in the central orogenic belt; however, two layers of anisotropy improve the fit to the data. At the bottom layer, the fast polarization directions are subparallel to the absolute plate motion, indicating the presence of the current strain within the subcratonic asthenosphere related to the interactions between the surrounding plates. In contrast, at the top layer, the fast polarization directions are consistent with the strikes of the local surface faults, confirming a combined contribution of the past orogenies in the lithosphere. [Copyright &y& Elsevier]

Abstract: We have developed and implemented a new grid-based earthquake analysis system that continuously monitors long-period seismic wavefield of 20–50s recorded by broadband seismometers. The new analysis system automatically and simultaneously determines the origin time, location and seismic moment tensor of seismic events within 3min of their occurrence. This system has been in operation since 2003 at the Earthquake Research Institute (http://wwweic.eri.u-tokyo.ac.jp/GRiD MT/), and the locations and origin times are usually obtained within 3s and 20km away from the earthquake catalog values determined by the Japan Meteorological Agency (JMA). In addition, moment tensor solutions are comparable to the network solutions manually obtained. This new system should enable us to monitor long-period seismic wavefield continuously further to help identifying long-period (or low-frequency) events which are undetectable by the conventional monitoring of short-period seismic wavefields. [Copyright &y& Elsevier]

The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the [ithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in melt-rich mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions. [ABSTRACT FROM AUTHOR]

The vibration of a fluid-filled crack is considered to be one of the most plausible source mechanisms for the long-period events and volcanic tremors occurring around volcanoes. As a tool for the quantitative interpretation of source process of such volcanic seismic signals, we propose a method to numerically simulate the dynamic response of a fluid-filled crack. In this method, we formulate the motions of the fluid inside and the elastic solid outside of the crack, using boundary integrals in the frequency domain and solve the dynamic interactions between the fluid and the elastic solid using the point collocation method. The present method is more efficient compared with the time-domain finite difference method, which has been used in simulations of a fluid-filled crack and enables us to study the dynamics of a fluid-filled crack over a wide range of physical parameters. The method also allows us direct calculation of the attenuation quality factor of the crack resonance, which is an indispensable parameter for estimating the properties of the fluid inside the crack. The method is also designed to be flexible to many applications, which may be encountered in volcano seismology, and thus, extensions of the method to more complicated problems are promising. [ABSTRACT FROM AUTHOR]

We report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern Japan. Our data indicate that water is released from the hydrated oceanic crust at shallow depths (≤100 kilometers) and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle. Our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment. [ABSTRACT FROM AUTHOR]

Discusses the discovery of a discontinuity in the mantle beneath the subduction zones in Tonga and the Japan and Flores seas. Data analysis using of the J-array of Japan; Identification of 920-kilometer discontinuity; Speculation as a global feature; Significance.

Discusses the monitoring of volcano activity. How broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir that continually resonates with periods as long as 15 seconds; How the hydrothermal reservoir appears to help control the surface activity at Aso volcano.

The time reversal operation in seismic source estimation is considered. We show that the time reversal operation, equally the adjoint operation, for seismic source imaging gives an approximate solution to more conventional seismic source inverse problem through the ‘happy approximation’ underlined by Claerbout. Practical applications of such methods in a long-period range to monitor earth's activities in realtime are also discussed. [ABSTRACT FROM AUTHOR]

The hemispherical nature of heterogeneities of the isotropic velocity and attenuation of the inner core has been extensively studied by seismic observations, but consensus on the character of the boundary between two hemispheres has not yet been achieved. To investigate the hemispherical boundary in detail, we analyze core phase data whose ray paths propagate beneath the northern Pacific. We employ a waveform inversion approach based on simulated annealing to measure the differential traveltime and the attenuation parameter of the core phases. Measured differential traveltimes for data from European stations for events in the Fiji-Tonga region show positive anomalies consistent with the eastern hemisphere model, while those from USArray for events in Indonesia show relatively small values. Ray-theory based forward modeling of the differential traveltimes is conducted by varying the shape and the transition width of the boundary. An arc-like shape boundary that connects points (0°N, 159°W) on the equator and (79°N, 110°E) in far north with a transition width of ∼600 km best explains the observed differential traveltimes rather than a sharp boundary. This model also accounts for the observed variation of attenuation parameters. • The nature of hemispherical boundary of the inner core beneath the northern Pacific was investigated. • Core phase data were analyzed via a non-linear waveform inversion method. • Forward modeling of the differential traveltime anomaly was conducted for constraining the boundary. • The arc-like-shaped boundary with ∼600 km-wide velocity transition was observed. [ABSTRACT FROM AUTHOR]

We present a new reference model for the evolution of oceanic lithosphere, which incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling. The new reference model is based solely on thermal conduction, that is, without involving the occurrence of small‐scale convection, and unlike the plate model, it does not contain unphysical boundary conditions. Yet, our model can explain both bathymetry and the heat flow data on the normal seafloor. The success of the new model owes to the use of realistic material properties in conduction modeling as well as the consideration of all of major processes that take place ubiquitously beneath seafloor. The effect of secular cooling on the bathymetry of old seafloor is particularly notable. Whereas secular cooling brings only weak temperature variations with an amplitude of ∼20 K, it can nonetheless affect global bathymetry substantially owing to the deep sensitivity of long‐wavelength topography kernels. We suggest that the well‐known fact that Earth has been cooling, which was not considered in any of previous reference models, may be the key to the long‐standing puzzle of seafloor flattening. The new reference model is expected to be useful to better quantify the impact of the emplacement of hotspot islands and oceanic plateaus, the effect of small‐scale convection, and the regional history of secular cooling in the convecting mantle. Plain Language Summary: Seafloor deepens as it moves away from mid‐ocean ridges, and this subsidence reflects how the suboceanic mantle loses its heat by conduction. When seafloor becomes older than 70 million years old, however, it starts to deviate upward from what is predicted by the simple law of thermal conduction. A common approach to model such a deviation is to adopt the so‐called plate model, which can suppress seafloor subsidence with a constant temperature boundary condition at a shallow depth (120–140 km), although the real Earth does not contain such a boundary. Here we show that, by taking into account all of major processes intrinsic to the suboceanic mantle, from the cold shallow part to the hotter deep part, it is possible to explain the evolution of seafloor topography as well as heat loss, without invoking an unphysical boundary condition. In particular, this study illustrates that the fact that Earth is cooling, which is long known in Earth sciences, can have considerable effects on the large‐scale behavior of ocean basins. Key Points: A new reference model for the evolution of normal oceanic lithosphere is proposedThe model is based solely on thermal conduction and yet free of unphysical boundary conditionsThe model incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling [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]

Investigation of fluid distribution along megathrust faults is an important issue, since the fluid affects frictional properties and thus slip behavior on faults. Scattered teleseismic phases, or receiver functions (RFs), have made significant contributions to understanding the fluid content of subducting plates beneath the onshore regions but have been rarely applied in offshore settings. In this study, we conducted receiver function inversion analysis to investigate detailed seismic properties near the megathrust fault using ocean bottom seismometers deployed off the Kii Peninsula, southwest Japan. RFs were calculated at high frequencies (up to 4 Hz), removing the effect of water reverberations from vertical component records. Our inversion was performed in two steps: first, we modeled sediment layer by a simple stacking method and then solved for deeper structure by a waveform inversion. The results indicate the presence of a thin low-velocity zone (LVZ) of a thickness of 0.2-1.2 km with a S wave velocity of 0.7-2.4 km/s along the plate interface. We interpret this LVZ as thin fluid-rich sediment layer between the overriding and subducting plates that acts as a pathway of fluid migration. [ABSTRACT FROM AUTHOR]

High frequency energy radiation of large earthquakes is a key to evaluating shaking damage and is an important source characteristic for understanding rupture dynamics. We developed a new inversion method, Image Deconvolution Back-Projection (IDBP) to retrieve high frequency energy radiation of seismic sources by linear inversion of observed images from a back-projection approach. The observed back-projection image for multiple sources is considered as a convolution of the image of the true radiated energy and the array response for a point source. The array response that spreads energy both in space and time is evaluated by using data of a smaller reference earthquake that can be assumed to be a point source. The synthetic test of the method shows that the spatial and temporal resolution of the source is much better than that for the conventional back-projection method. We applied this new method to the 2001 M w 7.8 Kunlun earthquake using data recorded by Hi-net in Japan. The new method resolves a sharp image of the high frequency energy radiation with a significant portion of supershear rupture. [ABSTRACT FROM AUTHOR]

Abstract: The Indochina block is located at the junction of four major plates. It has a close relationship with the uplift of Tibet plateau and the sea-floor spreading of the South China Sea in the geological evolution histories. We deployed a temporary broadband seismograph array in Vietnam from February 2000 to October 2005, and measured shear-wave splitting of core-refracted phases to investigate the upper mantle anisotropy beneath the Indochina block and adjacent regions. The observed delay times of up to 1.5s are large, indicating a mantle source for the anisotropy. Two distinct regimes of seismic anisotropy are revealed. The fast polarization directions in the northern Indochina block with a main trend of N100°E are almost consistent with the direction of both the surface displacement and the absolute plate motion. These observations suggest the anisotropy is possibly generated by the present-day asthenospheric flow caused by the India–Eurasia collision. The fast polarization directions in the southern Indochina block and the Hainan island are nearly N60°E. The anisotropy in this area may reflect a combination of the India–Eurasia collision and the past orogenies in the lithosphere, or an abrupt variation in the asthenospheric flow. [Copyright &y& Elsevier]

Abstract: At Aso volcano, Kyushu, Japan, several different types of volcanic tremor have been observed for many years. One of them is the continuous tremor, the ground vibration which has dominant frequency between 3 and 10 Hz and has approximately constant amplitudes without any clear beginning and ending. We observed the continuous tremor at Aso using short period seismometer arrays for 3 days in 1999. We locate the source of the continuous tremor by seismic array data processing. We use the semblance coefficients in order to estimate the arrival azimuth and apparent slowness by grid search. The epicenters of the continuous tremor are located around the currently active crater, and the source depths are likely to be shallower than 600 m. We find that the estimated epicenter clearly migrates synchronized with the change in the tremor amplitude. The migration often occurs periodically with a period of about 80 s, but aperiodic occurrence of the migration is also often seen. In both cases, the epicenter is located southeastward (northwestward) when the amplitude is larger (smaller). We propose that there are two or more independent tremor sources with fixed locations, and that their amplitudes modulate either aperiodically or periodically with periods nearly 80 s. The tremor signals from those sources are mixed at the arrays, and the estimated epicenter parameters vary according to which signal dominates the seismograms. The simplest model is that there are two point sources, one at west and the other at south of the crater (we call the two sources as “NW source” and “SE source”, respectively), and the amplitude of the SE source changes with time. Consequently, the SE source dominates the seismogram when the observed amplitude is larger, whereas the NW source dominates when the amplitude is smaller. We generate synthetic seismograms, and apply the location technique to them to verify the validity of the two point source model and to search the locations of two sources which can explain the observed synchronization between the amplitude and the apparent epicenter location. We find that the distance between the two sources needs to be more than 400 m to agree with the observation. We also analyze the seismic array data observed in 2001, and infer that the NW source in 1999 may be identical to the tremor source of the 2001 data. [Copyright &y& Elsevier]

Examines the broad-band recordings of deep events for the presence of signals from the lower-mantle discontinuities. Use of the novel technique; Important of the discontinuities for geodynamics, geochemistry and mineral physics; Details on the seismograms findings.

Strong long‐period seismic signals at periods around 25 and 18 s appear in the ambient noise cross‐correlation functions recorded by an array of ocean bottom seismometers (OBSs) in the western‐central Pacific. The signal amplitude varies from time to time, and the apparent travel times of the signals are typically smaller than those expected for the Rayleigh waves propagating along the great circle connecting the station pairs. From the cross‐correlation functions, the signal sources are located in the Vanuatu Arc. Local data analysis suggests the signals originate from two different sources possibly located at depths of ~0–1 km below the sea level beneath the active cones of Ambrym volcano. Plain Language Summary: Spatially localized sources that persistently generate long‐period seismic waves at periods longer than 10 s are reported worldwide. For some sources, the excitation mechanisms have been well‐investigated. A volcano in Japan generates such waves that travel more than thousands of kilometers. For others, however, the excitation origins are still poorly understood. These include the one observed in the Vanuatu Arc. A temporal observation network of seismometers installed on the seafloor in the western‐central Pacific observed persistent waves peaking at around two different periods. From the data analysis, we find the signals originate from an active volcano, Ambrym volcano, in the Vanuatu Arc. Besides, we find that those seismic waves are most likely originated from two different sources possibly located at shallow depths beneath the volcano. Key Points: Persistent long‐period (25 and 18 s) seismic signals appear in ambient noise cross‐correlations of OBS data in the western‐central PacificThe signals are originated from Ambrym volcano in the Vanuatu ArcSource locations of the two signals may be different, but both are possibly located at shallow depths right beneath the active cones [ABSTRACT FROM AUTHOR]

We found that SH diff phases generated by earthquakes in the Fiji–Tonga, recorded in India, are accompanied by secondary pulses. We interpreted them as a consequence of multipathing of S waves caused by the Pacific large low-shear-velocity province (LLSVP). We analysed the differential traveltimes between SH diff and the secondary pulse, together with the absolute SH diff arrival times, to constrain the thickness and velocity perturbations in the western end of the Pacific LLSVP. Our preferred model shows a lateral variation in the thickness of the LLSVP; the southern part reveals a thicker (300 km) low-velocity region compared to the northern part (200 km). However, the velocity perturbations of the LLSVP appear to be comparable (⁠|$-1.5\hbox{ per cent}$|⁠). The results are consistent with a scenario that the LLSVP is a chemically distinct pile with significant surface topography. [ABSTRACT FROM AUTHOR]

We measured shear wave splitting from SKS data recorded by the transcontinental NECESSArray in NE China to constrain lithosphere deformation and sublithospheric flows beneath the area. We selected several hundreds of high quality SKS/SKKS waveforms from 32 teleseismic earthquakes occurring between 09/01/2009 and 08/31/2011 recorded by 125 broadband stations. These stations cover a variety of tectonic terranes, including the Songliao basin, the Changbaishan mountain range and Zhangguancai range in the east, the Great Xing'an range in the west and the Yanshan orogenic belt in the southwest. We assumed each station is underlaid by a single anisotropic layer and employed a signal-to-noise ratio (SNR) weighted multi-event stacking method to estimate the two splitting parameters (the fast polarization direction φ , and delay time, δt ) that gives the best fit to all the SKS/SKKS waveforms recorded at each station. Overall, the measured fast polarization direction lies more or less along the NW–SE direction, which significantly differs from the absolute plate motion direction, but is roughly consistent with the regional extension direction. This suggests that lithosphere deformation is likely the general cause of the observed seismic anisotropy. The most complicated anisotropic structure is observed beneath the southern Great Xing'an range and southwest Songliao basin. The observed large variations in splitting parameters and the seismic tomographic images of the area are consistent with ongoing lithospheric delamination beneath this region. [ABSTRACT FROM AUTHOR]

We used ambient noise data recorded by the NECESSArray, a portable seismic array deployed in Northeast China, to construct Rayleigh-wave phase velocity dispersion curves and obtain phase velocity maps for periods from 6 to 40 s. S-wave velocity structure beneath NE China is obtained by joint inversion of the phase velocities and receiver functions. Our results show good correlation between crustal velocity structure and geological features. At shallow depths the Songliao Basin is separated into two small sub-basins by a fast velocity anomaly corresponding to a southwest uplifted horst and the southern sub-basin is associated with the Kailu depression. A low-velocity anomaly extending from the lower-crust to uppermost mantle beneath the Changbaishan mountain range is consistent with P- and S-wave tomography models. The mantle low velocity has been interpreted as a local mantle upwelling attributed to the subducting Pacific slab induced return flow. Finally, the S-wave velocity in the mid-and-lower crust is lower beneath the Xing'an–Mongolia Orogenic Belt (XMOB) than beneath the Changbaishan mountain range. This was not expected since the Changbaishan mountain range has experienced more extensive volcanic activity in the Cenozoic, including the currently active Changbaishan and Jingpohu volcanoes, than the XMOB. We infer that the mid-and-lower crust of the XMOB is mainly felsic in composition and propose that the low shear-wave velocity of the mid-and-lower crust beneath the XMOB could be the consequence of removal of deep mafic crust through a continental accretion and subsequent delamination event in the Paleozoic, or via convection removal induced by the rollback of the flat subducted Paleo-Pacific slab during the middle Mesozoic. [ABSTRACT FROM AUTHOR]

In order to better understand the deep subduction geometry of the Pacific plate and genesis of intraplate volcanism in northeast China (NE China), we computed a total of 45,505 receiver functions from 788 teleseismic events recorded by 255 stations (NECESSArray temporal and permanent stations) in NE China. We used a common-conversion-point stacking (CCP) method to generate a 3D reflectivity volume beneath the study area. To position the P-to-S conversions to the correct depths, we employed 3D crustal and mantle models as references to make time to depth conversion. The 3D reflectivity volume was generated in an area between 115°–135°E and 40°–49°N, in the depth range of 300 to 800 km. We found significant topographic relief on the 660-km discontinuity across the study area. In particular, in a westward Pacific plate subduction section between 40°N and ∼45.5°N, the 660-km discontinuity is depressed by as much as ∼30–40 km along the western extension of the deep seismicity. The depression is elongated along the strike of the deep seismicity and is confined to a 200–300 km region in the E–W direction of subduction. To the west of this depression the 660-km discontinuity is uplifted by 5–10 km in a rectangular area of ∼100 km by 200 km centered at about 125°E and 43°N. In the north, the 660-km discontinuity is moderately depressed (∼20 km) in a broad area that extends further west. The high and low regions in the 660-km topographic map correlate, respectively, with low- and high-velocity anomalies in the P- and S-wave tomographic velocity images at the same depth. Our results suggest that slab stagnation might not be occurring in the southern part of the NE China, where the Changbaishan volcanic complex is located, thus the magmatism is unlikely caused by dehydration of the flat-lying Pacific slab in the transition zone. The low velocity mantle upwelling arising from a gap of stagnant slabs is a likely source that feeds the volcanic complex in NE China. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed tens of thousands of receiver-function data recorded by 185 broadband seismic stations to study the crustal structure beneath the northeast China. Moho depth and average crustal ratio were measured at each station using the H–κ grid searching technique. For stations located above unconsolidated sediments, we applied the H–β method to remove strong shallow reverberations and generate subsurface receiver functions that allow for effective H–κ analysis. The measurements show that the Songliao basin has a relatively thin crust (∼31 km), and the Moho depth increases significantly from southeast (∼27 km) to northwest (∼35 km). The northwestward tilting of the Songliao basin may suggest that it was initiated by lithospheric flexure due to the load of the Great Xing'an range in the Jurassic before the large-scale extension in the Cretaceous. Moho depth varies from 26.7 km to 42.3 km across the study area, with the shallowest and deepest Moho being located at the eastern flank of the Songliao basin and the Great Xing'an range, respectively. The Moho depth correlates well with the surface topography in the western part, but not the central and eastern parts of the study area. The residual topography computed based on the Airy's isostasy model indicates that the high topography at the eastern flank of the Songliao basin, the Changbaishan volcanic center, and the southern end of the Great Xing'an range is likely dynamically supported by the upper mantle. [Copyright &y& Elsevier]

Volcanism that occurs far from plate margins is difficult to explain with the current paradigm of plate tectonics. The Changbaishan volcanic complex, located on the border between China and North Korea, lies approximately 1,300 km away from the Japan Trench subduction zone and is unlikely to result from a mantle plume rising from a thermal boundary layer at the base of the mantle. Here we use seismic images and three-dimensional waveform modelling results obtained from the NECESSArray experiment to identify a slow, continuous seismic anomaly in the mantle beneath Changbaishan. The anomaly extends from just below 660 km depth to the surface beneath Changbaishan and occurs within a gap in the stagnant subducted Pacific Plate. We propose that the anomaly represents hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific Plate and is now escaping through a gap in the subducting slab. We suggest that this subduction-induced upwelling process produces decompression melting that feeds the Changbaishan volcanoes. Subduction-induced upwelling may also explain back-arc volcanism observed at other subduction zones. [ABSTRACT FROM AUTHOR]

We have developed a real-time moment tensor monitoring system (RMT) which takes advantage of a grid-based moment tensor inversion technique and real-time broad-band seismic recordings to automatically monitor earthquake activities in the vicinity of Taiwan. The centroid moment tensor (CMT) inversion technique and a grid search scheme are applied to obtain the information of earthquake source parameters, including the event origin time, hypocentral location, moment magnitude and focal mechanism. All of these source parameters can be determined simultaneously within 117 s after the occurrence of an earthquake. The monitoring area involves the entire Taiwan Island and the offshore region, which covers the area of 119.3°E to 123.0°E and 21.0°N to 26.0°N, with a depth from 6 to 136 km. A 3-D grid system is implemented in the monitoring area with a uniform horizontal interval of 0.1° and a vertical interval of 10 km. The inversion procedure is based on a 1-D Green's function database calculated by the frequency–wavenumber (fk) method. We compare our results with the Central Weather Bureau (CWB) catalogue data for earthquakes occurred between 2010 and 2012. The average differences between event origin time and hypocentral location are less than 2 s and 10 km, respectively. The focal mechanisms determined by RMT are also comparable with the Broadband Array in Taiwan for Seismology (BATS) CMT solutions. These results indicate that the RMT system is realizable and efficient to monitor local seismic activities. In addition, the time needed to obtain all the point source parameters is reduced substantially compared to routine earthquake reports. By connecting RMT with a real-time online earthquake simulation (ROS) system, all the source parameters will be forwarded to the ROS to make the real-time earthquake simulation feasible. The RMT has operated offline (2010–2011) and online (since January 2012 to present) at the Institute of Earth Sciences (IES), Academia Sinica (http://rmt.earth.sinica.edu.tw). The long-term goal of this system is to provide real-time source information for rapid seismic hazard assessment during large earthquakes. [ABSTRACT FROM AUTHOR]

The rapid detection and characterization of megathrust earthquakes is a difficult task given their large rupture zone and duration. These events produce very strong ground vibrations in the near field that can cause weak motion instruments to clip, and they are also capable of generating large-scale tsunamis. The 2011 M 9 Tohoku-oki earthquake that occurred offshore Japan is one member of a series of great earthquakes for which extended geophysical observations are available. Here, we test an automated scanning algorithm for great earthquakes using continuous very long-period (100–200 s) seismic records from K-NET strong-motion seismograms of the earthquake. By continuously performing the cross-correlation of data and Green's functions (GFs) in a moment tensor analysis, we show that the algorithm automatically detects, locates and determines source parameters including the moment magnitude and mechanism of the great Tohoku-oki earthquake within 8 min of its origin time. The method does not saturate. We also show that quasi-finite-source GFs, which take into account the effects of a finite-source, in a single-point source moment tensor algorithm better fit the data, especially in the near-field. We show that this technique allows the correct characterization of the earthquake using a limited number of stations. This can yield information usable for tsunami early warning. [ABSTRACT FROM AUTHOR]

The article is a report on global surface wave telegraphy using seismic hum. Earth's seismic hum signals, which are background free oscillations, are continuously excited by the ocean and atmosphere. The authors used cross-correlation (CC) analysis of hum signals that showed background Rayleigh waves which were propagated globally. Topics include how the waves open the possibility for global surface wave tomography without references to earthquakes.

Abstract: We determined the depths of the 410-km and 660-km seismic discontinuities (called “410” and “660,” respectively, hereafter) beneath the Philippine Sea and the northwestern Pacific by the receiver function method; we used the broadband data obtained from broadband ocean-bottom seismographs. We found a very deep “660” at 691km in the stagnant slab beneath the Philippine Sea. In the surrounding Philippine Sea regions and northwestern Pacific, the “660” appears at depths of 659–674km. Comparison of the “660” depth and P-velocity anomalies indicated that temperatures in the stagnant slab are lower than that in the mantle transition zone (MTZ) beneath the northwestern Pacific by about 500K. The temperature in the MTZ beneath the Parece Vela Basin and West Philippine Sea Basin, which surrounds the stagnant slab, is lower than that in the MTZ beneath the Pacific by about 200K. The water content in the stagnant slab (∼0.2wt% H2O) is higher than that in the other regions; however, the large uncertainty in the estimated water content (±0.2–0.3wt% H2O) prevents us from arriving at a definitive conclusion regarding the presence of water in the stagnant slab. Therefore, it is desirable to use other geophysical parameters (e.g., electrical conductivity) along with the seismic parameters in order to obtain an accurate estimate of the water content in the MTZ. [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate the continuous volcanic tremor of Aso Volcano in Japan, we performed a series of temporary short-period seismic array observations near the Nakadake first crater (the active crater) during five years from 1999 to 2003. We deployed in all of the temporary observations a seismic array at the same location about 700 m west of the active crater, in order to investigate long-term changes in the tremor activity. In 1999 and 2003, another array was simultaneously deployed at a different location 700 m north of the crater to help locate the tremor sources. We developed a frequency domain semblance method and applied it to the waveform data of the frequency range where the continuous tremor is dominant (3–6 Hz). We measured arrival azimuths and slownesses of the continuous tremor signals as functions of frequency, which are then used to locate the epicenters of the tremor signals corresponding to the principal peaks of the power spectra. For the observations in 1999 and 2002, the continuous tremor amplitudes are relatively small, and the slowness of the tremor signal observed at the west array takes a local minimum (0.5 to 0.6 s/km) near the frequency (∼4.7 Hz for 1999 and ∼4.8 Hz for 2002) which corresponds to the highest spectral peak. This implies that body waves dominate the tremor signals at the west array around the frequency. The tremor epicenters corresponding to 4.7 Hz for the observation in 1999 are located at the west rim of the currently active crater. While the surface crater activity of Aso remains low and the tremor activity is not clearly linked with the surface activity until early 2003, a close link between the tremor and crater activity appears in the middle of 2003, when a small phreatic eruption occurred a month before the array observation (July 10, 2003). Tremor signals of the observation in 2003 show a large spectral peak (4.2 Hz) where the slowness measured for the west array is very large (1.1 s/km), clearly suggesting that surface waves are dominant. The epicenter is again located at the western rim of the active crater. We interpret these observations as follows: in 1999 and 2002 when the surface activities of Aso were low, the continuous tremor excitation was deep and inactive. In the middle of 2003 when Aso Volcano became active with a series of phreatic eruptions a shallower tremor source was activated, possibly masking the deeper sources. This shallowing of the dominant tremor source could be due to the increase in the volcanic gas flow rate triggered by the phreatic eruptions. [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]

We installed five broadband seismometers around Mt. Usu, Hokkaido, Japan, just before the first surface eruption on March 31, 2000. By using these broadband data with short-period seismograms recorded by Japan Meteorological Agency and Institute of Seismology and Volcanology of Hokkaido University, we located 590 earthquakes associated with the 2000 eruption of Mt. Usu from March 31, 12:58 to April 5, 2000, including 41 low-frequency earthquakes. Low-frequency earthquakes have clear predominant frequencies of about 1.0–1.5 Hz. While the seismicity of tectonic earthquakes was still active on April 5, there were almost no low-frequency events after April 2. All the low-frequency earthquakes occurred within a vertical and narrow zone near the craters of eruption, and they are shallower than 4 km. In contrast, the active area of tectonic earthquakes spreads out in a large area, particularly deep in the south of the craters. From these temporal and spatial patterns, together with GPS and gravity measurements that support the termination of magma activities before April 2, we suggest that low-frequency earthquakes were caused by magma or hydrothermal activities beneath the volcano while tectonic ones were caused by the regional tectonic stress affected by the intrusion of magma body near the craters. Related to long-term magma activities of this eruption, four deep crustal low-frequency events were identified on October 17, 1998 just in the south of Mt. Usu in the depth range of 20–30 km. As proposed for other active volcanoes, these deep crustal events may represent a part of deep magma activities beneath Mt. Usu. [Copyright &y& Elsevier]