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We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth. Plain Language Summary: The deployment of campaigns and permanent ocean bottom seismometers (OBSs) has enabled us to investigate the activity and physical properties of offshore seismic phenomena. Our knowledge of offshore subsurface structures is still limited; consequently, many studies have used conventional analysis methods with the simplest assumptions. Using observed and synthetic seismograms near the Nankai Trough, we found a limitation in the conventional analysis method applied to OBS data. Thick sedimentary layers, which have been confirmed by seismological studies along the Nankai Trough just below the OBSs, cause an approximately 1‐order overestimation of source parameters for seismic phenomena occurring around the shallow plate boundary. This overestimation may have occurred during the seismic energy estimation of shallow slow earthquakes in Hikurangi, Costa Rica, and Mexico. After correcting for the effects of thick sedimentary layers, we found that the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. This suggests that the physical mechanisms governing seismic slow earthquakes can be the same, regardless of region and source depth. Key Points: Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigatedThe assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremorsScaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10 to 10−9 [ABSTRACT FROM AUTHOR]

For the study of the random velocity fluctuation of the solid Earth medium, it is useful to measure the collapse of a seismic wavelet with increasing travel distance and the excitation of coda waves. Radiative transfer theory (RTT) is a powerful tool for synthesizing the propagation of a seismic wavelet in random media statistically characterized by the power spectral density function (PSDF) of the fractional velocity fluctuation. The Born scattering coefficient is a key building block of RTT. As the centre wavenumber of a wavelet increases, the phase shift across the correlation length increases and the Born approximation leads to an extremely large forward scattering exceeding the applicable range of the perturbation method. In such a case, the Eikonal approximation is able to explain the envelope broadening with increasing travel distance; however, it can not explain the coda excitation. To overcome the difficulty, we have proposed a hybrid Monte Carlo (MC) simulation for scalar waves. In the case of von Kármán-type random media, when the centre wavenumber is higher than the corner wavenumber, taking the centre wavenumber as a reference, we divide the PSDF into two spectral components. Applying the Born and Eikonal approximations to the high- and low-wavenumber spectral components, we statistically evaluate the wide-angle scattering and the narrow-angle ray bending, respectively. The proposed MC simulation serially using two kinds of scattering processes successfully synthesizes the time trace of the wave energy density from the onset to the late coda. The travel-distance fluctuation derived from the one-way propagation of the Eikonal approximation is also important. This paper extends this method for the propagation of a vector wavelet in random elastic media. We suppose that random fractional fluctuations of the P - and S -wave velocities and the mass density are linearly proportional to each other, which maintains the linear polarization of an S wave throughout the scattering process. Using the hybrid MC simulation with the spectrum division, we synthesize three-component energy density time traces for the anisotropic radiation from a moment tensor source, from which we derive three-component root mean square (RMS) velocity amplitude time traces at different azimuths. In parallel, we synthesize the propagation of a vector wavelet in many realized random elastic media by the finite-difference simulation, then we calculate three-component RMS velocity amplitude time traces. Using them as a benchmark, we confirm the validity of the proposed MC simulation for specific cases. [ABSTRACT FROM AUTHOR]

To clarify the characteristics of high-frequency (> 1 Hz) S and S-coda waves at ocean-bottom seismometers (OBSs), we analyzed seismograms observed at permanent OBSs and inland broadband seismometers around the Kii Peninsula in southwest Japan along the Nankai Trough. The coda amplitudes (both horizontal and vertical) at the OBSs were much larger than those at the inland rock-site stations. Because coda amplitudes relative to those at inland rock-site stations have been used as site-amplification factors, large site amplifications for both components can be expected due to the presence of thick oceanic sediments just below the OBSs; however, the observed maximum S-wave amplitudes in the vertical component exhibited similar attenuation trends against epicentral distances at both OBS and inland stations. To clarify the causes of this discrepancy, we conducted numerical simulations of seismic wave propagation using various three-dimensional seismic velocity structure models. The results demonstrated that coda waves at OBSs mostly comprise multiple scattered waves within a thick (> 2 km) sedimentary layer; consequently, coda amplitudes at OBSs become much larger than those at inland rock-site stations. Our numerical simulations also confirmed the generation of large coda amplitudes at regions with seawater depths ≥ 4 km, where no OBS was deployed. However, the thick sedimentary layer and seawater have limited effects on maximum S-wave amplitudes at the OBSs. Given that the effects of a thick sedimentary layer and seawater on S and S-coda waves differ, we concluded that the coda-normalization technique for site-amplification correction against a rock-site station could not be applied if stations are located within regions above the thick sedimentary layer or deeper sea depths. The site amplifications at the OBSs were corrected according to the horizontal-to-vertical ratios at each OBS; we adjusted the simulated horizontal envelopes at the OBSs using these ratios of the observed S-coda waves. As well as inland seismometers, the site-corrected simulation results practically reproduced the observed high-frequency envelopes at OBSs. [ABSTRACT FROM AUTHOR]

The spatial autocorrelation (SPAC) method has been applied to ambient seismic noise measured by arrays of translational seismometers for inverting phase-velocity dispersion curves of Rayleigh or Love waves for shallow S -wave velocity structure. Recently, it is becoming possible to observe wave spatial gradients such as strain, rotation and tilt owing to the development of dense seismic networks and improving measurement technologies. Therefore, it is desirable to extend the formulation of the SPAC method to strain, rotation and tilt. This study presents analytical expressions of cross-spectra and coherence of the strain, rotation and tilt components that are measured on the free surface. According to the results, both Rayleigh and Love waves contribute to most components of strains. The exceptions are the areal strain and the vertical axial strain (e zz ) on the free surface that are affected by only Rayleigh waves. Only Rayleigh waves contribute to the tilts and rotations around the horizontal axes on the free surface, too. On the other hand, only Love waves contribute to the rotation around the vertical axis. Therefore, different kinds of wave spatial gradients are helpful to separate Rayleigh and Love waves correctly. For practical applications, the analytical expression for the radial axial strain (e rr ) component will be applied directly to distributed acoustic sensing data measured with straight sections of a fibre-optic cable. On the other hand, dense observations of rotation and tilt may still be difficult to carry out at present. However, an application of analytical formulations in this study to arrays of at least several three-component rotational seismometers is attractive because it enables us to separately estimate the phase velocities of Rayleigh and Love waves. [ABSTRACT FROM AUTHOR]

We present one of the first studies on source location determination for volcanic earthquakes and characterization of volcanic subsurfaces using data from a distributed acoustic sensing (DAS) system. Using the arrival time difference estimated from well-correlated waveforms and a dense spatial distribution of seismic amplitudes recorded along the fiber-optic cable, we determine the hypocenters of volcanic earthquakes recorded at Azuma volcano, Japan. The sources are located at a shallow depth beneath active volcanic areas with a range of approximately 1 km. Spatial distribution of the site amplification factors determined from coda waves of regional tectonic earthquakes are well correlated with old lava flow distributions and volcano topography. Since DAS observation can be performed remotely and buried fiber-optic cables are not damaged by volcanic ash or bombs during eruptions, this new observation system is suitable for monitoring of volcanoes without risk of system damage and for evaluating volcanic structures. [ABSTRACT FROM AUTHOR]

In high-frequency seismograms of a small earthquake, coda waves continue for a long time and the S -wavelet is broadened as travel distance increases. Those phenomena can be interpreted as results of scattering by random inhomogeneities distributed in the solid earth. Recent measurements show that the power spectrum of the random medium heterogeneities decreases according to some power of wavenumber. As a mathematical model, we study the propagation of a scalar wavelet through von Kármán-type random media. Our objective is to propose a stochastic method to synthesize intensity time traces, the mean square amplitude traces, by using a few number of parameters statistically characterizing the random media. The most conventional method is known to use the Born approximation in the radiative transfer equation. However, as the centre wavenumber of a wavelet increases higher than the corner wavenumber, the Born approximation becomes inapplicable because the phase shift increases. We proposed a spectrum division method to avoid the difficulty in our previous papers. Taking the centre wavenumber as a reference, we divide the power spectrum into two components. The short-scale component is chosen to satisfy the applicable condition of the Born approximation, which leads to wide-angle scattering per volume. Here, we newly propose to use the phase screen approximation for the long-scale component, which leads to narrow-angle ray bending per length. We simultaneously introduce both the wide-angle scattering due to the short-scale component and the narrow-angle ray bending due to the long-scale component into the Monte Carlo simulation code in the framework of the radiative transfer theory. Synthesized wavelet intensity time traces show envelope broadening and peak delay with increasing travel distance and long lasting coda waves at a short distance. If the power spectrum is properly divided, synthesized intensity time traces well explain the averaged intensity time traces calculated by the finite difference simulation of wavelets in random media from the onset through the peak to coda for a wide dynamic range. The proposed method has a potential to adopt easily the spatial variation of randomness and intrinsic absorption. [ABSTRACT FROM AUTHOR]

Random velocity fluctuations distributed in the solid Earth function as sources of seismic wave scattering. Scattering effects are often observed in high-frequency seismograms of earthquakes as the broadening of the apparent duration of an S -wavelet and the emergence of coda waves. We conduct large-scale 3-D finite difference (FD) simulations of the scalar wave equation to analyse the intensities of scattered waves propagating through random small-scale heterogeneous media. First, we compare ensemble averaged intensities (mean square amplitudes) derived by the FD simulation with those synthesized based on statistical methods such as the radiative transfer equation with the Born approximation and the newly developed spectrum division methods. We consider several types of random media characterized by von Kármán type autocorrelation functions with different characteristic distances and mean square fractional fluctuations. In the case of a large characteristic distance, the forward scattering is dominant and the fluctuation of the traveltime is large. Even in that case, the newly developed spectrum division method can reproduce the average intensity derived by FD simulations in the entire lapse time range. We further investigate the characteristics of scattered waves. To know the property of the fluctuation of intensities due to the small-scale heterogeneity is important for the ground motion prediction. We reveal the gradual shift of intensity fluctuations from the log-normal distribution to the exponential one with the increase of lapse time. The timing of the shift varies depending on the random medium parameters. This can be explained as the shift from the multiple forward scattering regime to the incoherent wide-angle scattering one. The decay rate of the intensity changes from r −2 to r −4 with the increase of the propagation distance r, which corresponds to the empirical relationship of the observed intensity. This timing of the change also depends on the random medium parameters. [ABSTRACT FROM AUTHOR]

Recently, coda-wave interferometry has been used to monitor temporal changes in subsurface structures. Seismic velocity changes have been detected by coda-wave interferometry in association with large earthquakes and volcanic eruptions. To constrain the spatial extent of the velocity changes, spatial homogeneity is often assumed. However, it is important to locate the region of the velocity changes correctly to understand physical mechanisms causing them. In this paper, we are concerned with the sensitivity kernels relating travel times of coda waves to velocity changes. In previous studies, sensitivity kernels have been formulated for two-dimensional single scattering and multiple scattering, three-dimensional multiple scattering, and diffusion. In this paper, we formulate and derive analytical expressions of the sensitivity kernels for three-dimensional single-scattering case. These sensitivity kernels show two peaks at both source and receiver locations, which is similar to the previous studies using different scattering models. The two peaks are more pronounced for later lapse time. We validate our formulation by comparing it with finite-difference simulations of acoustic wave propagation. Our formulation enables us to evaluate the sensitivity kernels analytically, which is particularly useful for the analysis of body waves from deeper earthquakes. [ABSTRACT FROM AUTHOR]

In the heterogeneous earth medium, short period seismograms of an earthquake are well characterized by their smooth envelopes with random phases. The Markov approximation has often been used for the practical synthesis of their envelopes for a given frequency band. It is a stochastic extension of the phase screen method to synthesize wave envelopes in media with random fluctuations under the condition that the wavelength is shorter than the correlation distance of the fluctuation. We propose an extension of the Markov approximation for the envelope synthesis to the case that an isotropically outgoing wavelet is radiated from a point source in horizontal layered random elastic media, where different layers have different randomness and different background velocities. In each layer, we solve the master equation for the two frequency mutual coherence function (TFMCF) which contains the information of the intensity in the frequency domain. Just below each layer boundary, we calculate the angular spectrum which is the expression of the TFMCF in the transverse wavenumber domain for up-going wavelets. The angular spectrum shows the ray angle distribution of intensities of scattered waves. Multiplying it by the square of transmission coefficients calculated from the background velocity contrast at the boundary, we evaluate the angular spectrum just above it. We neglect P to S (S to P) conversion scattering inside of each layer; however, we take into account the mode conversion at the layer boundary. Different from the vertical incidence of a plane wavelet, the wavefront expands with time and its curvature is modified at the layer boundary due to the Snell's law. Approximating the wavefront in the second layer by a circle for a small incidence angle, we may shift the real origin to the pseudo-origin of the wavefront circle, which leads to the change in geometrical spreading factor. Finally, we calculate the mean square envelope from the TFMCF by using an FFT. By multiplying the angular spectrum by conversion or reflection coefficients and calculate the TFMCF for the converted or reflected wavelets at layer boundaries, we can calculate any phase generated due to velocity discontinuities. For the reflected wavelets, we solve the master equation of the TFMCF downward. To confirm the validity of the method, we directly synthesize mean square envelopes in 2-D two-layered random elastic media and compare them with the averaged envelopes calculated by finite difference (FD) simulations of the wave propagation in random elastic media. We find that the Markov envelopes well agree with the FD envelopes not only for a transmitted wavelet but also for a P to S converted wavelet and a reflected wavelet at a layer boundary. [ABSTRACT FROM AUTHOR]

SUMMARY The Markov approximation is a powerful stochastic method for the direct synthesis of wave envelopes in random velocity fluctuated media when the wavelength is shorter than their correlation distance. To apply the Markov approximation to realistic cases, we consider horizontal layered random media characterized by an anisotropic autocorrelation function (ACF), where different layers have different randomnesses and different background velocities having step-like changes. Solving the parabolic master equation for the two frequency mutual coherence function (TFMCF) in random elastic media for the vertical incidence of a plane wavelet from the bottom, we calculate the angular spectrum just before the first velocity boundary. Multiplying transmission or PS conversion coefficients of the boundary by the angular spectrum, we calculate the angular spectrum and TFMCF on the other side of the boundary. Then we solve the master equation for the forward propagating wavelet in the second layer. Taking the same procedure for each layer boundary, we finally obtain the mean square (MS) vector envelopes on the free surface on the top layer. For the practical simulation, we use 2-D random media characterized by a Gaussian ACF. We numerically confirm the validity of the envelope synthesis for a specific case of layered random media with anisotropic ACF by comparing with finite difference (FD) simulations of elastic waves. Considering the Earth structure, the horizontal correlation distance is larger than the vertical one and the velocity fractional fluctuation becomes weak as depth increases, the Markov approximation is good for modelling the primary wavelet and also the converted wavelet for the vertical incident wavelet. We derive an applicable range of the Markov approximation for random media with anisotropic ACF by comparing with FD simulations. The results show that the Markov approximation is accurate when the wavelength is comparable or shorter than the both of vertical and horizontal correlation distances and the MS fractional fluctuation is much smaller than the ratio of squared horizontal correlation distance to the product of the vertical correlation distance and propagation distance. [ABSTRACT FROM AUTHOR]

The purpose of this study was to ascertain the usefulness of preoperative evaluations of donors by computed tomography (CT) volumetry and CT cholangiography for prevention of unexpected liver failure and biliary complications after donor right hepatectomy for adult-to-adult living donor liver transplantation. Fifty-two donors who underwent right hepatectomy without the middle hepatic vein were enrolled in this study. The values of graft weight (GW) were significantly correlated with those of estimated graft volume (GV; P < 0.0001). GW was predicted by the following formula: GW = 155.25 + 0.658 × GV; r2 = 0.489. CT cholangiography revealed anatomical variants of biliary structure in one-third of the donors and also clearly showed one or two small biliary branches from the caudate lobe to the right hepatic ducts or the confluence in 58% of the donors. Biliary leakage, which was treated by conservative therapy, occurred in only one donor (1.9%). No donors received homologous blood transfusion. Hyperbilirubinemia (serum total bilirubin >5 mg/dl) occurred in 5.8% of the donors during their early postoperative periods. Precise evaluations of liver remnant volume by CT volumetry and biliary variation by CT cholangiography are essential for performing safe donor hepatectomy, preventing hepatic insufficiency and minimizing the risk of biliary tract complications. [ABSTRACT FROM AUTHOR]

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along‐strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties. Plain Language Summary: Slow earthquakes are characterized as slips much slower than similar‐size regular earthquakes. Although interactions between the shallow slow earthquakes and large tsunamigenic earthquakes have often been discussed, our knowledge of the source characteristics and spatial variations of the shallow slow earthquakes is still limited. In this study, we quantitatively investigated the activity characteristics of shallow, very low frequency earthquakes (VLFEs) along the Nankai Trough. Activity areas and released cumulative moments of shallow VLFE swarms exhibited a similar scaling law irrespective of regions. However, the duration and cumulative moments of the swarms varied in each region. These characteristics can provide key information on the faulting conditions of slow earthquakes in shallow plate boundaries. Key Points: Comprehensive detection and source parameter estimations of shallow very low frequency earthquake swarms along the Nankai TroughThe scaling relationship between the activity areas and cumulative moments of the swarms roughly follows Mo ∝ A3/2Along‐strike variations of the scaling law for swarm durations reflect differences in the faulting conditions of slow earthquakes [ABSTRACT FROM AUTHOR]

We examined a relationship between rupture propagation directions and the distribution of fault strength by analyzing seismological data from the earthquake swarm on the Yamagata-Fukushima border, NE Japan. This earthquake swarm exhibits a distinct hypocenter migration behavior and was estimated to be triggered by upward fluid movement after the 2011 Tohoku-Oki earthquake. We utilized the dense nationwide seismic network in Japan to estimate apparent source time functions of >1500 small earthquakes (M JMA ≥ 2). We found clear directional dependences of the peak amplitude and the pulse-width in the apparent source time functions, suggesting the earthquake rupture directivity, for half of the earthquakes. Rupture directions mostly avoid the directions of the hypocenter migration. The difference between the microscopic and macroscopic propagations of rupture might be explained by the spatial variation in the fault strength affected by pore pressure along the fault; ruptures of each earthquake are hindered from developing toward the region with higher fault strength ahead of the pore-pressure front. Estimates of stress drop systematically increased on taking the effects of rupture directivity into account. We observed a temporal increase in stress drop from 3 MPa to 10 MPa during the pore-pressure migration. • Strength gradient caused by upwelling fluid controls each rupture in the swarm. • Estimates of stress drop systematically increased on taking the effects of rupture directivity into account. • Individual earthquakes share the common macroscopic plane structures in the swarm diffusion. [ABSTRACT FROM AUTHOR]

Stress accumulation and release inside the plate remains poorly understood compared to that at the plate boundaries. Spatiotemporal variations in foreshock and aftershock activities can provide key constraints on time‐dependent stress and deformation processes inside the plate. The 2017 M5.2 Akita‐Daisen intraplate earthquake in NE Japan was preceded by intense foreshock activity and triggered a strong sequence of aftershocks. We examine the spatiotemporal distributions of foreshocks and aftershocks and determine the coseismic slip distribution of the mainshock. Our results indicate that seismicity both before and after the mainshock was concentrated on a planar structure with N‐S strike that dips steeply eastward. We observe a migration of foreshocks toward the mainshock rupture area, suggesting the possibility that foreshocks were triggered by aseismic phenomena preceding the mainshock rupture. The mainshock rupture propagated toward the north, showing less slip beneath foreshock regions. The stress drop of the mainshock was 1.4 MPa, and the radiation efficiency was 0.72. Aftershocks were intensely triggered near the edge of large coseismic slip regions where shear stress increased. The aftershock region expanded along the fault strike, which can be attributed to the postseismic aseismic slip of the mainshock. We find that the foreshocks, mainshock, aftershocks, and postseismic slip released stress at different segments along the fault, which may reflect differences in frictional properties. Obtained results were similar to those observed for interplate earthquakes, which supports the hypothesis that the deformation processes along plate boundaries and intraplate faults are fundamentally the same. Key Points: Relocated hypocenters and focal mechanisms indicate that the mainshock, foreshocks, and aftershocks occur on the same intraplate faultThe foreshocks, mainshock, aftershocks, and postseismic slip released stress on different fault segmentsForeshock and aftershock seismicity migrate along the fault plane, suggesting that aseismic slip occurs before and after the mainshock [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to an article related to liver diseases published in a previous issue.

Background: This study aimed to evaluate the therapeutic potential of intrasplenic transplantation of culture-propagated homologous hepatocytes in rats suffering from acute liver failure (ALF). Methods: ALF was induced in dipeptidyl peptidase IV-negative (DPPIV−) Fischer 344 rats by totally removing the two anterior liver lobes (68% of the liver) and ligating the pedicle of the right lobe (24% of the liver). Hepatocytes isolated from DPPIV+ Fischer 344 rats were cultured for 11 d to propagate 3-fold, and the resulting hepatocytes were dubbed “culture-propagated hepatocytes (CPHEPs)”. A total of 1.5 × 107 cells of CPHEPs were transplanted intrasplenically before ALF induction (CPHEP group). Similarly, freshly isolated hepatocytes (FIHEPs) were transplanted as a positive control (FIHEP group), and culture medium (CM) was injected into rats as a negative control (CM group). Results: The survival of the CPHEP group was comparable to that of the FIHEP group and longer than that of the CM group (P < 0.01). Both CPHEP and FIHEP transplantation improved blood parameters such as ammonia, total bilirubin, glutamic pyruvic transaminase, and glutamic oxaloacetic transaminase; transplantation also affected liver tissue parameters such as apoptosis rate and bromodeoxyuridine-labeling index. Conclusions: Transplantation of culture-propagated homologous hepatocytes has a remarkable therapeutic potential for ALF in rats. [ABSTRACT FROM AUTHOR]