Kazuo Yoshimoto

Short-wavelength crustal heterogeneities in the Nikko area, central Japan, revealed from the three-component seismogram envelope analysis

Abstract The short-wavelength crustal heterogeneity is investigated in the Nikko area (Central Japan), using a three-component seismogram envelope analysis applied to small local earthquakes. We stochastically model fluctuations in crustal density and seismic wave velocities by using an exponential auto-correlation function. The model parameters are correlation length a and root mean square (rms) fractional fluctuation strength e. Free surface and non-spherical source radiation effects are incorporated in the synthesis of three-component seismogram envelopes. We use 149 short-period seismograms recorded by array stations from 10 local earthquakes with magnitudes 1 ∼ 2 and epicentral distances less than 15 km. A best fit minimum residual between observed and synthetic envelopes in the frequency band of 8 ∼ 16 Hz gives e 2 a ⋍ 8 × 10 −6 m −1 . With the additional information on the strength of direct wave attenuation and the excitation of coda waves at low frequencies, we estimate that a = 300 ∼ 800 m and e = 5 ∼ 8%. This strong heterogeneity and short correlation length probably reflect the geological condition of the Nikko area which is located in the vicinity of the volcanic front.

Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

An M=5 earthquake occurred on September 15, 1998, in the Nagamachi-Rifu fault (NRF), northeastern Japan. In the aftermath of this event, many seismograph stations were constructed temporarily around the fault, forming a dense network of stations with a spatial separation of 5 km. We report here our estimation of the three-dimensional velocity structures of the P and S waves using arrival-time data recorded at these stations with the aim of understanding the heterogeneous structure around the NRF. Low-Velocity and high Poisson’s ratio anomalies are imaged in the lower crust beneath the volcanic area, which are probably associated with the partially molten materials conveyed through the upwelling flow in the mantle wedge. A distinct low-velocity anomaly, which is explainable by the existence of H2O-filled pores, is observed in the mid crust at the deeper extension of the NRF. Two low-velocity anomalies that are probably associated with the remnants of magmatic activity that formed the Shirasawa caldera and with the existence of thick late-Cenozoic sedimentary layers are observed at depths shallower than 10 km in the hanging wall of the NRF. Our results successfully characterize the major features of the complex velocity structure around the NRF, with implications for the existence of fluid-rich regions in the mid to lower crust.

Three-component seismogram envelope synthesis in randomly inhomogeneous semi-infinite media based on the single scattering approximation

Abstract We present a method for synthesizing three-component seismogram envelopes of local earthquake in a randomly inhomogeneous semi-infinite medium. The method extends the single scattering model of Sato [Sato, H., 1984. Attenuation and envelope formation of three-component seismograms of small local earthquakes in randomly inhomogeneous lithosphere. J. Geophys. Res. 89: 1221–1241.] by incorporating the effects of a free surface, frequency-dependent non-spherical radiation from a double-couple point source, and non-isotropic scattering including wave-type conversion. We synthesize seismogram envelopes—the root-mean-square (rms) amplitude of seismograms—at a receiver located on the free surface by dividing an inhomogeneous medium into many small cells and summing the energy of scattered waves from the cells on the isochronal scattering shells of different scattering modes for a given lapse time. Our main focus is the free surface effect, which alters the shape of isochronal scattering shells and the amplitudes of incident waves. Synthesizing three-component seismogram envelopes for different source-receiver configurations, we find that these effects on seismogram envelopes are not negligible and are pronounced at early S coda. P coda envelope shape is not very sensitive to free-surface incorporation because of dominant SP scattered waves which come from the source direction.

Moho and Philippine Sea plate structure beneath central Honshu Island, Japan, from teleseismic receiver functions

We used teleseismic P waves recorded by the J-array, the Hi-net and a temporal local seismic network to investigate the three-dimensional topography of the Moho and the Philippine Sea plate beneath central Honshu Island, Japan. An image of the subsurface discontinuities beneath the region, derived from receiver function analysis, depicts the Philippine Sea plate dipping toward the north with complex local curvatures. The Moho is clearly detected in the northern part of the area studied, and its depth increases to the center of the island. Receiver functions from the stations adjacent to the Itoigawa-Shizuoka tectonic line indicate the step-like topography of the Moho directly beneath this tectonic line.

The Separation of Intrinsic and Scattering Seismic Attenuation in South Korea

Abstract For the separation of intrinsic () and scattering attenuation () in South Korea, coda observations represented by the multiple-lapse time windows method were numerically simulated using the direct simulation Monte Carlo method. The simulation showed that the depth-dependent velocity model separated by crust and mantle fit better than the uniform velocity model. Under a uniform attenuation assumption, the and total in South Korea represent the lowest values among current global measurements. These low attenuation values can be explained by the inactive seismicity of South Korea corresponding to those of the seismically stable area such as a shield.

Frequency-dependent attenuation of S-waves in the Kanto region, Japan

Apparent, scattering, and intrinsic S-wave attenuations (QS−1, Qscat−1 and Qint−1) of the upper lithosphere in the Kanto region of Japan were measured in the 1- to 32-Hz frequency range using Multiple Lapse Time Window Analysis (MLTWA) for 115 borehole seismograms of local earthquakes. A new set of time windows for MLTWA, in which multiple isotropic scattering is assumed, was proposed and employed to estimate the frequency dependence of S-wave attenuation parameters. Scattering attenuation was found to dominate intrinsic attenuation in the S-wave attenuation mechanism at low frequencies (<;2 Hz), whereas the opposite relation was observed at high frequencies. The transition is caused by the different frequency dependences of Qscat−1 (∝ f−1.5) and Qint−1 (∝ f−0.7) at this frequency. Interestingly, Qscat−1 is almost frequency independent at frequencies >8 Hz, which implies the self-similar nature of short-wavelength heterogeneities in the upper lithosphere. In terms of the upper lithosphere of the Kanto region, these results may indicate that the random heterogeneities characterized by the Gaussian autocorrelation function with a fractional fluctuation ε ≈ 10% and a correlation length a ≈ 2 km are superimposed on the weak background self-similar heterogeneity.

Prediction of maximum P - and S -wave amplitude distributions incorporating frequency- and distance-dependent characteristics of the observed apparent radiation patterns

Frequency-dependent model of the apparent radiation pattern has been extensively incorporated into engineering and scientific applications for high-frequency seismic waves, but distance-dependent properties have not yet been fully taken into account. We investigated the unified characteristics of frequency and distance dependences in both apparent P- and S-wave radiation patterns during local crustal earthquakes. Observed distortions of the apparent P- and S-wave radiation patterns could be simply modeled by using a function of the normalized hypocentral distance, which is a product of the wave number and hypocentral distance. This behavior suggests that major cause of distortion of the apparent radiation pattern is seismic wave scattering and diffraction within the heterogeneous crust. On the basis of observed normalized hypocentral distance dependency, we proposed a method for prediction of spatial distributions of maximum P- and S-wave amplitudes. Our method incorporating normalized hypocentral distance dependence of the apparent radiation pattern reproduced the observed spatial distributions of maximum P- and S-wave amplitudes over a wide frequency and distance ranges successfully.Graphical abstract.

Surface wave excitation at the northern edge of the Kanto Basin, Japan

In this paper, we analyze the seismograms of a local earthquake (MW = 5.8) and find a clear surface wave excitation at the northern edge of the Kanto Basin, Japan. To conduct three-dimensional (3-D) finite difference method simulations for this observation, we investigate the characteristics of the S wave velocity in the Kanto Basin and construct a realistic sedimentary structure model. The S wave velocities derived from the vertical seismic profiling measurements in the Kanto Basin show smooth depth gradients rather than step-like increases and can be successfully modeled by an exponential asymptotically bounded velocity function. Our simulation using the modified Japan integrated velocity structure model by incorporating a sedimentary layer with S wave velocity-depth gradients well reproduces the excitation and dispersion of the observed surface waves. This result indicates that realistic modeling of the impedance contrast at the sediment-bedrock interface is indispensable for the precise evaluation of surface wave excitation at the basin edge.

Application of scattering theory to P-wave amplitude fluctuations in the crust

The amplitudes of high-frequency seismic waves generated by local and/or regional earthquakes vary from site to site, even at similar hypocentral distances. It had been suggested that, in addition to local site effects (e.g., variable attenuation and amplification in surficial layers), complex wave propagation in inhomogeneous crustal media is responsible for this observation. To quantitatively investigate this effect, we performed observational, theoretical, and numerical studies on the characteristics of seismic amplitude fluctuations in inhomogeneous crust. Our observations of P-wave amplitude for small to moderately sized crustal earthquakes revealed that fluctuations in P-wave amplitude increase with increasing frequency and hypocentral distance, with large fluctuations showing up to ten-times difference between the largest and the smallest P-wave amplitudes. Based on our theoretical investigation, we developed an equation to evaluate the amplitude fluctuations of time-harmonic waves that radiated isotropically from a point source and propagated spherically in acoustic von Kármán-type random media. Our equation predicted relationships between amplitude fluctuations and observational parameters (e.g., wave frequency and hypocentral distance). Our numerical investigation, which was based on the finite difference method, enabled us to investigate the characteristics of wave propagation in both acoustic and elastic random inhomogeneous media using a variety of source time functions. The numerical simulations indicate that amplitude fluctuation characteristics differ a little between medium types (i.e., acoustic or elastic) or source time function durations. These results confirm the applicability of our analytical equation to practical seismic data analysis.

Direct simulation Monte Carlo method with a focal mechanism algorithm

To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo (DSMC) method is modified by implanting a focal mechanism algorithm. We compare the results of the modified DSMC method (DSMC-2) with those of the original DSMC method (DSMC-1). DSMC-2 shows more or similarly reliable results compared to those of DSMC-1, for events with 12 or more recorded stations, by weighting twice for hypocentral distance of less than 80 km. Not only the number of stations, but also other factors such as rough topography, magnitude of event, and the analysis method influence the reliability of DSMC-2. The most reliable result by DSMC-2 is obtained by the best azimuthal coverage by the largest number of stations. The DSMC-2 method requires shorter time steps and a larger number of particles than those of DSMC-1 to capture a sufficient number of arrived particles in the small-sized receiver. To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo method is modified by implanting a focal mechanism algorithm. The modified method shows more reliable results compared to those of the original one, for events with more than 12 recorded stations.