Toshiki Watanabe

Variations of fluid pressure within the subducting oceanic crust and slow earthquakes

[1] We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes.

Seismic traveltime tomography using Fresnel volume approach

Summary A Fresnel volume approach is applied to represent wave propagation for seismic traveltime tomography instead of rays. A Fresnel volume is defined as a set of many waves delayed after the shortest traveltime by less than half a period. It is derived by calculating traveltimes both from a source and from a receiver. Tracing rays from sources to receivers is completely avoided. This considerably reduces computational time. We solved the eikonal equation by using a finite-difference method to calculate traveltimes. The advantage of this approach is as follows; First, the frequency of wave can be introduced into analysis. Therefore, we can evaluate the resolution of seismic tomography. Next, The smoothing feature can be naturally introduced. Finally, Fresnel volumes with finite bandwidth considerably reduces the sparseness of ray distribution (data kernel). These advantages make 3-D tomography analysis possible.

A marine deep-towed DC resistivity survey in a methane hydrate area, Japan Sea

Abstract We have developed a new deep-towed marine DC resistivity survey system. It was designed to detect the top boundary of the methane hydrate zone, which is not imaged well by seismic reflection surveys. Our system, with a transmitter and a 160-m-long tail with eight source electrodes and a receiver dipole, is towed from a research vessel near the seafloor. Numerical calculations show that our marine DC resistivity survey system can effectively image the top surface of the methane hydrate layer. A survey was carried out off Joetsu, in the Japan Sea, where outcrops of methane hydrate are observed. We successfully obtained DC resistivity data along a profile ~3.5u2009km long, and detected relatively high apparent resistivity values. Particularly in areas with methane hydrate exposure, anomalously high apparent resistivity was observed, and we interpret these high apparent resistivities to be due to the methane hydrate zone below the seafloor. Marine DC resistivity surveys will be a new tool to image sub-seafloor structures within methane hydrate zones.

Seismic attenuation tomography and its application to rock mass evaluation

Abstract We developed two methods of ray-based seismic attenuation tomography. One is amplitude attenuation tomography, which uses amplitude attenuation of the first arrival P-wave. As amplitude is easily influenced by various factors other than viscosity, we removed the effects of velocity structure on wave amplitude, such as transmission loss and diffraction, in the analysis. Decrease of wave frequency during propagation is estimated to obtain the Q-value directly. The other method is pulse broadening tomography, which uses broadening of rise-time or pulse-width of the first arrival P-wave. This method is based on the rise-time principle, which describes pulse broadening with travel time in viscous media. In this method, attenuation information can be obtained by using time data only. The combined use of velocity and attenuation characteristics can be a powerful tool to characterize an in situ rock mass. Therefore we integrated the seismic travel time tomography and the seismic attenuation tomography to a series procedure, and applied it to field data obtained in a mine. The result of travel time tomography agrees with the rock condition estimated by the geological observation at the sites. Faults and fractured zones were detected by the attenuation tomography.

Differential waveform tomography for time-lapse crosswell seismic data with application to gas hy- drate production monitoring

Summary To detect the change of physical properties in small areas, a series of high-resolution waveform inversions is applied to timelapse seismic data. A procedure by use of differentiation between the time-lapse data and normalization using reference data is proposed in this study. The procedure is derived as a straightforward extension of waveform inversion as the scatterer imaging. Through numerical tests, the proposed approach was found to be more accurate than the conventional approach in obtaining the velocity change in small areas. The method was applied to the time-lapse crosswell seismic data obtained during the Mallik 2002 gas production test. A small area showing a velocity decrease near the production zone were found using the proposed method, indicating the existence of dissociated methane gas in the sand layers.

Velocity and amplitude of P-waves transmitted through fractured zones composed of multiple thin low-velocity layers

Abstract Fractured zones and faults in rock masses influence the velocity and amplitude of P-waves propagated through them. In the frequency range of seismic exploration, wavelengths of P-waves are comparable to or longer than the thickness of fractures or faults. Fractures can be modeled as thin low-velocity layers in homogeneous high velocity material. We carried out model experiments on P-wave propagation through a low-velocity zone composed of many thin low-velocity layers, with the direction of P-wave propagation normal to the layers. The experimental models were made of aluminum rods and acrylic resin disks: they correspond, respectively, to rock and materials filling fractures such as clay. Next, we numerically simulated one-dimensional wave propagation through the low-velocity zone by using the communication matrix method. Waveforms obtained by this model calculation show good agreement with those obtained by the model experiments. The waveform and the amplitude of the transmitted P-waves vary with the number and the thickness of low-velocity layers. The time-average equation does not provide a good velocity estimation, because the velocity decreases with the increase in the number of low-velocity layers in spite that the total thickness of layers remaining the same. The distribution of low-velocity layers has little effect on velocity and waveform. Effects of low-velocity zones on waveform, amplitude and velocity of the transmitted P-wave vary with frequency of the incident P-wave. We concluded that the wave transmitted through the low-velocity zone was formed by the superposition of the direct wave and the many waves having multiple reflections at the interfaces of the layers in their path.

Viscoacoustic wave form inversion of transmission data for velocity and attenuation

This study investigates the performance of a frequency domain viscoacoustic full wave form nonlinear inversion to obtain high resolution images of velocity and attenuation. An efficient frequency domain implementation is applied that consists of performing a series of single frequency inversions sweeping from low to high frequency. A cascaded inversion was adopted in which the real part of the velocity is first imaged using the phase information, then the quality factor (Q) is imaged using the amplitude information. Tests with synthetic data indicate that our approach yielded better images than the simultaneous determination of the real and imaginary parts of the complex velocity. The method is applied to laboratory data obtained in a water tank with suspended acrylic bars. Broadband 200 kHz data are obtained for a crosshole configuration with a computer-controlled scanning system and piezofilm source and detector. The velocity image produced by the full wave form inversion is compared to a curved ray travel time tomography velocity image, and was observed to possess higher resolution and more precise locations of the acrylic bars. The Q image shows a lower resolution than the velocity image, but recovers the correct Q for acrylic. This method can be applied for geophysical applications targeted to soil, unconsolidated rocks, and marine sediments and also nondestructive evaluation and medical applications.

Seismic traveltime tomography in anisotropic heterogeneous media

Abstract This paper presents a seismic traveltime tomography method for the analysis of anisotropic heterogeneous media. In this study, anisotropy of slowness is approximated to be sinusoidal with respect to the angle from the anisotropic axis. Anisotropy is represented by the minimum slowness, the maximum slowness and its orientation for every cell. Our method can simultaneously obtain these three parameters. In our first formulation of the anisotropic tomography, the orientation of anisotropy is determined directly from the traveltime data. In the second formulation, the orientation of anisotropy is modified iteratively as well as the maximum and the minimum slowness. The result of the numerical simulation shows that the anisotropic tomography successfully reconstructs the velocity structure having 1–10% velocity anisotropy in case of noise free data. On the other hand, the ordinary isotropic tomography fails to reconstruct the anisotropic structure. Though both the restriction of source-receiver arrangement and the existence of noise affect the accuracy of the reconstructed images, our method can obtain anisotropic information for better understanding of subsurface structures.

Active monitoring at an active volcano: amplitude-distance dependence of ACROSS at Sakurajima Volcano, Japan

First testing of volcanic activity monitoring with a system of continuously operatable seismic sources, named ACROSS, was started at Sakurajima Volcano, Japan. Two vibrators were deployed on the northwestern flank of the volcano, with a distance of 3.6xa0km from the main crater. We successfully completed the testing of continuous operation from 12 June to 18 September 2012, with a single frequency at 10.01xa0Hz and frequency modulation from 10 to 15xa0Hz. The signal was detected even at a station that is 28xa0km from the source, establishing the amplitude decay relation as a function of distance in the region in and around Sakurajima Volcano. We compare the observed amplitude decay with the prediction that was made before the deployment as a feasible study. In the prediction, we used the existing datasets by an explosion experiment in Sakurajima and the distance-dependent amplitude decay model that was established for the ACROSS source in the Tokai region. The predicted amplitude in Sakurajima is systematically smaller than that actually observed, but the dependence on distance is consistent with the observation. On the basis of the comparison of the noise level in Sakurajima Volcano, only 1-day stacking of data is necessary to reduce the noise to the level that is comparable to the signal level at the stations in the island.

Continuous observation of seismic wave velocity and apparent velocity using a precise seismic array and ACROSS seismic source

We report the results of continuous monitoring—using a seismometer array—of the travel time of seismic waves generated by an ACROSS artificial seismic source. The seismometer array, which was deployed in a surface vault located 2.4 km from the source, recorded both direct P- and S-waves and refracted P- and S-waves that traveled along a velocity boundary between the granite basement and overlying sedimentary rocks. We analyzed temporal variation in differential travel time and apparent velocity for these phases for a period of 1 month and found significant temporal variation in the differential travel time. Most of the variation can be attributed to changes in environmental conditions, such as atmospheric temperature and rainfall. Variation is even observed in the seismogram that is located 50 m from the vibration source, although much smaller variation is observed in the vibration of the foundation to which the source is attached. The spectral study revealed that the effects of temperature and rainfall depend strongly on the frequency range used by ACROSS and that a large variation occurs in the 15- to 20-Hz range, especially between 17 and 20 Hz. The environmental effect on the temporal variation is comparable to the record of refracted S waves and that of a distance of 50 m, whereas a larger variation was observed in the direct S wave. This result shows that the signal is affected by the environmental change near the vibration source. The environmental effect can be drastically reduced when the signal from the 15- to 20-Hz range is eliminated in the analysis.