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Ocean bottom distributed acoustic sensing (OBDAS) is emerging as a new measurement method providing dense, high-fidelity and broad-band seismic observations from fibre-optic cables deployed offshore. In this study, we focus on 35.7 km of a linear telecommunication cable located offshore the Sanriku region, Japan, and apply seismic interferometry to obtain a high-resolution 2-D shear wave velocity (VS) model below the cable. We first show that the processing steps applied to 13 d of continuous data prior to computing cross-correlation functions (CCFs) impact the modal content of surface waves. Continuous data pre-processed with 1-bit normalization allow us to retrieve dispersion images with high Scholte-wave energy between 0.5 and 5 Hz, whereas spatial aliasing dominates dispersion images above 3 Hz for non-1-bit CCFs. Moreover, the number of receiver channels considered to compute dispersion images also greatly affects the resolution of extracted surface-wave modes. To better understand the remarkably rich modal nature of OBDAS data (i.e. up to 30 higher modes in some regions), we simulate Scholte-wave dispersion curves for stepwise linear VS gradient media. For soft marine sediments, simulations confirm that a large number of modes can be generated in gradient media. Based on pre-processing and theoretical considerations, we extract surface wave dispersion curves from 1-bit CCFs spanning over 400 channels (i.e. ∼2 km) along the array and invert them to image the subsurface. The 2-D velocity profile generally exhibits slow shear wave velocities near the ocean floor that gradually increase with depth. Lateral variations are also observed. Flat bathymetry regions, where sediments tend to accumulate, reveal a larger number of Scholte-wave modes and lower shallow velocity layers than regions with steeper bathymetry. We also compare and discuss the velocity model with that from a previous study and finally discuss the combined effect of bathymetry and shallow VS layers on earthquake wavefields. Our results provide new constraints on the shallow submarine structure in the area and further demonstrate the potential of OBDAS for high-resolution offshore geophysical prospecting. [ABSTRACT FROM AUTHOR]

We theoretically and empirically demonstrate the usability of the zero-crossing method for quick microtremor surveys in earthquake engineering (i.e. microtremor array surveys), namely shallow (< a few kilometres) surveys with small-scale (< 1 kilometre in radius) mobile seismic arrays with a short observation time (< a few hours). The zero-crossing method is a type of spatial autocorrelation (SPAC) method that determines phase velocities based on multiple frequencies at which the SPAC coefficient curve crosses zero. It is theoretically shown that the zero-crossing method is robust against incoherent noise and that the use of the first zero crossings (i.e. those at the lowest frequencies) is more robust against inadequate conditions of the microtremor wavefield than the use of later zero crossings (i.e. those at higher frequencies). We used microtremor array data with maximum array radii and observation durations of 400 m and 120 min on average, respectively, at 445 observation sites in the Kanto Plain, Japan, for validating the practicality of using the first zero crossings. As an illustration of the robustness against low signal-to-noise ratios (SNRs), we show that with the zero-crossing method, low-sensitivity (i.e. low-SNR) seismometers provide the same analysis results as those obtained with high-sensitivity seismometers, even when the power spectral densities for the low-sensitivity seismometers are close to the self-noise level. We then show that a reference phase velocity dispersion curve (RPVDC), created mainly based on the first zero crossings at each site, has a spatial distribution that well corresponds to the geology and topography and is consistent with that obtained in a previous study. We inverted five RPVDCs to model 1-D S -wave profiles and validated them using S -wave profiles obtained from velocity logs at nearby deep (e.g. hundreds of metres) boring wells. The accuracy of phase velocities at the later zero crossings for three-sensor/four-sensor arrays and all zero crossings for two-sensor arrays are statistically examined (maximum of 9805 data) based on a comparison with the RPVDCs. The disadvantage of the zero-crossing method is that it can only provide information on phase velocities at discrete wavelengths up to a maximum wavelength of 2.6 r (i.e. corresponding to the first zero-crossing point), where r is the radius of a seismic array. Therefore, the RPVDCs were then used to examine the upper limit of the analysable wavelength ranges for the conventional SPAC method for microtremor array surveys. Based on a few hundred three-sensor/four-sensor arrays, it was found that for arrays with radii larger than several tens of metres, three-quarters of the upper limit wavelengths (ULWs) stayed within 5 r. For arrays with radii smaller than this value, the ULWs strongly depended on the array radius; the ULWs dramatically increased with decreasing array radius. For example, for arrays with an r value of 0.6 m, half of 336 data ranged between 26 r and 54 r , and the maximum ULW reached 186 r. This strong size dependence can be explained by differences in SNR. [ABSTRACT FROM AUTHOR]

P and S waves radiating from repeating earthquakes are analysed to identify subtle changes in seismic wave velocity associated with the Mw 9.0 Tohoku-Oki earthquake at shallow and deep zones in NE Japan. Accurate measurement of the changes in traveltimes of P and S waves needs high precision of the repeating earthquake's hypocentre parameters, but a large area of NE Japan was damaged so that usual hypocentre relocation methods may not be applicable. Therefore, we develop a new inversion method to simultaneously determine velocity changes of the structure as well as hypocentre parameters of repeating earthquakes. We represent arrival time differences of body waves from a pair of repeating earthquakes occurring before and after a target large earthquake by linear equations of relative locations, origin times and station correction factors, and determine these parameters by using a least-squares method. Arrival time differences at stations for a set of 25 repeating earthquakes are measured by comparing the observed waveforms. Applying the inversion method to these data, we find that traveltime delays up to about 0.04 s for the S wave are widely distributed in NE Japan close to the large slip area of the Tohoku-Oki earthquake. Traveltime delays for P waves are much smaller than those for S waves. The spatial change of traveltime delays are not well correlated with those of strong motions. This implies that the traveltime delays are not caused only in the shallow subsurface. To investigate where the seismic velocity changes occur, we separately analyse the repeating earthquakes occurring in the north and south regions of NE Japan, and observe a spatial shift of large station correction factors for the S wave: large station correction factors estimated from the repeating earthquakes located in the north are shifted to the south area of NE Japan, while those from the repeating earthquakes of the south are shifted to the north area. From comparisons of these spatial changes in the observed time differences with results obtained from forward modeling of direct S wave, the velocity decrease of about 0.1 per cent is inferred to extend in N-S direction in the middle of NE Japan with a depth of approximately 25 km. Our results provide important evidence that the medium changes after a megathrust earthquake such as the 2011 Tohoku-Oki earthquake are not limited to surface layers due to the effects of strong ground motion, but extend deeper in the crust. [ABSTRACT FROM AUTHOR]