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The correct absolute time of a seismogram is an important prerequisite for many seismological processing methods. Recently, seismic interferometry has been applied to continuously estimate instrumental clock errors in certain regions. However, previous studies have focused on estimating clock errors in only one target region for each study, and data processing methods that provide more stable results in different regions have not been explored. This study presents a new method to estimate clock errors that are more stable in various regions based on seismic interferometry with windowed cross-correlation and least absolute deviation regression. The applicability of this method was validated by its application to 50 active volcanoes in Japan. We found clock errors at 23 stations at 13 volcanoes between January 2017 and December 2021. The maximum amplitude of the clock errors ranged between 0.24 and 18.27 s. This value range is significantly larger than the amplitude of the background fluctuation in the estimated values of clock errors, which is less than 0.05 s for most volcanoes. A comparison of different approaches for the estimation of clock error shows that our proposed method that uses windowed cross-correlation and least absolute deviation regression can reduce the effect of waveform changes in seismic ambient noise cross-correlation functions, probably because of noise source fluctuations. Managing the time stamps of data using the method proposed in this study will contribute to data quality assurance and ensure the reliability of analysis using time information in various study regions. [ABSTRACT FROM AUTHOR]

The S-wave velocity (Vs) structure of sediments and the uppermost crust in the landward slope of a subduction zone are important for determining the dynamics of the overriding plate. Although distributed acoustic sensing (DAS) measurements have improved the horizontal resolution of Vs structure in marine areas, the estimations have been limited to the uppermost sedimentary layers. In the present study, we applied seismic interferometry to DAS data of 13 h duration to image the sedimentary and crustal structure offshore of Sanriku, Japan with a spatial horizontal resolution of 2.5 km and > 3.0 km depth. We grouped the DAS data into 10 km long subarrays with 75% overlaps. We first applied a frequency-wavenumber filter to the DAS data to remove DAS instrumental noise and to allow effective extraction of surface waves from short-time records. We then applied a seismic interferometry method and estimated the phase velocities at each subarray. The estimated phase velocities of the fundamental-mode and first higher-mode Rayleigh waves were then used to determine one-dimensional Vs structures for each subarray. The resultant 2-D Vs structure was interpreted as representing sediments and crust. The upper sedimentary layers thicken seaward, while the entire sedimentary unit shows complex lateral variations in depth. The boundary between the sedimentary layers and the uppermost crust varies in depth from 1.8 to 6.8 km and is the deepest in the middle of the profile. Combining this result with the P-wave velocity (Vp) structure along the nearest survey line, determined in previous studies, allowed us to estimate Vp/Vs = 3.12, on average, for the lower sedimentary layers. Our method of applying seismic interferometry to marine DAS data broadens the techniques for estimating Vs and Vp/Vs structure of sedimentary layers and the upper crust across subduction zones. These results show that application of the frequency-wavenumber filtering and seismic interferometry to marine DAS data can estimate the Vs structure and the Vp/Vs structure, together with standard marine geophysical surveys of sedimentary layers and the upper crust across subduction zones. [ABSTRACT FROM AUTHOR]

Autocorrelation functions (ACFs) of vertically incident seismic waves are used to image subsurface reflectors. However, the reflection responses derived from ACFs usually contain many false signals. We present a method to quantify the errors in ACFs and extract true reflectors with high reliability. We estimated the errors for each earthquake at each station as follows. We calculated the amplitude of the observed waveform within the noise window and generated 1000 random noise traces that have this amplitude. By subtracting the random noise traces from the observed waveform, we created 1000 candidate earthquake waveforms. We computed the ACF for each of the 1000 waveforms and calculated the ensemble average and standard deviation of the 1000 different ACF amplitudes at each lag time. Then, we applied weighted stacking to the ACFs of many earthquakes to obtain the reflection response at the station. We calculated the standard deviation of the weighted stack to estimate errors in the reflection response. We evaluated the method by applying it to seismic data from the metropolitan area of Japan. The subsurface structure of the study area has been studied extensively and consists of a strong velocity discontinuity between sedimentary and basement layers. Following our method, the discontinuity was imaged as a clear reflector with an amplitude that was substantially greater than three times the standard deviation, which corresponds to statistical significance at the 99% confidence level. At other depths where reflectors are not expected to be present, the amplitudes of the peaks were less than or close to three times the standard deviation. The signal of the discontinuity was clearly visible at frequencies below 10 Hz and was less prominent at higher frequencies. [ABSTRACT FROM AUTHOR]

Assessing the relative seismic properties of volcanoes is essential because it improves our understanding of the similarity and diversity of processes occurring in magmatic and hydrothermal systems, thereby contributing to a comprehensive understanding of these systems. Scattering and absorption are among the most challenging but useful parameters to obtain to discriminate volcanic structures. However, the separation of scattering from absorption generally relies on active seismic data or earthquake activity. We estimated seismic scattering and intrinsic absorption parameters of Rayleigh waves at 18 active volcanoes in Japan using seismic ambient noise. The use of cross-correlation functions enables the estimation of these parameters in volcanoes where there are no active seismic data available and with insufficient earthquake activity. The scattering and intrinsic absorption parameters from the 18 volcanoes are about two orders of magnitude larger than those in non-volcanic regions, indicating strong medium heterogeneity in the shallow regions of those volcanoes, as the Rayleigh waves in our target frequency are mainly sensitive to subsurface structures shallower than 1 km in depth. Volcanic islands and volcanoes near the coastline or lakes show the highest intrinsic attenuation and frequency dependence, likely caused by widespread water-saturated regions beneath the volcanoes. Our study highlights both similarities and differences in scattering and absorption properties across the 18 volcanoes. Further developing passive estimation capabilities to better constrain scattering and absorption parameters will allow us to distinguish differences in fracturing and pore space properties of the subsurface medium and will contribute to detailed studies of complex subsurface structures at active volcanic systems. [ABSTRACT FROM AUTHOR]

To continuously monitor crustal behavior associated with earthquakes, magmatic activities and other environmental effects (e.g., tides and rain precipitation), we have developed a continuous monitoring system of seismic velocity of the Japanese Islands. The system includes four main processing procedures to obtain spatio-temporal velocity changes: (1) preparing ambient-noise data; (2) creating virtual seismograms between pairs of seismometer stations by applying seismic interferometry; (3) estimating temporal velocity variations from virtual seismograms by stretching interpolation approach, and (4) mapping spatio-temporal velocity variations. We developed a data-processing scheme that removes unstable stretching interpolation results by using the median absolute deviation technique and a median filter. To map velocity changes with high stability and high temporal resolution during long-term (i.e., longer-term monitoring), we proposed the "sliding reference method". We also developed evaluation method to select the optimum parameters related to stability and temporal resolution. To reduce computation time for continuous monitoring, we applied parallel computation methods, such as shared memory and hybrid distributed memory parallelization. Using our efficient and stable monitoring system, we succeeded to continuously monitor the spatio-temporal velocity variation of the whole Japanese Islands using ambient-noise data from 767 seismometers. Finally, we developed a web application that displays spatio-temporal velocity changes. In the monitoring results that we open through the website, we identified velocity variation (e.g., pore pressure variation) that could be related to earthquake, aftershock, magmatic activities and environmental effects in a stable manner. [ABSTRACT FROM AUTHOR]

In this study, we utilize acceleration time histories data at eight KiK-net vertical arrays in Japan, recorded between January 2009 to June 2014, to analyze nonlinear seismic site response. Ground motion records provided by seismic vertical arrays allow estimation of in situ soil dynamic parameters. First, the shear modulus is computed from the shear wave velocity using seismic interferometry by deconvolution. Considering each record as an in situ cyclic test under seismic loading, the shear strain is computed as the velocity ratio between two sensors and the shear stress is assumed as a spatial function of accelerations. Using classic hyperbolic regression models, we quantitatively assess the relationships between the in situ soil dynamic parameters and the level of ground motion. We found nonlinearity starts at a quite low level of shear strain of 5 × 10−6–5 × 10−5, and becomes much more significant beyond a relatively high level of strain of 1 × 10−4. The threshold strains from ground motion observations are well consistent with those from previous laboratory tests. Finally, a parametric analysis is presented to discuss the influence of plasticity index on nonlinear soil behavior evaluations. It is concluded that the threshold acceleration of soil nonlinear behavior (generally ranging from 20 to 100 cm/s2) is mainly controlled by the stiffness and plasticity index, and the degree of soil nonlinear behavior is mainly controlled by the stiffness, plasticity index and the level of ground motion. [ABSTRACT FROM AUTHOR]

A series of uplifts and subsidences of a volcanic complex in the Kussharo caldera in eastern Hokkaido (Japan) has been revealed by interferometric analysis using archived satellite synthetic aperture radar data. A time series of interferograms from 1993 to 1998 showed the temporal evolution of a ground deformation process. The horizontal dimension of the deformation field was about 10 km in diameter, and the maximum amplitude of the deformation was >20 cm. Uplift started in 1994, and concurrent earthquake swarm activity was observed around the uplift area; however, no other phenomena were observed during this period. A subsidence process then followed, with the shape of the deformation forming a mirror image of the uplift. Model simulations suggest deformation was caused by a source at the depth of about 6 km and that the position of the source remained static throughout the episode. Subsidence of the volcanic complex was also observed by another satellite from 2007 to 2010, and likely continued for more than 10 years. In addition to the main uplift-subsidence sequence, small deformation patterns with short spatial wavelengths were observed at the center of the deforming area. Data from three satellites recorded small-scale subsidence of the Atosanupuri and Rishiri lava domes at a constant rate of approx. 1 cm/year from 1993 to 2016. [ABSTRACT FROM AUTHOR]

Coda-wave interferometry is a technique to detect small seismic velocity changes using phase changes in similar waveforms from repeating natural or artificial sources. Seismic interferometry is another technique for detecting seismic velocity changes from cross-correlation functions of ambient seismic noise. We simultaneously use these two techniques to clarify seismic velocity changes at Sakurajima volcano, one of the most active volcanoes in Japan, examining the two methods. We apply coda-wave interferometry to the records of repeated active seismic experiments conducted once a year from 2011 to 2014, and seismic interferometry to the ambient seismic noise data. We directly compare seismic velocity changes from these two techniques. In coda-wave interferometry analyses, we detect significant seismic velocity increases between 2011 and 2013, and seismic velocity decreases between 2013 and 2014 at the northern and eastern flanks of the volcano. The absolute values are at a maximum 0.47 ± 0.06% for 2-4 Hz, 0.24 ± 0.03% for 4-8 Hz, and 0.15 ± 0.03% for 8-16 Hz, respectively. In seismic interferometry analyses, vertical-vertical cross-correlations in 1-2, 2-4, and 4-8 Hz bands indicate seismic velocity increases and decreases during 3 years of 2012-2014 with the maximum amplitudes of velocity change of ±0.3% for 1-2 Hz, ±0.4% for 2-4 Hz, and ±0.2% for 4-8 Hz, respectively. Relative velocity changes indicate the almost annual change. These periodical changes are well matched with volcano deformation detected by GNSS receivers deployed around the volcano. We compare the results from coda-wave interferometry with those from seismic interferometry on the shot days and find that most of them are consistent. This study illustrates that the combined use of coda-wave interferometry and seismic interferometry is useful to obtain accurate and continuous measurements of seismic velocity changes. [ABSTRACT FROM AUTHOR]

Continuous, very long baseline interferometry (VLBI) campaigns over 2 weeks have been carried out repeatedly, i.e., CONT02 in October 2002, CONT05 in September 2005, CONT08 in August 2008, and CONT11 in September 2011, to demonstrate the highest accuracy the current VLBI was capable at that time. In this study, we have compared zenith total delays (ZTD) and troposphere gradients as consistently estimated from the observations of VLBI, Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) at VLBI sites participating in the CONT campaigns. We analyzed the CONT campaigns using the state-of-the-art software following common processing strategies as closely as possible. In parallel, ZTD and gradients were derived from numerical weather models, i.e., from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields, the High Resolution Limited Area Model (European sites), the Japan Meteorological Agency-Operational Meso-Analysis Field (MANAL, over Japan), and the Cloud Resolving Storm Simulator (Tsukuba, Japan). Finally, zenith wet delays were estimated from the observations of water vapor radiometers (WVR) at sites where the WVR observables are available during the CONT sessions. The best ZTD agreement, interpreted as the smallest standard deviation, was found between GNSS and VLBI techniques to be about 5-6 mm at most of the co-located sites and CONT campaigns. We did not detect any significant improvement in the ZTD agreement between various techniques over time, except for DORIS and MANAL. On the other hand, the agreement and thus the accuracy of the troposphere parameters mainly depend on the amount of humidity in the atmosphere. [ABSTRACT FROM AUTHOR]

One-baseline 1-h Very Long Baseline Interferometry (VLBI) Intensive sessions are carried out every day to determine Universal Time (UT1). Azimuthal asymmetry of tropospheric delays around the stations is usually ignored and not estimated because of the small number of observations. In this study we use external information about the asymmetry for the Intensive sessions between Tsukuba (Japan) and Wettzell (Germany), which are carried out on Saturdays and Sundays (1) from direct ray-tracing for each observation at Tsukuba and (2) in the form of linear horizontal north and east gradients every 6 h at both stations. The change of the UT1 estimates is at the 10 μs level with maximum differences of up to 50 μs, which is clearly above the formal uncertainties of the UT1 estimates (between 5 and 20 μs). Spectral analysis reveals that delays from direct ray-tracing for the station Tsukuba add significant power at short periods (1–2 weeks) w.r.t. the state-of-the-art approach, and comparisons with length-of-day (LOD) estimates from Global Positioning System (GPS) indicate that these ray-traced delays slightly improve the UT1 estimates from Intensive sessions. [ABSTRACT FROM AUTHOR]

Mentions the successful opening of the parabolic antenna on the Japanese radiotelescope satellite Highly Advanced Laboratory for Communications and Astronomy (HALCA). The observations being made in Very Long Baseline Interferometry.