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We propose a method for capturing the near-surface elastic wave velocity variation using microtremors. The proposed method does not directly estimate the velocity of the medium; instead, it measures the spatial velocity change. We apply the stretching method, which is often used to detect the time variation of shear wave velocity based on the monitoring of seismic ambient noise, to microtremor recordings. Because spatially dense ambient noise records are unavailable, we performed microtremor measurements using multiple linear arrays with multiple sensors along the survey line to collect as many recordings as possible. The synchronised microtremor recordings of each array were used to compute the horizontal-to-vertical spectral ratio (H/V), cross-correlation (CC), autocorrelation (AC), and single-station cross-correlation (SC). The H/V ratio and the waveforms of these correlation functions showed significant spatial variation. The velocity variation was measured quantitatively using the stretching method, which correlates a compressed and stretched reference function with each function on the line. The velocity variation of the vertical and radial components of CC compares well with the variation of the H/V peak, indicating that the radial components of CC retrieved the Rayleigh-wave Green's function. The waveforms of AC and SC also showed spatial variation but the measured value of velocity change was smaller than that for CC. Further investigation is awaited to conclude if this is due to low-quality data or inappropriate measurement conditions. The reliability of the velocity change was confirmed by comparing the results to dispersion curves obtained using the conventional array method. [ABSTRACT FROM AUTHOR]

1D velocity profiles at a strong motion station in the northern part of the Kumamoto plain, Japan, were submitted in Step-1 of the blind prediction exercise of strong ground motions in the sixth international symposium on effects of surface geology on seismic motion (ESG6). Individual participants were requested to estimate a 1D S-wave velocity profile of sedimentary layers from the given data obtained by microtremor array explorations and surface-wave explorations at the site. This paper reports the target site, methods used by the individual participants, and the submitted results. More than half of the participants estimated the phase velocities of the Rayleigh-wave in the frequency range from 0.53 to 29.8 Hz. The statistical analysis of the phase velocity dispersion curves indicates that the standard deviation was below 40 m/s at the frequencies above 3.4 Hz, and it was below 20 m/s above 20 Hz. The S-wave velocity profiles are also similar to a depth of 20 m. The standard deviation was below 45 m/s. The average S-wave velocity in the top 30 m from the surface is 207.3 ± 60.7 m/s for the submitted profiles. The large variation is related to the introduction of the near-surface low velocity layers. The large variation of the S-wave velocities was found in the deep part. The average S-wave velocity at a depth of 1500 m was 2674 m/s with the standard deviation of 786 m/s. We compared 1D amplifications for the submitted profiles. Common peaks can be identified at 0.3–0.4 Hz and 1–2 Hz, excluding two teams. However, the amplifications vary much in the frequency range higher than 4 Hz. Through the experiment, it was found that the dispersion curves and the shallow S-wave velocity structures are estimated with a low standard deviation among the participants. Further development of the techniques for deep S-wave velocity profiling was found to be required. [ABSTRACT FROM AUTHOR]

We propose a new waveform inversion method to estimate the 2D S-wave velocity structure of deep sedimentary layers using broadband Love waves. As a preprocessing operation in our inversion scheme, we decompose earthquake observation records into velocity waveforms for periods of 1 s each. Then, we include in the inversion only those periods for which the assumption of 2D propagation holds, which we propose to determine through a principal component analysis. A linearized iterative inversion analysis for the selected Love wave segments filtered for periods of 1 s each allows a detailed estimation of the boundary shapes of interfaces over the seismic bedrock with an S-wave velocity of approximately 3 km/s. We demonstrate the effectiveness of the technique with applications to observed seismograms in the Kanto Plain, Japan. The differences between the estimated and existing velocity structure models are remarkable at the basin edges. Our results show remarkable differences from previous existing structural models, particularly near the basin edges while being in good agreement with the surface geology. Since a subsurface structure at a basin edge strongly affects the earthquake ground motions in a basin with the generation of surface waves, our method can provide a detailed model of a complex S-wave velocity structure at an edge part for strong ground motion prediction. [ABSTRACT FROM AUTHOR]

If seismic interferometry is to be applied to a strong ground motion record, the number of available records is limited, compared with the number of ambient noise records. To begin with, we investigated the characteristics of the autocorrelation function for various strong motion records, and the influence of the component of strong motion records to be used for autocorrelation analysis, in order to maximise the number of records available. We found that the autocorrelation function is not influenced by the event or the component of the strong motion record to be used for the analysis, so that reflected waves can be emphasised when a number of autocorrelation function images are prepared without selecting specific types of record. During autocorrelation analysis, we used spectral whitening that worked to remove frequency-dependent source and path effects. In this way, reflected waves from the seismic bedrock could be observed clearly in the whitened autocorrelation function. In addition, we showed the possibility of using the autocorrelation function for exploration by tuning the S-wave velocity structure model to fit the theoretical Green's function to the stacked autocorrelation function. We also found a relationship between the bandpass filter range in the autocorrelation process and the contribution of the multilayer boundary to the reflected wave, from both theoretical and observational examination. We understand the procedure for selecting an appropriate filter range to observe reflected waves from a desired layer boundary. [ABSTRACT FROM AUTHOR]

A large acceleration with a peak amplitude of 2700 cm/s2 was recorded at the K-NET Tsukidate station (MYG004) during the 2011 off Pacific Coast of Tohoku Earthquake. Nonetheless, there was little structural damage around the station. In this study, we estimate the engineering bedrock's motion variability due to the main shock, deconvolved from the observed surface motions, considering uncertainties in shallow S-wave velocity (Vs) profiles. The Vs profile uncertainty at the strong motion station is estimated with observed Rayleigh wave phase velocities, from microtremor explorations, using the Markov-chain Monte Carlo inversion. We obtain upper and lower possible values of the estimated bedrock spectral accelerations and the ground motion proxies at MYG004 using the uncertainty of the Vs profile. The estimated bedrock peak ground acceleration ranges are 610–930 cm/s2 and 2000–3100 cm/s2 for the east–west (EW) and north–south (NS) directions, respectively. Meanwhile, the ranges of the peak ground velocities are about 35–40 cm/s and 90–120 cm/s for the EW and NS components, respectively. The results indicate that the minimum and maximum possible values of the deconvolved spectral acceleration and the ground motion proxies are not only affected by Vs profile uncertainty but also by surface ground motion characteristics. Moreover, the spectral velocities at a station in the main part of Tsukidate Town are also estimated with the observed phase velocity. They have predominant peaks at periods shorter than 0.5 s, suggesting only minor structural damage in the city center. [ABSTRACT FROM AUTHOR]

On 24 May 2014, a Mw 6.9 earthquake occurred in the west of Gokceada Island, northern Aegean Sea. The earthquake was close to Canakkale, Enez, Tekirdag cities, and damaged 300 buildings in the Marmara Region, NW Turkey. We simulated its broadband (0.1-10 Hz) ground motions including 1D deep and shallow structures soil amplification effects at the 12 strong ground motion stations in the western Marmara Region. The 1D deep velocity structures from the focal layer to the engineering bedrock with an S-wave velocity of 0.78 km/s in different azimuthal directions were tuned by comparing the observed group-velocity dispersion curves of Rayleigh and Love waves from the mainshock with theoretical ones. We also added the shallow parts from previous surveys into the 1D models. Synthetic seismograms on the engineering bedrock were generated using the discrete wave number method with a source model and the 1D deep velocity structures. Then the surface motion was generated considering shallow soil amplification. The synthetic seismograms are generally in good agreement with the observed low and high-frequency parts at most of the stations indicating an appropriateness of the source model and the 1D structural model. [ABSTRACT FROM AUTHOR]

A procedure for estimation of Rayleigh wave phase velocities from microtremor observations, using an array with a reference station, is investigated in this study. Simultaneous observation of microtremors at a reference station and at a strong motion observation array in the Kanto Basin, Japan, was carried out. We first calculated cross correlations between records at the reference station and those at stations in the array using a seismic interferometric processing method on a 4300-h data series. After identifying dispersive Rayleigh waves from results of multiple filtering analysis of the cross correlations, semblance analysis of the cross correlations for different segments was carried out to estimate phase velocities for fundamental and higher-mode Rayleigh waves. The phase velocities from the proposed method are more appropriate than those from conventional methods at long periods as they avoid contamination by higher mode Rayleigh waves. The fundamental Rayleigh wave phase velocities were inverted to an S-wave velocity profile for deep sedimentary layers. We also examined the variations in the phase velocity with decreasing data duration. The phase velocities at periods less than 3 s from 6-h records are similar to those from 4300-h records, suggesting that our method is possibly applicable in microtremor exploration. [ABSTRACT FROM AUTHOR]

In the Kanto Basin, Japan, it has been reported that the dominant periods of long-period ground motions vary depending on the areas where earthquakes occurred. This suggests that the characteristics of long-period ground motions are closely related to their propagation paths in the sedimentary layers. We, firstly, investigated the azimuthal dependence of amplifications for long-period ground motions based on the spectral ratios of ground motions on the surface to those on the basement, using both the observation data and the simulation data in the Kanto Basin. In both the data for periods of more than 3 s, the seismic stations where the dominant periods vary for individual earthquakes are concentrated in the central part of the Tokyo Metropolitan Area. The azimuthal dependence on amplifications for long-period ground motions suggests that the wave propagation features of long-period ground motions are related to the laterally complex three-dimensional subsurface structures along the propagation paths between stations and epicenters. We, secondly, investigated the amplification characteristics for body and surface waves, to evaluate their contributions to earthquake ground motions for periods of more than 1 s observed in the basin. It is found that body and surface waves tend to dominate at periods of less than 2 s and more than 3 s, respectively. We, finally, understood the amplification characteristics of body and surface waves including the dominant periods and those amplification factors, to appropriately predict broadband earthquake ground motions. Highlights: The characteristics of long-period ground motions including the dominant periods and those amplification factors were quantitatively investigated based on the spectral ratio by seismic data observed on the surface and the bottoms of the deep boreholes. The azimuthal dependence of amplifications for long-period ground motions propagating in the Kanto Basin was demonstrated by analyzing both the observation data and the simulation data. The dominant periods of the spectral ratios vary for individual earthquakes in the central part of the Tokyo Metropolitan Area. This suggests that the wave propagation features of the long-period ground motions depend on the laterally complex subsurface structures in the Kanto Basin along the propagation paths between stations and epicenters. The amplification characteristics for body and surface waves for periods of more than 1 s observed were investigated to evaluate their contributions to earthquake ground motions. Surface and body waves significantly influence earthquake ground motions at periods of more than 3 s and less than 2 s in the Kanto Basin, respectively. [ABSTRACT FROM AUTHOR]

Microtremor array measurements, and passive surface wave methods in general, have been increasingly used to non-invasively estimate shear-wave velocity structures for various purposes. The methods estimate dispersion curves and invert them for retrieving S-wave velocity profiles. This paper summarizes principles, limitations, data collection, and processing methods. It intends to enable students and practitioners to understand the principles needed to plan a microtremor array investigation, record and process the data, and evaluate the quality of investigation result. The paper focuses on the spatial autocorrelation processing method among microtremor array processing methods because of its relatively simple calculation and stable applicability. Highlights: 1. A summary of fundamental principles of calculating phase velocity from ambient noise 2. General recommendations for MAM data collection and processing using SPAC methods 3. A discussion of limitations and uncertainties in the methods [ABSTRACT FROM AUTHOR]