Kosuke Chimoto

Estimation of shallow S-wave velocity structure using microtremor array exploration at temporary strong motion observation stations for aftershocks of the 2016 Kumamoto earthquake

Shallow S-wave velocity VS profiles were estimated for 26 temporary strong motion observation sites surrounding the epicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather the dispersion characteristics of Rayleigh waves. VS profiles were obtained by inverting the dispersion curves for each site and those of three permanent strong motion stations that recorded the sequence of seismic events. The shallow VS profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the VS profiles at other stations varied. The VS profiles were found to have the common feature of the uppermost low-velocity layer being widely distributed from Mashiki to the village of Minami-Aso, and it was especially thick in the areas that suffered heavy damage. This low-velocity layer was a major contributor to the site amplification. The horizontal-to-vertical spectral ratios of the microtremors indicate that both the shallow soil and deep sedimentary layers may control the site response characteristics over a broad frequency range.Graphical abstract.

Microtremor exploration for shallow S-wave velocity profiles at stations in local strong motion network in Bursa, Yalova, and Kocaeli in north-western Turkey

We conducted microtremor array surveys for shallow S-wave velocity profiles at 20 sites in Bursa, Yalova and Kocaeli provinces in the north-western part of Turkey to provide fundamental data to assess the seismic hazard in the area. All of the measurement sites were positioned very close to strong motion stations belonging to the Disaster and Emergency Management Presidency of Turkey (AFAD) in order to further understand site amplification factors in strong motion records. Of the 20 study sites, two were located in Yalova, four in Bursa and 14 in Kocaeli. We temporarily installed two small arrays to obtain simultaneous records of vertical microtremors. Then, the spatial autocorrelation method was applied to retrieve Rayleigh wave phase velocity curves in a frequency range from 1 to 30 Hz from the array records. The phase velocities in the western part of the Kocaeli area are low across a wide frequency range, while relatively high phase velocities are found in the eastern part of the Kocaeli province. The phase velocities in the Yalova and Bursa provinces are widely distributed suggesting large variations in soil conditions. The observed phase velocity curve at each site was inverted to a one-dimensional (1D) S-wave velocity profile to a depth of 100 m, using a hybrid heuristic inversion method. All the S-wave velocity profiles in the eastern Kocaeli area are similar; however, the sites in the western Kocaeli and Yalova-Bursa areas have profiles with different features from the others. Finally, we discuss amplification factors for S-waves using the inverted profiles. The dominant fundamental periods of the amplification factors were distributed in a frequency range from 0.7 to 5 Hz. The profiles obtained are also used to map average S-wave velocities in the study area, with an addition of existing data at strong motion stations of the AFAD. We conducted microtremor array surveys for shallow Vs profiles at strong motion stations in Bursa, Yalova and Kocaeli provinces in Turkey. The Vs profiles to a depth of 100 m were deduced from Rayleigh wave phase velocities in frequencies from 1 to 30 Hz. Site amplification factors were discussed with the profiles.

Microtremor exploration for shallow S-wave velocity structure in Bandung Basin, Indonesia

We have conducted a microtremor survey for shallow S-wave velocity profiles to be used for seismic hazard evaluation in the Bandung Basin, Indonesia. In the survey, two arrays were deployed temporarily at each of 29 sites, by installing seven vertical sensors in triangular configurations with side lengths from 1 to 16 m. Records of vertical microtremors from each array were used to estimate Rayleigh wave phase velocity spectra using the spatial autocorrelation method, as well as the horizontal-to-vertical spectral ratio obtained at the centre of the arrays. Phase velocities at sites on the basin margin exhibit higher values than those obtained in the central part of the basin, in a frequency range of 7 to 30 Hz. The phase velocity data were used to deduce S-wave velocity profiles of shallow soil using a hybrid heuristic inversion method. We validated our inversion models by comparing observed horizontal-to-vertical spectral ratios with ellipticities of the fundamental mode of Rayleigh waves, calculated for the inversion models. The S-wave velocity profiles in the area can be characterised by two soft layers over a firm engineering basement that has an S-wave velocity of 500 m/s. The S-wave velocities of the two layers are 120 and 280 m/s on average. The distribution of the averaged S-wave velocity in the top 30 m clearly indicates low values in the eastern central part and high values in the edge of the basin. The amplification is large in the areas with low velocity layers. In addition, we have proposed an empirical relation between the amplification factor and the topographical slope in the area. We obtained the shallow S-wave velocity profile distributions in the Bandung Basin, Indonesia, from the inversion of the observed Rayleigh wave phase velocity spectra using a microtremor survey. The profiles will be used for calculation of the average S-wave velocity of the upper 30 m and site amplification factors for seismic hazard evaluation in the area.

Determination of Local Site Soil Conditions by Microtremor Measurements for Sustainable Buildings

As well known, local soil condition at a specific site affects the significant features of strong ground motion such as amplitude, frequency range and time duration. The effects of site condition depend on the properties of the motion characteristics, material properties, topography and geometry of the field. The bedrock slope at the basin sides also is one of the most important factors in the response analyzing of a specific site in geotechnical earthquake engineering applications. The reflection and refraction waves from the basin edges induce the wave transmission and two dimensional effects. In this case two dimensional site response analyses are required to estimate the behavior of the soil layers. So, local geotechnical site condition should be into account in the design of earthquake resistant building. The bedrock slope of the basin edges should be determined for suitable method in two dimensional dynamic response analyses. In this study, the result of a specific site response based on shear wave velocity profiles determined by microtremor array measurements are presented. The geotechnical properties of the soil layers were obtained from the previous subsurface explorations. Seismic bedrock depth and bedrock slope of the edge were defined by microtremor array measurements. Shear wave velocity value is from ten to hundred meters was obtained from passive surface wave method by employed Spatial Auto Correlation algorithm. Established variation of shear wave velocity with depth models were examined in terms of local site amplification. In order to verify consistency of obtained amplification numbers, empirical and measuremental results were compared for each site. As a conclusion, dynamic properties of local site conditions under earthquake excitation were described in terms of maximum amplification, resonance frequency and predominant period.

Estimation of surface-wave phase velocity from microtremor observation using an array with a reference station

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. An interferometric procedure for Rayleigh wave phase velocities from cross correlations of microtremors in an array with a reference station is investigated in this study. After identifying dispersive Rayleigh waves, semblance analysis of the cross correlations was carried out to estimate phase velocities for fundamental and higher-mode Rayleigh waves.