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SUMMARYThe average S-wave velocity (VS) in the upper 30 m (VS30) is a proxy for seismic wave amplification. Microtremor array exploration is one of the available methods for site characterization, but the recorded data require complicated processing that can lead to different estimations of VS30 depending on the analyst and processing software. We propose a method of estimating VS30 by using derivatives obtained in the early stages of microtremor array data processing. Statistical analysis with 2376 virtually generated subsurface VS structure models revealed that the frequencies at which the spatial-autocorrelation (SPAC) coefficients and centreless circular array (CCA) coefficients take specific values strongly correlate with VS30, which we used to develop formulas for estimating VS30. Numerical validations using actual VS profiles at 616 sites in Japan showed that the proposed method could estimate VS30 with a root-mean-square deviation (RMSD) of 57–80 m/s with SPAC coefficients and 56m/s with CCA coefficients. Our proposed methods were applicable for 98–100 per cent of theVS profiles when we limited our estimation to sites with VS30 < 760 m/s. The results indicated that SPAC coefficients from arrays with radii of 8–20 m can be used for VS30 estimation and are less affected by incoherent noise. In contrast, CCA coefficients are much more sensitive to incoherent noise, which resulted in the overestimation of VS30. The estimated VS30 values from the recorded microtremor array data were in good agreement with the reference values from the actual VS profiles. The proposed method allows for robust and efficient VS30 estimation without relying on the analyst’s skills or software.

We performed the spectral inversion to separate the source, path, and site effects using strong ground motion records from essential facilities such as powerhouses and hydroelectric dams, distributed all over Costa Rica. The spectral inversion technique was adapted to a sparsely observed strong motion data set. We used the results to estimate strong ground motions at sites where there were no observation records in and around the intensity anomaly that appeared during the 2012 Sámara earthquake of Mw 7.6. To ensure the quality, we incorporated a broadband station to estimate levels of acceleration source spectra for six events 5.3 ≤ Mw ≤ 6.5, using them as the reference events for the spectral inversion. In this paper, we introduced two consistency checks: confirmation of the correctness of the input data, and verification of the output using synthetic spectra. We used them as an iterative implementation of sequences of elimination of suspicious data and inversions. We regulated the source spectra of the reference events for the final inversion, considering the estimate of site effect for one-dimensional SH wave propagation at the site of the lowest site amplification. We obtained the frequency-dependent quality factor Q = 179f0.5598 for the northern and central parts of Costa Rica. From the output of the spectral inversion, we reproduced the acceleration spectra of earthquakes in the sites where the event was not recorded. We applied this formulation using the earthquake mentioned above. The resulting synthetic spectra are consistent with the anomalous intensity distribution reported at that time.

In order to distinguish between shallow earthquakes and quarrying activity, we evaluated 3069 seismic signals from 413 occurrences (112 explosions and 301 shallow earthquakes, depths 5 km), collected by the Aswan seismic sub-network in southern Egypt from 2010 to 2021. The spectral properties and related source parameters (e.g., seismic moment and corner frequency) using the estimated P-wave source spectra for both earthquakes and quarry blasts were investigated. The analysis showed that the P-wave source spectra of shallow earthquakes have higher corner frequencies (8.8–23 Hz) than quarry blasts (1.0–2.6 Hz) within the same magnitude range. The source spectra of quarry blasts exhibited significant misfits with the omega-square model and had steeper falloffs at high frequencies. The selected quarry blasts have a narrow seismic moment range, from 2.03 × 1011 to 1.35 × 1012 Nm. Our results demonstrate that the evaluation of misfit of P-wave spectra from the omega-square source model, based on spectral amplitude characteristics of high- and low-frequency bands, is the most reliable discriminant method in the routine data analysis of the target area.

This study analyzed weekday shopping behavior from a home base to a destination using data from the 4th Keihanshin metropolitan area person trip survey. We first analyzed the relationships between occupation and transportation means, transportation means and travel time, and transportation means and duration of time at the destination. Results of a chi-square test, residual analysis, and correspondence analysis confirmed that employed persons tend to travel by cars while unemployed persons tend to travel by bus or walk. The relationship between travel time and duration of time at the destination was also revealed according to transportation means. Results of a cluster analysis then classified shopping behaviors to expose four patterns. Finally, multiple regression analyzed the degree to which certain variables were related to duration of time at the destination. Results confirmed a strong relationship between duration of time at the destination and travel time. The degree of this factor’s influence on other variables was also clarified. The value of these findings is that the relationship between travel time by means of transportation and the duration of time at the destination was clarified using statistical analysis. We then found a highly accurate equation that estimates the duration of time at a destination from the travel time. If the duration of time at a destination can be estimated, it may be possible to more accurately develop the structure of rest facilities, the number of parking lots, the degree of congestion, and so on, which can be associated with the customer’s usage behavior in a shopping site. This research in this paper contributes to the field of urban analysis and marketing by presenting a new effective method for person trip survey and analysis.

The 2018 Mw 6.4 Hualien earthquake generated a large peak-to-peak velocity of over 2 m/s, with a period of 3 s at the south end of the Milun fault, which resulted in the collapse of five buildings. To investigate the shallow subsurface soil structure and evaluate possible effects on the ground motion and building damage, we performed microtremor measurements in the Hualien basin. Based on the velocity structure jointly inverted from both Rayleigh-wave dispersion curves and microtremor horizontal-to-vertical spectral ratio data, we found that the shallow subsurface structure generally deepens from west to east. Close to the Milun fault, the structure becomes shallower, which is consistent with faulting during the 2018 earthquake and the long-term tectonic displacement. There is no significant variation for the site conditions in the north–south direction that can explain the large peak ground velocity in the south. As a result of the dense measurements in the heavily damaged area, where three high-rise buildings totally collapsed, these locations have the average S-wave velocity of the upper 30 m (AVS30) values and are relatively high compared to the more distant area from the Meilun River. This is somewhat unusual, because lower AVS30 values indicating softer ground conditions are expected close to the river. We did not find any characteristic subsurface soil structure that may contribute to the building collapses. The large 3 s pulse was probably generated by source effects, rather than subsurface soil amplification.

Understanding the cause/effect relationship between tectonic earthquakes and volcanic eruptions is a striking topic in Earth Sciences. Volcanoes may erupt due to the impact of seismic waves (i.e. dynamic stress) and changes in the stress field (i.e. static stress) with variable reaction times. In 2012, three large (Mw≥7.3) subduction earthquakes struck Central America within ten weeks; some volcanoes in the region erupted days after, meanwhile for others it took months to years to erupt. Here we show that the three earthquakes contributed to the increase in the number of volcanic eruptions during the seven years that followed. We found out that only those volcanoes that were already in a critical state of unrest effectively erupted, indicating that the earthquakes only prompted the eruptions. We recommend the permanent monitoring of active volcanoes to reveal which are more susceptible to culminate into eruption when the next large-magnitude earthquake hits a region.

Understanding the relationship cause/effect between tectonic earthquakes and volcanic eruptions is a striking topic in Earth Sciences. Volcanoes erupt with variable reaction times as a consequence of the impact of seismic waves (i.e. dynamic stress) and changes in the stress field (i.e. static stress).In2012, three large(Mw≥7.3)subductionearthquakes struck Central Americawithin a period of 10 weeks;subsequently, some volcanoesin the regionerupted a few days after, whileotherstook months or even years to erupt. Here, we show thatthese threeearthquakes contributed to the increase in the number of volcanic eruptionsduring the 7 years that followed these seismic events. We found that onlythosevolcanoes that were already in a critical state ofunrest eventually erupted, which indicates that the earthquakes only prompted the eruptions. Therefore, we recommend the permanent monitoring of active volcanoes toreveal which are more susceptible to culminate into eruption in the aftermath of the next large-magnitude earthquake hits a region., Funding was provided by Japan International Cooperation Agency (JICA) and Istitute Nazionale di Geofisica e Vulcanologia (INGV).

Seismic ambient vibration array measurements were carried out at seven sites in Ismailia, Egypt in order to obtain one-dimensional (1D) shear wave velocity (Vs) profiles and horizontal-to-vertical spectral ratios (HVSR). We applied the spatial autocorrelation (SPAC) method to the ambient vibration data for the estimating Rayleigh-wave dispersion curves and obtained 1D Vs profiles. The deeper parts (90–350 m) of the estimated models have Vs values ranging between 500 and 700 m/s, while the subsurface parts have Vs varying between 140 and 200 m/s. The averaged Vs in the upper 30 m based on the results indicate that site classes of all the sites are classified as C or D according to NEHRP soil classification. The computed site amplification factors derived from the estimated Vs profiles show that the resonant frequencies at sites on the east side of the Suez Canal are below 1 Hz, while those at sites on the west side vary between 1 and 2 Hz, indicating the difference of subsurface geology across the Suez Canal. The HVSR curves at each site are generally similar for all sensors, indicating nearly flat subsurface layers beneath each site. We also compared the dominant frequencies of HVSRs and those derived from 1D amplification factors. The comparisons show that both the frequencies are similar to each other, but there are slight differences possibly due to the lack of deeper sedimentary layer structures and lower resolution of HVSRs in the lower frequency range, indicating the importance of further ambient vibration survey with broadband sensors. The results show that the combination of SPAC and HVSR methods is advantageous to estimate proper Vs profiles to evaluate site effects for earthquake disaster mitigation.

On 2013 April 13, an inland earthquake of Mw 5.8 occurred in Awaji Island, which forms the western boundary of the Osaka sedimentary basin in western Japan. The strong ground motion data were collected from more than 100 stations within the basin and it was found that in the Osaka Plain, the pseudo velocity response spectra at a period of around 6.5 s were significantly larger than at other stations of similar epicentral distance outside the basin. The ground motion lasted longer than 3 min in the Osaka Plain where its bedrock depth spatially varies from approximately 1 to 2 km. We modelled long-period (higher than 2 s) ground motions excited by this earthquake, using the finite difference method assuming a point source, to validate the present velocity structure model and to obtain better constraint of the attenuation factor of the sedimentary part of the basin. The effect of attenuation in the simulation was included in the form of Q(f) = Q0(f/f0), where Q0 at a reference frequency f0 was given by a function of the S-wave velocity, Q0 = αVS. We searched for appropriate Q0 values by changing α for a fixed value of f0 = 0.2 Hz. It was found that values of α from 0.2 to 0.5 fitted the observations reasonably well, but that the value of α = 0.3 performed best. Good agreement between the observed and simulated velocity waveforms was obtained for most stations within the Osaka Basin in terms of both amplitude and ground motion duration. However, underestimation of the pseudo velocity response spectra in the period range of 5–7 s was recognized in the central part of the Osaka Plain, which was caused by the inadequate modelling of later phases or wave packets in this period range observed approximately 2 min after the direct S-wave arrival. We analysed this observed later phase and concluded that it was a Love wave originating from the direction of the east coast of Awaji Island.

In order to investigate the seismic velocity structure in the region of concentrated severe damage during the 2016 Kumamoto earthquake sequence, we conducted continuous seismic observations in the central area of Mashiki Town, Kumamoto Prefecture. During 4 days of observations at eight temporary sites, 2 months after the mainshock, recordings from 30 aftershocks (1.7 ≤ M j ≤ 4.3, 1.9 km ≤ depth ≤ 13.5 km) were obtained. The aftershock data showed that site amplifications at approximately 1 Hz are dominant in a zone where almost no buildings were damaged along the Akitsu riverside, whereas site amplifications at higher than 3 Hz are observed in the heavily damaged zones. Our data also showed that the peak acceleration and velocity amplitudes, as well as seismic intensities for the small events in the less damaged zone, are clearly larger than those in the damaged zones, implying that the damage distribution is inconsistent with site response based on linear site amplifications. The estimated phase velocities of Rayleigh waves using the aftershock and microtremor data show dispersive characteristics in the lower frequency range (0.26 ≤ f ≤ 1.27 Hz), but the values are substantially smaller than those derived from the P–S logging model at the nearest KiK-net strong-motion observation station KMMH16. The derived microtremor horizontal-to-vertical spectral ratios and earthquake radial-to-vertical (R/V) spectral ratios show common distinct peaks at around 0.4 Hz, which are possibly related to the response of deep sedimentary layers beneath the area. The refined velocity structure model that better accounts for both the phase velocity and common dominant peak indicates that the values of S wave velocity (Vs) above the bedrock layer (Vs = 2700 m/s) are smaller than those inferred from the logging model and the depth to the bedrock layer could be much deeper (about 600 m) in comparison with the logging model (234 m). The derived R/V spectral ratio at station KMMH16 also shows a distinct peak at 0.4 Hz, suggesting that there is no large difference of deep sedimentary structure between the observation area and station KMMH16.

We performed a damage survey of buildings and carried out microtremor observations in the source region of the 2015 Gorkha earthquake. Our survey area spans the Kathmandu valley and areas to the east and north of the valley. Damage of buildings in the Kathmandu valley was localized, and the percentage of the totally collapsed buildings was less than 5 %. East of the Kathmandu valley, especially in Sindhupalchok district, damage of buildings was more severe. In the center of Chautara and Bahrabise, towns in Sindhupalchok district, the percentage of the totally collapsed houses exceeded 40 %. North of the Kathmandu valley, the damage was moderate, and 20–30 % of the buildings were totally collapsed in Dhunche. Based on the past studies and our microtremor observations near the strong motion station, the H/V spectrum in Kathmandu has a peak at around 0.3 Hz, which reflects the velocity contrast of the deep sedimentary basin. The H/V spectra in Bahrabise, Chautara, and Dhunche do not show clear peaks, which suggests that the sites have stiff soil conditions. Therefore, the more severe damage outside the Kathmandu valley compared with the relatively light damage levels in the valley is probably due to the source characteristics of the earthquake and/or the seismic performance of buildings, rather than the local site conditions.

Intraslab earthquakes generate sP phases that are clearly observed at epicentral distances of 60 km or farther in southwestern Japan, while the phases are less easily seen in other regions of Japan. The amplitudes of the sP phase are sometimes larger than those of the direct S , especially at a distance of 130 km or farther. We show that the strong sP -phase arrivals in southwestern Japan are due to the shallow crustal structure of the Chugoku area, at the bounce point of the depth phase. In this study we use sP phases to refine the slab and crustal structures in this region. The comparison of data to synthetic seismograms calculated with existing one-dimensional (1D) models show some discrepancies in terms of the travel times and amplitudes. The synthetics calculated with an improved 1D model better match the observed waveform data. The revised 1D models are used to construct a 2D model that better represents the configuration of the slab and variation of velocity discontinuity depths. For stations where the travel times still show large differences using the improved 1D model, the synthetics calculated with a 2D model better account for the travel times of sP and S waves in southwestern Japan.

[1] Recent three-dimensional (3-D) travel-time tomographic models of southwestern (SW) Japan image the subducting Philippine Sea plate (PHSP) and low velocity anomalies with long-wavelength spatial resolution. However, the agreement between the synthetics calculated with the existing 3-D model and data is not satisfactory forsP and S waves in the higher frequency range (≥0.2 Hz). The unsatisfactory agreement can be attributed to the parameters of the initial wave speed model for the tomographic inversion, as well as the difficulty in detecting S-wave travel times accurately. In order to improve the wave speed model, we first constructed a 3-D reference model by forward modeling using waveform data from an intraslab earthquake and then performed repeated travel-time inversions using the 3-D reference model as the initial model. The newly derived high-resolution model 3DM_SWJ can produce synthetics that are substantially in better agreement with the data than previous models. The distribution of highP-wave toS-wave speed ratios (VP/VS) calculated from model 3DM_SWJ shows short-wavelength heterogeneities that could have been formed in relation to the subduction of the Philippine Sea Plate beneath the Eurasian plate. This combined use of forward waveform modeling for slab and crust configuration and repeated tomographic inversion of travel-times provides a better 3-D structural model of seismic properties for understanding tectonic features related to the subduction process, in addition to better estimating ground motions.

Estimated seismic intensities using conventional relationships are not always in agreement with those observed at specific sites. We investigate subsurface amplification factors using the KiK-net strong-motion data recorded both in the borehole and on the ground surface and attempt to estimate more accurate intensities at a given site. Here the amplification factors are evaluated using the ratios of peak velocity amplitudesVsurf/Vborthat are calculated from records on the ground surface and in the borehole. The ratios (amplification factors) decrease with hypocentral distance in general, but the attenuation characteristics vary from station to station. At stations where large amplifications are observed, the ratios show large scatter relative to the empirical amplification formulas. The variation is also affected by the incident direction of seismic waves as well as the hypocentral distance. We derived empirical amplification formulas ofVsurf/VbedwhereVbedis the peak velocity on the engineering bedrock as a function of hypocentral distance at individual stations (i.e. specific sites) in place of the conventionally used amplification constants. The estimated intensities using the site-effective formulas show substantial improvement at most of the stations in comparison with those calculated using conventional formulas.