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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]

This article investigates the stress drop variability of shallow normal-faulting earthquakes in western Anatolia through strong-motion simulations. For this purpose, source characteristics of three moderate to large magnitude events are constrained by the empirical Green's function simulation in a broadband frequency range. Recordings of ten strong-motion stations in 78-km epicentral distance range are utilized for the simulation. Estimated strong-motion generation areas (SMGAs) are 22 km2, 66 km2, and 110 km2 where rise times are 0.6 s, 0.7 s, and 0.6 s, respectively, for the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw 6.3), and 2017 Bodrum-Kos (Mw 6.6) earthquakes. Those values are found to be consistent with global source-scaling relationships. One particular observation is that stress drop ratios between the mainshock and aftershock for all events are relatively large compared with those previously calculated for strike-slip events in Turkey. Stress drops of SMGAs for the Simav and Bodrum earthquakes are in the range of 25 MPa, and this value drops to 19 MPa for the Lesvos earthquake. To further investigate the stress drop variation of earthquakes in Western Anatolia, an earthquake source database (fc-Mo) offered by Yamanaka et al. (IAG-IASPEI 2017, S07-1-03, 2017) is utilized. Brune (Journal of Geophysical Research, 76:5002, 1971) stress drop values of > 360 small to moderate earthquakes (Mw 3.0–6.0) are calculated with the given corner frequency and seismic moment information assuming a constant shear wave velocity. Results indicate that the majority of the earthquakes have a stress drop value < 5 MPa. This value changes to between 5 and 57 MPa for the remaining earthquakes. These high stress drop values support the former findings stating that normal-faulting earthquakes may release higher stress than strike-slip earthquakes. This indicates that the regional stress regime in western Turkey may cause relatively larger stress release during the normal-faulting mainshocks. [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]

We estimate the wave field in the reclaimed zone of Tokyo Bay by analyzing the three components of surface and downhole strong-motion accelerograms recorded at two borehole stations during five medium earthquakes. The target period range is between 0.1 and 2.0 sec. First, wave types of observed seismic motions are identified by calculating theoretical amplification of body waves and surface waves and cross-correlation functions between the surface and downhole recordings. Then, the horizontal components of the surface and downhole recordings are separated into S-wave and surface-wave accelerograms. Finally, 2D wave-propagation simulations for basin structures are performed to interpret the recordings. The conclusions are as follows. (1) Main seismic motions are composed of not only S waves, but also Love waves. Surface waves are found to be more dominant than S waves in the period range of more than 1.0 sec. (2) Love waves are more heavily amplified than S waves at the theoretical predominant periods of ground in the soft surficial deposits. (3) The large surface-wave amplification is excited by the surficial deposits, together with a deep basin. (4) In the surficial deposits, quality factors are found to be about 10 around the theoretical predominant periods of ground. (5) The wave-propagation simulations suggest strong heterogeneities near the ground surface and can explain observed seismic motions qualitatively. [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]

We investigated the cause of destructive ground motion during the 2018 Hokkaido eastern Iburi earthquake. We conducted strong motion observations of aftershocks and microtremors and the surface wave method in the damaged areas of the town of Mukawa, Hokkaido prefecture, Japan. The ground accelerations were continuously recorded during a period of approximately 3 months after the main shock on September 6, 2018. The heavily damaged buildings were mainly situated around the strong motion station (HKD126) in Mukawa town. Such concentration of damage can be explained by the strong power that was observed in the 1–2 s period of the response spectrum at this station. We estimated the S-wave velocity profiles of this station site and a temporary station site that was installed on a nearby hill. The estimated S-wave velocity, which was inverted from phase velocity structures with the microtremor array and the surface wave method observations explained the difference in the SH-wave amplification characteristics between the two sites. An analysis of HKD126 and the temporarily observed records clearly indicates the strong effects of the local geological conditions on the heavily damaged area of Mukawa. The strong ground motion power generated during the main shock in Mukawa for 1–2 s period was mainly amplified by this shallow underground velocity structure. [ABSTRACT FROM AUTHOR]

The promotion of earthquake research in Japan covers comprehensive basic policies for thepromotion of seismic research through the observation, measurement and survey ofearthquakes. HERP (Headquarter for Earthquake Research Promotion) proposed a frameworkof survey and observation plan of earthquakes in Japan. At the first stage of this framework, itconsists of observation of crustal activities and surveying of underground structures. Nextstep is to improve the methods of predicting strong ground motion and increase accuracy,and thus the improvement of seismic hazard maps. In order to fully capture theseismic hazard of a region, we must understand the effects of the regional 2D/3Dunderground structure on wave propagation. Selection and modification of groundmotion time series to represent a specific hazard at a site has a large impact on theresults of nonlinear response history analyses using these input motions, indicatingthe need to establish rational procedures for ground motion selection, scaling andmodification. Validated seismological methods may be used to generate ground motiontime series that incorporate near-fault rupture directivity effects and basin effects,and appropriately represent the duration and long period energy content of theselarge design events. These effects cannot be fully predicted by simply adoptingGMPEs that are developed for other regions. The main objective of this project is todetermine time-histories for distant and near-field earthquakes through ground motionsimulations. This joint project proposes to investigate the validation of the modeling procedure ofdetermination of geological models proposed by HERP for seismic hazard map in Japan, andoutput of this validation will be applied to the Eskisehir basin in Turkey. We canunderstand the performance of the procedure through the application to the Kumamotoarea by the Japanese side (using HERP procedure), and will be compared with theresults from Eskisehir by Turkish side. We also incorporate the methods used in theJapanese procedure into estimation of geological models in the Eskisehir basin. Inthis frame, we attempt to calculate broadband period ground motion time seriesfrom near-field and far-field earthquakes by using source modeling and groundmotion techniques. The effects of ground motions having a broadband period rangecan be studied in two parts; first one is the rupture directivity effects and secondlybasin edge effects. The prediction and simulation of such ground motions needsource modeling, determination of 2D/3D basin geometry and S-wave velocitystructure. We performed both array- and single-station microtremor measurements at a number oflocations so as to characterize the area to determine 1-D S-wave velocity structures down tothe bedrock depth beneath the strong motion sites for the broadband simulation. Finalgeological models obtained will be verified by seismic interferometry method andmicrogravity measurements. As next step, we make ground-motion simulations to get timehistories to analyze the effects of ground motions to buildings. In spite of increasing numberof buildings having different heights and characteristics in urbanized areas in Turkey, a fewstudies exist on the interaction of such buildings with ground motions having broadbandperiods. [ABSTRACT FROM AUTHOR]

Recent studies suggests that variation in stress drop may be dependent to faulting type. In addition, Akkar et al. (2018) performed residual analyses of strong-motion data compiled from the Marmara and Aegean regions in Turkey and stated that between-event residuals of the Aegean region indicate that the ground motion prediction equation tends to overestimate at 1 s and shorter periods. They explain such a discrepancy may be due to the fact that the regional differences in stress drop were not taken into account in the current ground motion prediction equations. These indications motivate us to examine the stress parameters of the well recorded normal faulting earthquakes. The Aegean region is one of the most rapidly moved and seismically active parts of the world. As a result, continuous seismic activity is observed at the region. We selected and analyzed three earthquakes, the 2011 Simav earthquake (Mw 5.8), the 2017 Lesvos earthquake (Mw 6.3) and the 2017 Bodrum-Kos earthquake, (Mw 6.6) in Western Turkey. Source characteristics of these three normal faulting events are investigated by performing empirical Green's function simulation in broadband frequency range. Relationship between strong motion generation area, rise time, and stress drop as a function of seismic moment are examined. Broadband ground motion simulations of the earthquakes revealed that the stress drop ratio between the mainshock as a large event and the aftershock as a small event, C, is relatively large in comparison to those previously calculated for strike slip events in Turkey. This indicates that the regional stress regime in Western Turkey may affect relative larger stress release during the normal-faulting potential mainshocks. [ABSTRACT FROM AUTHOR]