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Seismic interferometry relates the Green’s function between two observation sites to the cross correlation of recordings at these sites. In the formulation of seismic interferometry to retrieve the exact Green’s function, an assumption that the energy density ratio of S-to-P waves is 2(α/β)3 is used. Thus, understanding how and under what conditions a wavefield satisfies this energy partition ratio is important for these studies. Previous studies revealed that multiple scattering by random inhomogeneities of the medium equilibrates the S- to P-wave energy densities, and the ratio is 2(α/β)3. However, the effect of the free surface on the energy partition ratio is not fully understood. This study analyzes the effect of the free surface to the energy density ratio in a diffuse wavefield. We employ the definition of a diffuse wavefield by a random superposition of plane waves. We consider a wavefield where plane waves travel inside a homogeneous solid with a free surface and repeat P-to-S and S-to-P conversions at the free surface. Under this condition, it is known that the energy density ratio of S-to-P waves tends to a specific value. However, the value proposed in a previous study slightly deviated from 2(α/β)3, and it was argued that the deviation was due to a numerical error. We prove that the ratio exactly equals 2(α/β)3. Our results show that repeated reflections at the free surface as well as scattering in random media equilibrate the energy ratio of S-to-P waves into 2(α/β)3 regardless of the energy partition of the source.

Pulse-like near-source ground motions were observed by the local network during the 2015 Mw 8.3 Illapel, Chile earthquake. Such ground motions can be quite damaging to a wide range of infrastructures. The primary objective of this study is to provide a source model that can explain such ground motions. The isolated nature of the pulses indicated that the rupture of some small isolated region on the fault contributed to the generation of the pulse. Therefore, we considered such regions and termed them as Strong Motion Pulse Generation Areas (SPGAs). We used the corrected empirical Green's function (EGF) method because this method has been successfully applied to near-source pulse-like ground motions in previous studies. We simulated synthetic waveforms using the frequency dependent quality factor Q=239f0.71 and empirical site amplification factors, which we obtained by applying a generalized inversion technique to local weak-motion data. The result indicated that the observed ground motions from the Mw 8.3 Illapel earthquake can readily be explained with a source model that involves two SPGAs with dimensions of several kilometers in spite of the huge rupture zone of the earthquake. The source model can reproduce velocity waveforms, acceleration Fourier amplitude spectra (FAS) and pseudoacceleration response spectra. It also reproduces the duration of strong ground motions quite accurately. No significant bias was found with respect to distance and frequency. In conclusion, the corrected EGF method is a very efficient tool to simulate near-source ground motions of a subduction earthquake when it is combined with higher stress-drop subevents whose sizes are adjusted to the observed pulse widths.

This article describes the methods employed by the author and the results for the ESG6 Blind Prediction Steps 2&3. The target event for Step 2 was an M5.9 earthquake in the Aso region, Japan. To simulate ground motions at the target site “KUMA”, the author used the site-effect substitution method. The Fourier amplitude spectrum was estimated from the spectral ratio between KUMA and a nearby station JMA. The Fourier phase spectrum was approximated by the spectrum of another event at KUMA. Comparison between the estimated and recorded ground motions after the blind prediction revealed that, except for the error in the arrival time, the estimated ground motions were fairly consistent with the observed ground motions, indicating the effectiveness of the site-effect substitution method when the rupture process of the target event is simple and the soil nonlinearity at the target site is not significant. The target events for Step 3 were the M6.5 foreshock and the M7.3 mainshock of the 2016 Kumamoto earthquake sequence. To simulate ground motions at KUMA, the author used a similar method to estimate the Fourier amplitude spectrum, however, the author simply used the Fourier phase spectrum at JMA for the target events. Then, the author conducted effective stress analyses using a program called “FLIP”. The parameters were determined from the PS logging results, N values and fine particle content. Comparison between the estimated and recorded ground motions after the blind prediction indicated that, although the estimated ground motions captured the main phases of the recorded ground motions, the low frequency components were overestimated and the high frequency components were underestimated. The strong soil nonlinearity considered in the effective stress analyses was the main cause of the discrepancy between the estimated and observed ground motions. One explanation for this result could be that the nonlinear soil behavior at KUMA during the foreshock and the mainshock was not a strong one. Another explanation could be that the effect of soil nonlinearity was already included in the records at JMA and the effect of soil nonlinearity was double counted in the results submitted by the author.

The 2019 Mw 7.1 mainshock of the Ridgecrest earthquake sequence, which was the first event exceeding Mw 7.0 in California since the 1999 Hector Mine earthquake, caused near-fault ground motions exceeding 0.5g and 70 cm/s. In this study, the rupture process and the generation mechanism of strong ground motions of the mainshock were investigated through waveform inversions of strong-motion data in the frequency range of 0.2–2.0 Hz using empirical Green’s functions (EGFs). The results suggest that the mainshock involved two large slip regions: the primary one with a maximum slip of approximately 4.4 m was centered ∼3 km northwest of the hypocenter, which was slightly shallower than the hypocenter, and the secondary one was centered ∼25 km southeast of the hypocenter. Outside these regions, the slip was rather small and restricted to deeper parts of the fault. A relatively small rupture velocity of 2.1 km/s was identified. The robustness of the slip model was examined by conducting additional inversion analyses with different combinations of EGF events and near-fault stations. In addition, using the preferred slip model, we synthesized strong motions at stations that were not used in the inversion analyses. The synthetic waveforms captured the timing of the main phases of observed waveforms, indicating the validity of the major spatiotemporal characteristics of the slip model. Our large slip regions are also generally visible in the models proposed by other researchers based on different datasets and focusing on lower frequency ranges (generally lower than 0.5 Hz). In particular, two large slip regions in our model are very consistent with two of the four subevents identified by Ross et al. (2019), which may indicate that part of the large slip regions that generated low-frequency ground motions also generated high-frequency ground motions up to 2.0 Hz during the Ridgecrest mainshock.

Near-fault fling steps might cause severe damage to near-fault structures such as bridges or base-isolated buildings. Therefore, the accurate simulation of ground displacements including fling steps based on fault models is an important issue not only for seismological but also for engineering purposes. The discrete wavenumber (DWN) method (e.g., Bouchon, 2003) has been established as a method to calculate complete elastic wavefield, including permanent displacement for a homogeneous or a layered half-space. However, the accuracy of the permanent displacements calculated by the DWN method is influenced by the selection of parameters, such as the imaginary part of the complex frequency and the subfault size in the case of extended sources. The objective of this study is to clarify the requirement for these parameters for the accurate simulation of fling-step displacements to further enhance the use of the DWN method. Honda and Yomogida (2003) also addressed the issue of calculating fling-step displacements using the DWN method; however, their study was focused on cases in which a large amount of seismic moment is released at depth. This study was focused on fling-step displacements due to rather shallow slip, in which the fault distance was as small as several meters in an extreme case, motivated by recent damaging earthquakes such as the 2016 Kumamoto, Japan, earthquakes. The ground displacements including fling steps were calculated by the DWN method and compared with the analytical solutions for the static displacements (Okada, 1985, 1992), both for point sources and extended sources in a homogeneous half-space. According to the results, following recommendations were made. For the imaginary part of the complex frequency, ωc=ω−λi, λ=ξπ/Tw with ξ≥2.0 can be recommended, with the understanding that the waveforms are effective only within the range of [0,Tw/ξ]. For extended sources, the subfault size should be as small as 0.5 times the fault distance to accurately simulate fling steps.

The pseudo point-source model approximates the rupture process on faults with multiple point sources for simulating strong ground motions. A simulation with this point-source model is conducted by combining a simple source spectrum following the omega-square model with a path spectrum, an empirical site amplification factor, and phase characteristics. Realistic waveforms can be synthesized using the empirical site amplification factor and phase models even though the source model is simple. The Kumamoto earthquake occurred on April 16, 2016, with M JMA 7.3. Many strong motions were recorded at stations around the source region. Some records were considered to be affected by the rupture directivity effect. This earthquake was suitable for investigating the applicability of the pseudo point-source model, the current version of which does not consider the rupture directivity effect. Three subevents (point sources) were located on the fault plane, and the parameters of the simulation were determined. The simulated results were compared with the observed records at K-NET and KiK-net stations. It was found that the synthetic Fourier spectra and velocity waveforms generally explained the characteristics of the observed records, except for underestimation in the low frequency range. Troughs in the observed Fourier spectra were also well reproduced by placing multiple subevents near the hypocenter. The underestimation is presumably due to the following two reasons. The first is that the pseudo point-source model targets subevents that generate strong ground motions and does not consider the shallow large slip. The second reason is that the current version of the pseudo point-source model does not consider the rupture directivity effect. Consequently, strong pulses were not reproduced enough at stations northeast of Subevent 3 such as KMM004, where the effect of rupture directivity was significant, while the amplitude was well reproduced at most of the other stations. This result indicates the necessity for improving the pseudo point-source model, by introducing azimuth-dependent corner frequency for example, so that it can incorporate the effect of rupture directivity.

At present, dense strong motion observation networks have been established in Japan. One of the important findings based on these networks is that strong ground motions are quite site-dependent. Characteristics of observed ground motions at nearby stations can exhibit a significant variation, even when the stations are within several hundreds of meters. These observations raise one important question; if characteristics of strong ground motions exhibit large variations even for smaller regions, we should be concerned about the application of observed or predicted ground motions for the assessment of structures. In particular, if ground-motion parameters such as PGA, PGV, Spectral Intensity, etc., exhibit large variations for smaller regions, their use for the seismic design and practice will be subject to restriction. In other words, the evaluation of variation of these parameters is an important issue. From such a point of view, the authors investigated the variation of observed ground motions within a very small distance in this study. First of all, fifteen couples of adjacent strong motion stations in Japan, where the distance is within 100 m, were listed up based on our field reconnaissance. Then, microtremor measurements were carried out at each of the station pairs. Next, variation of recorded earthquake ground motions for the station pairs was examined based on various ground motion parameters and response spectra. Moreover, we investigated the key factor which is affecting the variation of observed ground motions.

Serious damage due to ground liquefaction occurred in Urayasu City, located in the coastal area around Tokyo Bay, during to the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). In particular, extensive liquefaction was observed in the Imagawa District of Urayasu City. In order to investigate the mechanism of damage and to establish appropriate countermeasures, it is very important to evaluate the strong ground motion in the Imagawa District. For this purpose, aftershock observations and microtremor measurements were carried out in and around the Imagawa District. Then, site effects in the Imagawa District were evaluated based on the microtremor H/V spectral ratios and the aftershock records.

On March 11, 2011, the largest earthquake ever recorded in Japan struck off the coast of the Tohoku region of the country, and was subsequently named the 2011 off the Pacific coast of Tohoku Earthquake by the Japan Metrological Agency. The M w 9.0 earthquake generated strong motions that affected the island of Honshu from Tokyo Bay to the northern extent of the island, and induced a series of tsunamis that devastated coastal communities throughout the region. Significant aftershocks ( M w >7) were experienced that further contributed to damage in the coastal Tohoku region during emergency response and recovery efforts. This paper summarizes the findings of the Port and Airport Research Institute (PARI) research team as it investigated coastal structures along approximately 600 km of coastline as well as the team׳s follow-up experiments and analyses. Two characteristics of the ground motion observed were the long duration and the high-frequency component. For this reason, the degree of damage caused to coastal facilities by the ground motion was relatively small. The wide-ranging investigation by the PARI team facilitated the interpretation of damage patterns across the entire region affected by the earthquake, with a primary goal of distinguishing damage to coastal structures resulting from strong ground shaking and secondary effects (such as liquefaction, ground failures, and settlement) from that caused by the subsequent and significant tsunami inundation.

Serious damage due to soil liquefaction occurred in a wide area of the Tohoku and the Kanto regions in Japan during 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0). The farthest liquefied site from the epicenter of the 2011 main shock is Ikenouchi, Minamiboso City, whose epicentral distance is approximately 440 km. Evaluation of strong ground motions at Ikenouchi is very important to understand why liquefaction was observed in such a far site. For this purpose, in-situ geotechnical investigations including aftershock observation were carried out at Ikenouchi. Then, the ground motions at Ikenouchi during the 2011 main shock was estimated based on the site-effect substitution method considering the microtremor H/V spectral ratios and the aftershock observation results. Finally, 3-D effective stress analyses were carried out using a FEM code ‘FLIP’ to understand how site-specific characteristics of strong ground motions affected the behavior of the ground including liquefaction. A striking feature of the estimated waveforms at the target site is that the waveforms include a much larger number of cycles compared to the records at permanent observation stations around the target site. Such a site-specific feature of strong ground motions was one of the main causes of the occurrence of liquefaction 440 km away from the epicenter. The result suggests the importance of taking into account not only soil properties but also site-specific characteristics of strong ground motions for a rational assessment of liquefaction for future large earthquakes.

In this study, the rupture process of the main shock of the Kumamoto earthquake, particularly the generation of strong ground motions in the frequency range relevant to structural damage, was investigated based on the inversion of strong ground motions. Strong-motion records in the near-source region were mainly utilized because the authors were interested in the generation mechanism of damaging ground motions in the near-source region. Empirical Green’s functions (EGFs) were applied to avoid uncertainty in the subsurface structure model. Four cases of inversions with different combinations of small events were used to investigate the dependence of the inversion results on the selection of the small events. It was found that the dependence of the final slip distribution and peak slip velocity distribution on the selection of the EGF events is small. The results clearly indicate that a region of significantly large slip and slip velocity existed approximately 15 km northeast of the hypocenter. However, no “asperity” was observed between the hypocenter and Mashiki. Thus, it is not appropriate to conclude that the large-amplitude pulse-like ground motion in Mashiki was generated by the forward-directivity effect associated with the rupture of an asperity. As far as the source effect is concerned, the ground motion in Mashiki cannot be interpreted as the worst case scenario. On the other hand, the rupture of the “asperity” 15 km northeast of the hypocenter should have caused significantly large ground motions in regions close to the asperity. The significant damage of highway bridges in the region can potentially be attributed to the rupture of the asperity. The result of this study was compared with an inversion result obtained from numerical Green’s functions for a layered half-space. The two results share the main features in spite of the difference of the Green’s functions and stations used. Therefore, it can be concluded that these two source models capture the main features of the rupture process of the earthquake.

Damage at airports has been frequently reported as a result of large-scale earthquakes. This study focused on seismic damage to an airport runway due to ground shaking. The runway at Noto Airport in Japan developed cracks during the 2007 Noto Hanto Earthquake (Mw 6.7). Post-event evaluation of this crack occurrence using numerical analysis was used to evaluate the seismic performance of the runway embankment and to quantify the applicability of the proposed method. As a first step, strong motion at Noto Airport was evaluated based on the empirical site amplification and phase effects. Second, a 3-dimensional seismic response analysis for the runway, embankment and bedrock below the runway was carried out. Finally, based on the results, effective indices to evaluate the crack occurrence along the airport runway due to the earthquake were examined. The examination results enable the rational assessment of the seismic performance of airport runways in future practice.

2004年新潟県中越地震では, 地すべりが数多く発生した。震源域の東竹沢では, 大規模な地すべりが発生するなどの甚大な地震被害が発生しており, 東竹沢地すべりにおける強震動を, 地盤震動特性を考慮して精度良く推定することは非常に重要である。そこで本稿では, 経験的サイト増幅・位相特性を考慮した強震動評価手法を用いて, 東竹沢地すべりにおける強震動を推定した結果について報告する。その際, 東竹沢地すべりにおけるサイト特性を (新) 木篭公民館での地震観測記録に基づいて評価することで, 既存強震観測点のある山古志支所との地盤震動特性の差異を明確にした。

2011年東北地方太平洋沖地震(MW9.0)では，震源から420km離れた横浜市金沢区の柴町集合住宅において，地下ピット式駐車場の浮上がりなどの深刻な液状化被害が発生した．そこで本研究では，柴町集合住宅などでの常時微動計測結果および横浜市高密度強震計ネットワークによる観測記録などに基づいて，柴町集合住宅におけるサイト特性を評価した．そして，サイト特性置換手法を用いて柴町集合住宅(液状化範囲)の工学的基盤相当での地震動を推定した．その結果，柴町集合住宅地内における常時微動H/Vスペクトルの特性は，旧海岸部の地形との関係性が高いこと，柴町集合住宅に近い幸浦消防署や土木事務所における本震観測記録の転用は，柴町集合住宅地内(液状化範囲)での本震時の地震動を過小に評価している可能性が高いことなどを示した．

After a large earthquake, it is often required to estimate strong ground motions during the earthquake at a site where an engineering structure was damaged. In this article, a new practical method of strong-motion estimation is proposed, based on aftershock records both at the site of interest and at a nearby strong-motion station and on the record of the large event at the strong-motion station. The method is simply composed of three steps. First, the Fourier amplitude at the site of interest for the large event is evaluated by correcting the Fourier amplitude at the strong-motion station for the difference of the path effect and the site amplification factors at the two sites. Then, the Fourier phase at the site of interest during the large event is approximated by the Fourier phase for a small event that occurred close to the main rupture area of the large event. Finally, an inverse Fourier transform is conducted to obtain the time history of strong ground motions at the site of interest during the large event. The method is applied to strong-motion records during the 11 August 2009 Suruga Bay, Japan, earthquake ( M w 6.2) to confirm its applicability. An obvious advantage of the proposed method is its simplicity. Unlike full strong-motion simulations, it does not require fault models. Therefore, the method will contribute to a prompt estimation of strong ground motions at a site of engineering interest after a large earthquake. At the same time, unlike conventional simple techniques, the present method is focusing not only on the spectral amplitude but also on the Fourier phase. Therefore, it leads to high-quality estimation of strong ground motions.

A great geo-disaster occurred in the 2008 Iwate-Miyagi Nairiku Earthquake. Therefore, evaluation with high accuracy of the strong seismic motion at the damaged sites is very important to analyze the failure mechanism. However, there are no strong motion observation stations near the watercourse closure sites during this earthquake. In this study, the seismic waveforms at Ichinonohara, Nuruyu and Sakashita, where watercourse closure took place, were estimated based on empirical site amplification and phase effects. The estimated seismic motions will be useful for rational safety assessment of natural slopes.

設計入力地震動の合理的な評価を行うためには，深層地盤による増幅特性を適切に考慮する必要がある．強震観測が行なわれている地点ではスペクトルインバージョンによって増幅特性を評価することが可能であるが，強震観測の行なわれていない地点では別の何らかの方法で増幅特性を推定する必要がある．本研究では，強震観測が行なわれていない地点のうち，常時微動観測によって堆積層が薄いと判断される地点を対象に，増幅特性を簡易に評価する方法を検討した．提案する方法は低周波数側の振幅をエリア別に評価し，これに堆積層の地盤データに基づく伝達関数を考慮するものである．

シールドトンネルなど地下構造物の縦断方向の耐震検討では，構造物に沿った地震波のみかけの伝播速度の評価が重要である．既往の研究では表面波の位相速度を利用することで安全側の評価を行う方法などが提案されているが，地盤の軸ひずみに関して実状に即した評価を行うことを目的とする場合に，みかけの伝播速度としてどの程度の値を用いるべきであるかについては，これまで十分な検討が行われてこなかった．そこで，本研究では，M5.3～M8.0の13の地震に対するシールドトンネルにおける地震観測結果，および，セグメントリング間の相対変位の計測結果に基づき，地震波の見かけの伝播速度に関する検討を行い，見かけ入射角依存性など，見かけの伝播速度に関するいくつかの特徴を明らかにした．

想定地震に対する地震動の評価精度を高めることは地震災害対策を進める上で極めて重要な課題である．この点に関して著者らは経験的サイト増幅・位相特性を考慮した強震動評価手法の適用に関する研究をすすめており，震源モデルやサイト増幅特性の設定が適切であれば，本手法により既往の強震記録を再現できることを明らかにしてきている．しかしながら，これまでの方法には，ある時刻t0に対してt

A waveform inversion study was conducted to reveal the rupture process of the 2007 Noto Hanto, Japan, earthquake using near-source strong ground motion records. To avoid uncertainty in the subsurface structure in and around the source region when calculating Green’s functions, I used the aftershock records as empirical Green’s functions. To ensure that the path and the site effects are shared between the ground motions from the mainshock and those from aftershocks, the mainshock fault plane was divided into four domains, each of which was allocated to one of the aftershocks used as the empirical Green’s function events. The result of the inversion indicated that the rupture process included the break of two asperities. The first asperity, located close to the JMA hypocenter, was broken soon after the rupture started. This was followed by an extension of the rupture and the subsequent break of the second shallow asperity. The total moment of the mainshock was estimated to be 1.96E+19 N m, which is roughly consistent with the moment estimated by the F-net (1.36E+19 N m). Agreement between the observed and synthetic ground motions is quite satisfactory.

A waveform inversion with empirical Green's functions was conducted to estimate strong-motion generation areas (SMGAs) of the 2003 Tokachi-oki, Japan, earthquake (Mw 8.3). Although the rupture process of this great subduction-zone earthquake has been revealed with waveform inversions based on low-frequency (lower than 0.2 Hz) ground motions, it is much more important from an engineering point of view to investigate how strong ground motions with higher frequencies were generated from the earthquake. Waveform data with higher frequencies have been excluded from the conventional inversions because of the difficulty in computing realistic theoretical Green's functions at higher frequencies. In this study, with the aid of empirical Green's functions, we extended the applicability of the waveform inversion to higher-frequency ground motions up to 1 Hz. We selected three after- shocks for use in the analysis, referring to the similarity of the group delay time be- tween the mainshock and the aftershock records. The resultant slip model has three SMGAs, each of which is close to the hypocenter of one of the aftershocks used. Gen- erally speaking, locations of the SMGAs identified in this study agree well with as- perities identified from the inverted results using low-frequency (lower than 0.2 Hz) ground motions plus geodetic data and tsunamis. It implies that strong ground motions up to 1 Hz were generated from almost the same asperities producing lower-frequency ground motions. In terms of the complexity of slip models, although our analysis is focused on high frequencies, our slip model is at least as simple as conventional low- frequency slip models. Such results would be useful in constructing source models of future great subduction-zone earthquakes for strong-motion prediction.