Ground motion prediction equation for the Kathmandu Valley, Nepal based on strong motion records during the 2015 Gorkha Nepal earthquake sequence

Single-site ground motion prediction equations (GMPEs) for the acceleration response spectra of each site in the Kathmandu Valley was constructed using strong motion records of magnitude 5.0 through 7.3 the 2015 Gorkha Nepal earthquake aftershocks observed at eight sites in the Kathmandu Valley. The regression coefficient for the site term has a strong correlation with the bedrock depth at each site in the Kathmandu Valley. Therefore, a new GMPE applicable to the whole Kathmandu Valley in the long-period range of 1-10 s was generalized using the bedrock depth as a parameter. We applied this GMPE to the largest aftershock. Consequently, at each sedimentary station, the residuals of the predicted value by GMPE are smaller than those predicted by the existing GMPE, and the peaks of the observed response spectra are reproduced well

Examination of one-dimensional S-wave velocity structure using strong-motion data for high-seismic-intensity area during the 2018 Hokkaido Eastern Iburi earthquake

The Yufutsu Plain, a sedimentary basin surrounded by mountains, is located in the southern part of the Ishikari–Yufutsu Lowlands, Hokkaido, Japan. The Hidaka arc–arc collision zone, located in the eastern part of the Yufutsu Plain, forms the Hidaka Mountain range in central Hokkaido, with the Ishikari–Teichi–Toen Fault Zone of the Ishikari–Yufutsu Lowlands on the west side, which forms part of a major geological boundary that extends in the north–south direction. The 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6) occurred at 03:08 JST on September 6, 2018, in this arc–arc collision zone. The K-NET HKD126 station in Mukawa Town, which is close to the severely damaged basin margin area, recorded strong-motion data with strong power for a predominant frequency of 0.5–1.0 Hz during the main shock. The peak ground acceleration was 661 cm/s2 in the east–west direction. The site amplification characteristics of the shallow S-wave velocity structure, which was estimated from microtremor array observations and surface wave explorations, were one of the causes of this strong ground motion. It is essential to accurately estimate the depth of the seismic bedrock and basin margin to evaluate the long period of large earthquakes. In this study, we used strong-motion data recorded at HKD126 and three temporary strong-motion stations near the basin margin area to tune the deep S-wave velocity structure. First, we performed microtremor array observations and surface wave explorations to estimate the S-wave velocity structure to a depth of 1 km beneath the station at the hill site because a detailed shallow structure is not available for this site. Then, with a combination of the estimated S-wave velocity structure and the existing structure, we tuned the deep structures based on an autocorrelation function analysis using strong-motion data. The validity of the estimated structures from the shallowest depth to the seismic bedrock was verified based on the differences between the observed arrival time difference and theoretical travel time difference for the S-wave initial motion. We estimated the seismic bedrock of the four stations to be at a depth of 7–10 km. In addition, an autocorrelation function analysis suggests topological bedrock undulations

Strong-Motion Characteristics and Visual Damage Assessment Around Seismic Stations in Kathmandu After the 2015 Gorkha, Nepal, Earthquake

A rapid visual damage assessment of buildings around four strong-motion seismic stations in Kathmandu Valley was carried out after the damaging Gorkha, Nepal earthquake (Mw7.8) of 25 April 2015. The waveforms of the main shock recorded at these stations were compared with the damage to buildings around the stations. The damage was found to be related to strong-motion characteristics of the earthquake. A dominance of long-period oscillation could be observed in the records. The damage to low-rise buildings in the valley was less than anticipated from an earthquake of this magnitude given that the majority of buildings were built without proper engineering consideration. The acceleration response spectra of one of the sedimentary sites show high response in the 1–2 s period range, and nearly 10% of the buildings, which were all low-rise, suffered damage around this site

Examination of shallow and deep S-wave velocity structures from microtremor array measurements and receiver function analysis at strong-motion stations in Kathmandu basin, Nepal

The Himalayan collision zone, where the Indian Plate subducts beneath the Eurasian Plate at a low angle, has caused many devastating earthquakes. The Kathmandu basin, situated in this region, is surrounded by mountains on all sides and is filled with distinct soft lake sediments with a highly undulating bedrock topography. The basin has been experi- encing rapid urbanization, and the growing population in its major cities has increased the vulnerability to seismic risk during future earthquakes. Several strong-motion stations have recently been deployed in the Kathmandu basin. It is expected that the data captured by this strong-motion station array will further enhance our understanding of site amplification in sedimentary basins. Clear P-to-S converted waves have been observed in the strong-motion records. In this study, we investigate the medium boundary that generated these converted waves. First, we estimate the shal- low velocity structures, which correspond to the topographic slopes or surface geology, beneath the strong-motion stations. We then apply a receiver function analysis to the strong-motion records. The receiver function indicates that the interface between the soft sediment and seismic bedrock serves as a boundary that generates converted waves. The obtained results can be used for tuning three-dimensional velocity structures

Estimation of 1-D velocity models beneath strong-motion observation sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes

The Himalayan collision zone experiences many seismic activities with large earthquakes occurring at certain time intervals. The damming of the proto-Bagmati River as a result of rapid mountain-building processes created a lake in the Kathmandu Valley that eventually dried out, leaving thick unconsolidated lacustrine deposits. Previous stud-ies have shown that the sediments are ~600 m thick in the center. A location in a seismically active region, and the possible amplification of seismic waves due to thick sediments, have made Kathmandu Valley seismically vulnerable. It has suffered devastation due to earthquakes several times in the past. The development of the Kathmandu Valley into the largest urban agglomerate in Nepal has exposed a large population to seismic hazards. This vulnerability was apparent during the Gorkha Earthquake (Mw7.8) on April 25, 2015, when the main shock and ensuing aftershocks claimed more than 1700 lives and nearly 13% of buildings inside the valley were completely damaged. Preparing safe and up-to-date building codes to reduce seismic risk requires a thorough study of ground motion amplification. Characterizing subsurface velocity structure is a step toward achieving that goal. We used the records from an array of strong-motion accelerometers installed by Hokkaido University and Tribhuvan University to construct 1-D velocity models of station sites by forward modeling of low-frequency S-waves. Filtered records (0.1–0.5 Hz) from one of the accelerometers installed at a rock site during a moderate-sized (mb4.9) earthquake on August 30, 2013, and three moderate-sized (Mw5.1, Mw5.1, and Mw5.5) aftershocks of the 2015 Gorkha Earthquake were used as input motion for modeling of low-frequency S-waves. We consulted available geological maps, cross-sections, and borehole data as the basis for initial models for the sediment sites. This study shows that the basin has an undulating topography and sediment sites have deposits of varying thicknesses, from 155 to 440m. These models also show high velocity contrast at the bedrock depth which results in significant wave amplification

応答スペクトルの単一サイト予測式（SS-GMPE）：三陸沖アウターライズ地震における検討

We studied characteristics of strong ground motions from the Off-Sanriku outer-rise earthquakes; these earthquakes are activated after the 2011 Tohoku earthquake (Mw 9.0). First, we calculated pseudo velocity responses for five outer-rise earthquakes (Mw 6.0-7.6) and compared them with the attenuation formulae which were constructed for the intraslab earthquake. In spite of outer-rise's data being over the distance range of these formulae, the extrapolated formulae generally explained the attenuation relations of these observed data. However, there were considerable residuals between the observed and predicted values. These residuals result from the site and source effects, because the previous attenuation formulae were constructed based on many site's strong motion records from many earthquakes occurring at various source areas. Second, in order to overcome the weakness of the previous attenuation equations, we proposed empirical ground motion prediction equations for a single site; we named these equations “Single-Site GMPE (SS-GMPE)”. Our data for regression analysis were strong motion records at a single observation site from the five outer-rise earthquakes occurring at the narrow source area with the radius of about100km. We carried out a regression analysis with respect to Mw for each site assuming the previously estimated internal attenuation for intraslab earthquakes. The earthquake source, propagation path and site effects were well incorporated into these equations, and as a result, the residuals between the observed and predicted values were considerably decreased. Finally, we applied the SS-GMPE to earthquakes not used in the regression analysis and found the good correspondence of predicted and observed spectra.本論文は、三陸沖で発生するアウターライズ地震による地震動特性を研究したものである。最初に、5つのアウターライズ地震（MW 6.0-7.6）による擬似速度応答スペクトルに対して、火山フロントを境にした伝播経路特性を考慮した既往のスラブ内地震を対象とする距離減衰式による予測値と観測値との比較を行った。本研究のデータセットの距離範囲はこの距離減衰式の適用範囲外であるが、距離を外挿したこの距離減衰式はアウターライズ地震における観測値を定性的に説明している。しかし、依然として予測値と観測値は大きな残差を有しており、この残差にはサイトの地盤増幅特性、震源特性が含まれると考えられる。これは、用いた距離減衰式に限らず、一般的な既往の距離減衰式が、複数の地域で発生した多くの地震からの複数サイトの記録を用いた回帰分析によって作成されているためである。そこで、本研究では、三陸沖の特定の震源域を対象とした単一サイトにおける地震動予測式（SS-GMPE:Single-Site Ground Motion Prediction Equations）を提案した。ここで提案した三陸沖アウターライズ地震に対するSS-GMPEは、サイトごとに擬似速度応答スペクトルを予測するもので、MWと震源距離をパラメターとした回帰分析によって構築した。構築した予測式には、伝播経路特性、サイト特性、震源特性が適切に取り込まれており、回帰に含まれない地震を対象とした検証の結果、残差の大幅な低減が確認できた

Joint inversion of teleseismic, geodetic, and near-field waveform datasets for rupture process of the 2015 Gorkha, Nepal, earthquake

The 2015 Gorkha earthquake and its aftershocks caused severe damage mostly in Nepal, while countries around the Himalayan region were warned for decades about large Himalayan earthquakes and the seismic vulnerability of these countries. However, the magnitude of the Gorkha earthquake was smaller than those of historical earthquakes in Nepal, and the most severe damage occurred in the north and northeast of Kathmandu. We explore reasons for these unexpected features by performing a joint source inversion of teleseismic, geodetic, and near-field waveform datasets to investigate the rupture process. Results indicate that the source fault was limited to the northern part of central Nepal and did not reach the Main Frontal Thrust. The zone of large slip was located in the north of Kathmandu, and the fault rupture propagated eastward with an almost constant velocity. Changes in the Coulomb failure function (ΔCFF) due to the Gorkha earthquake were computed, indicating that southern and western regions neighboring the source fault are potential source regions for future earthquakes related to the Gorkha earthquake. These two regions may correspond to the historical earthquakes of 1866 and 1344. Possible future earthquakes in the regions are predicted, and the warning for Himalayan seismic hazards remains high even after the Gorkha earthquake

Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake

On 25 April 2015, a large earthquake of Mw 7.8 occurred along the Main Himalayan Thrust fault in central Nepal. It was caused by a collision of the Indian Plate beneath the Eurasian Plate. The epicenter was near the Gorkha region, 80 km northwest of Kathmandu, and the rupture propagated toward east from the epicentral region passing through the sediment-filled Kathmandu Valley. This event resulted in over 8000 fatalities, mostly in Kathmandu and the adjacent districts. We succeeded in observing strong ground motions at our four observation sites (one rock site and three sedimentary sites) in the Kathmandu Valley during this devastating earthquake. While the observed peak ground acceleration values were smaller than the predicted ones that were derived from the use of a ground motion prediction equation, the observed peak ground velocity values were slightly larger than the predicted ones. The ground velocities observed at the rock site (KTP) showed a simple velocity pulse, resulting in monotonic-step displacements associated with the permanent tectonic offset. The vertical ground velocities observed at the sedimentary sites had the same pulse motions that were observed at the rock site. In contrast, the horizontal ground velocities as well as accelerations observed at three sedimentary sites showed long duration with conspicuous long-period oscillations, due to the valley response. The horizontal valley response was characterized by large amplification (about 10) and prolonged oscillations. However, the predominant period and envelope shape of their oscillations differed from site to site, indicating a complicated basin structure. Finally, on the basis of the velocity response spectra, we show that the horizontal long-period oscillations on the sedimentary sites had enough destructive power to damage high-rise buildings with natural periods of 3 to 5 s

Aftershock activity of the 2015 Gorkha, Nepal, earthquake determined using the Kathmandu strong motion seismographic array

The characteristics of aftershock activity of the 2015 Gorkha, Nepal, earthquake (Mw 7.8) were evaluated. The mainshock and aftershocks were recorded continuously by the international Kathmandu strong motion seismographic array operated by Hokkaido University and Tribhuvan University. Full waveform data without saturation for all events enabled us to clarify aftershock locations and decay characteristics. The aftershock distribution was determined using the estimated local velocity structure. The hypocenter distribution in the Kathmandu metropolitan region was well determined and indicated earthquakes located shallower than 12 km depth, suggesting that aftershocks occurred at depths shallower than the Himalayan main thrust fault. Although numerical investigation suggested less resolution for the depth component, the regional aftershock epicentral distribution of the entire focal region clearly indicated earthquakes concentrated in the eastern margin of the major slip region of the mainshock. The calculated modified Omori law’s p value of 1.35 suggests rapid aftershock decay and a possible high temperature structure in the aftershock region