Kathmandu Basin as a local modulator of seismic waves: 2D modelling of nonlinear site response under obliquely incident waves

The 2015 Mw 7.8 Gorkha, Nepal earthquake is the largest event to have struck the capital city of Kathmandu in recent times. One of its surprising features was the frequency content of the recorded ground motion, exhibiting a notable amplification at low frequencies (< 2 Hz) and a contrasting depletion at higher frequencies. The latter has been partially attributed to the damper behaviour of the Kathmandu basin. While such weak high-frequency ground motion helped avoiding severe damage in the city, the catastrophic outcomes of earlier earthquakes in the region attest to a contrasting role of the Kathmandu basin as a broadband amplifier, in addition to possible source effects. Given the possibility of future strong events in the region, our main objective is to elucidate the seismic behaviour of the Kathmandu basin by focusing on site effects. We numerically model 2D P-SV wave propagation in a broad frequency band (up to 10 Hz), incorporating the most recent data for the Kathmandu basin geometry, soil stratigraphy and geotechnical soil properties, and accounting for the non-linear effect of multi-dimensional soil plasticity on wave propagation. We find that: 1) the Kathmandu basin generally amplifies low frequency ground motion (< 2 Hz); 2) waves with large incidence angles relative to vertical can dramatically amplify the high frequency ground motion with respect to bedrock despite the damping effect of soil nonlinearity; 3) the spatial distribution of peak ground motion amplitudes along the basin is highly sensitive to soil nonlinearity and wave incidence (angle and direction), favoring larger values near the basin edges located closer to the source, as observed during the 2015 event. Our modelling approach and findings can support the ongoing resilience practices in Nepal and can guide future seismic hazard assessment studies for other sites that feature similar complexities in basin geometry, soil stratigraphy and dynamic soil behaviour.

Effects of Nonlinear Soil Behavior on Kappa (κ): Observations from the KiK-Net Database

ABSTRACT Soil nonlinear behavior is often triggered in soft sedimentary deposits subjected to strong ground shaking and has led to catastrophic damage to civil infrastructure in many past earthquakes. Nonlinear behavior in soils is associated with large shear strains, increased material damping ratio, and reduced stiffness. However, most investigations of the high-frequency spectral decay parameter κ, which captures attenuation, have focused on low-intensity ground motions inducing only small shear strains. Because studies of the applicability of the κ model when larger deformations are induced are limited, this article investigates the behavior of κ (both κr per record and site-specific κ0 estimates) beyond the linear-elastic regime. About 20 stations from the Kiban–Kyoshin network database, with time-average shear-wave velocities in the upper 30 m between 213 and 626 m/s, are used in this study. We find that the classification scheme used to identify ground motions that trigger soil nonlinear behavior biases estimates of κ0 in the linear and nonlinear regimes. A hybrid method to overcome such bias is proposed considering proxies for in situ deformation (via the shear-strain index) and ground shaking intensity (via peak ground acceleration). Our findings show that soil nonlinearity affects κr and κ0 estimates, but this influence is station dependent. Most κ0 at our sites had a 5%–20% increase at the onset of soil nonlinear behavior. Velocity gradients and impedance contrasts influence the degree of soil nonlinearity and its effects on κr and κ0. Moreover, we observe that other complexities in the wave propagation phenomenon (e.g., scattering and amplifications in the high-frequency range) impose challenges to the application of the κ0 model, including the estimation of negative values of κr.

The effect of soil nonlinearity on high-frequency spectral decay and implications for site response analysis

This study uses recorded ground motions at soil sites over a range of shaking intensities to investigate the effects of soil nonlinearity on the high-frequency spectral decay, as quantified by the parameter [Formula: see text]. Equivalent-linear site response analyses indicate that [Formula: see text] should increase significantly with increasing shear strain and ground motion intensity due to increases in soil damping. However, using more than 2500 motions from 32 sites, this study shows that [Formula: see text] does not vary systematically with the induced shear strain but instead remains at its small-strain value. This observation indicates that high-frequency components of motion are consistent with small-strain damping, rather than the strain-compatible damping used in site response analysis. It is demonstrated that equivalent-linear site response analyses for large strains can be modified to generate surface motions with more realistic high-frequency content by scaling the predicted surface motion to fit the small-strain [Formula: see text] or by employing frequency-dependent soil properties that account for the frequency dependence of the induced strains.

Прогресс исследований за последние годы нелинейных свойств грунтов за последние годы в России и на Тайване

Soil nonlinearity has a significant influence on result seismic effect at strong motions which differ from weak and moderate ones. Practice of construction faced with adequate account of nonlinear effect in weak soils and demand techniques for design parameters assessment. Researches of recent years in the field of soil nonlinearity may enrich each other and find the main way for effective practices and building codes regularization. The aim of this work is allocation of parameters for nonlinearity description and corresponding techniques development. Methods.Field soil response analysis with sources of different power in conjunction with strong motion records were analyzed by means of regression analysis and other machine learning techniques. Mathematical modeling includes multiple reflected waves analysis technique and finite elements modeling. Results. The parameters that are closely related to the absorption and soil nonlinearity were identified. The empirical formulas connecting the areas of normalized and real spectra with the parameters of seismic loadings were obtained using regression analysis. The differences of absorption mechanism in dispersed (soft) and rocky soils were defined. Conclusion.The models of ground strata behavior in the case of variable intensity of dynamic action on the basis of consideration of the real area of the spectrum and the average value of the frequency, characterized by a linear and nonlinear elastic-inelastic deformation of the soils are offered. Degree of nonlinearity (DNL) metric may be efficiently used for stress-strain curve assessment, and in the absence of strong earthquakes records it can be applied for powerful seismic sources records analysis what determines the direction of future research. Нелинейные свойства грунтов оказывают существенное влияние на результат сейсмического воздействия при сильных движениях, которые отличаются от слабых и умеренных. Практика строительства столкнулась с необходимостью адекватного учета эффекта нелинейности в слабых грунтах и методики оценки проектных параметров. Исследования нелинейных свойств грунтов проведенные за последние годы могут обогатить друг друга и найти основной путь для эффективной практики и регуляризации строительных норм. Целью работы является выделение параметров для описания нелинейности и разработки соответствующих методик. Методы исследования. Был проанализирован отклик грунта в полевых условиях с источниками различной мощности в сочетании с записями сильных движений с помощью регрессионного анализа и других методов машинного обучения. Математическое моделирование включает в себя метод анализа многократно отраженных волн и моделирование методом конечных элементов. Результаты. Были определены параметры, тесно связанные с поглощением и нелинейностью грунтов. С помощью регрессионного анализа были получены эмпирические формулы, связывающие области нормированного и реального спектров с параметрами сейсмических нагрузок. Выявлены различия механизма поглощения в дисперсных (рыхлых) и каменистых грунтах. Выводы. Предложены модели поведения наземных пластов при переменной интенсивности динамического воздействия на основе учета реальной площади спектра и среднего значения частоты, характеризующейся линейной и нелинейной упруго-неупругой деформацией грунтов. Критерий степени нелинейности (DNL) может быть эффективно использован для оценки кривой напряжения-деформации, а в отсутствие записей сильных землетрясений он может применяться для анализа записей мощных сейсмических источников, определяющих направление будущих исследований.

A Site Amplification Model for Crustal Earthquakes

A global dataset which is composed of more than 20,000 records is used to develop an empirical nonlinear soil amplification model for crustal earthquakes. The model also includes the deep soil effect. The soil nonlinearity is formulated in terms of input rock motion and soil stiffness. The input rock motion is defined by the pseudo-spectral acceleration at rock site condition (PSArock) which is also modified with between-event residual. Application of PSArock simplifies the usage of the site model by diminishing the need of using the period-dependent correlation coefficients in hazard studies. The soil stiffness is expressed by a Gompertz sigmoid function which restricts the nonlinear effects at both of the very soft soil sites and very stiff soil sites. In order to surpass the effect of low magnitude and long-distant recordings on soil nonlinearity, the nonlinear site coefficients are constrained by using a limited dataset. The coefficients of linear site scaling and deep soil effect are obtained with the full database. The period average of site-variability is found to be 0.43. The sigma decreases with decreasing the soil stiffness or increasing input rock motion. After employing residual analysis, the region-dependent correction coefficients for linear site scaling are also obtained.