Bedrock Ground Motion Research Articles

Characteristic period of response spectrum for surface and borehole ground motions

Determining the characteristic period of ground motions (Tg) is an essential step in the seismic design of structures, but a critical problem that needs to be solved is figuring out how to do so when bedrock ground motion is utilized as an input. In this paper, the relationship between surface and borehole temperature data is investigated, and the main factors affecting the Tg are analyzed. Then, the algorithm for predicting Tg is presented. It is found that two factors, i.e., peak ground acceleration (PGA) and fault distance have significant effects on the value of Tg. By comparing the values of Tg for borehole and surface motions, we find that, contrary to expectation, Tg for borehole motions is usually larger than that for surface motions in most cases. Finally, the conversion equation between the Tg for borehole and surface motions is given using regression analysis.

Machine learning based ground motion site amplification prediction

Site condition impact on seismic ground motion has been a complex but important subject in earthquake hazard analysis. Traditional studies on site amplification effect are either based on site response via wave propagation simulation or regression analysis using parameters such as Vs30, bedrock ground motion and site response period. Ground Motion Prediction Equations (GMPEs) are used for regions where there is limited data of seismic records. The main issues with these approaches are that they cannot demonstrate the complex relationship between site amplification and its various affecting parameters, thus there exists large uncertainty in the results. Recent studies based on machine learning have shown significant improvement in predicting the site amplification, but the result is not well explained. This study assembled the information on 6 parameters including Vs30, magnitude, epicentral distance, earthquake source depth, bedrock ground motion, and altitude of 353,327 records observed during 1997 and 2019 from 698 KiK-net stations. Three machine learning algorithms of Random Forest (RF), XGBoost, and Deep Neural Networks (DNN) were implemented to predict the site amplification factor using these 6 selected parameters. Shapley Additive explanation (SHAP) was used to explain the importance of the 6 parameters. The results show that all three machine learning algorithms performed much better than the traditional GMPE approach with XGBoost’s performance the best. The explanation provided by the SHAP analysis further enhanced the reasonability of this study. It is anticipated that the combination of machine learning and SHAP analysis can provide better assessment for site amplification of ground motion with better potential of future application in seismic hazard analysis.

Earthquake Risk Probability Evaluation for Najin Lhasa in Southern Tibet

The probabilistic seismic hazard analysis (PSHA) method is effectively used in an earthquake risk probability evaluation in seismogenic regions with active faults. In this study, by focusing on the potential seismic source area in Najin Lhasa, southern Tibet, and by incorporating the PSHA method, we determined the seismic activity parameters and discussed the relationship of ground motion attenuation, the seismic hazard probability, and the horizontal bedrock ground motion acceleration peak value under different transcendence probabilities in this area. The calculation results show that the PSHA method divides the potential source area via specific tectonic scales and detailed tectonic markers, which reduces the scale of the potential source area and better reflects the uneven spatial distribution of seismic activity in the vicinity of Najin. The corrected attenuation relationship is also in line with the actual work requirements and is suitable for earthquake risk analysis. In addition, the major influences on the peak acceleration of ground motion in the study area are mainly in the potential source areas of Qushui (M7.5), Dangxiong (M8.5), and Kangma (M7.5). The peak horizontal ground motion acceleration (PGA) with a transcendence probability of 10% in 50 years is 185.9 cm/s2, and that with a transcendence probability of 2% in 50 years is 265.9 cm/s2.

Seismic responses of sea-crossing isolated continuous beam bridges considering seabed–pile–seawater–bridge interaction

To consider the multiple interactions between offshore sites and sea-crossing bridges under seismic actions, a seabed–pile–seawater–bridge interaction model is established by the ADINA software [1]. This study uses seismic fluctuation analysis as an input to bedrock ground motion. It reflects the influence of submarine sites and seawater layers on ground motion characteristics through the fluid-solid coupling of seawater to the seabed. It was found that the whole bridge piers considering the coupling effect are in an elastic state. The yielding of the isolated bearings occurs. Also, the seismic response of the bridge structure under the seabed-pile-seawater-bridge interaction is significantly higher than those under the direct input of the ground motions from the fixed basement assumption. In addition, when the pile cap is above the seabed, the average seismic response of the bridge structure is 1.2–1.3 times the response when the pile cap is below the seabed. Therefore, the seismic design of marine structures should take full account of the influence of the submarine site and the seawater layer. Otherwise, it will result in greater seismic risks for marine structures such as sea-crossing bridges. In addition, this study also suggests the influence of the pile cap location on the seismic response of the bridge.

Effective input velocity and depth for deep and shallow sites for site response analysis

ABSTRACT Ground motion input layer depth and Vs are crucial parameters in computing representative surface amplification factor, especially for deep deposits where bedrock depth is unknown. For many soil sites, seismic bedrock depth is unknown and randomly assigning the input motion to any layer may result in bias response. The aim of this study is to understand the effect of input layer velocity or depth on surface response parameters. Further determining the appropriate layer for giving the input ground motion for reliable estimation of response parameters by carrying out detailed site-response analysis. For the analysis, surface and bedrock ground motion recordings from KiK-Net downhole are used. Total stress nonlinear site-response analysis has been carried out by varying the velocity and depth to input the ground motion recorded at the bottom most layer for deep and shallow profiles. Using linear mixed effect models on residuals calculated from recorded and predicted surface spectra, fixed bias and σ are calculated. Layer having Vs ≥ 1500 (150) m/s is suitable for capturing the surface amplification spectra for both deep and shallow deposits.

Seismic response analysis of loess site under far-field bedrock ground motion of the Wenchuan earthquake.

In this paper, considering the far-field seismic input, an accelerogram recorded in the bedrock at Wuquan Mountain in Lanzhou city during the 2008 Wenchuan Ms8.0 earthquake was selected, and numerical dynamic analyses were conducted. The one-dimensional equivalent linear method was implemented to estimate the ground motion effects in the loess regions. Thereafter, slope topographic effects on ground motion were studied by applying the dynamic finite-element method. The results revealed the relationship between the PGA amplification coefficients and the soil layer thickness, which confirmed that the dynamic response of the sites had obvious nonlinear characteristics. The results also showed that there was an obvious difference in the dynamic magnification factor between the short-period and long-period structures. Moreover, it was found that the amplification coefficient of the observation point at the free surface was greater than the point inside the soil at the same depth, which mainly occurred in the upper slope. Through this study, the quantitative assessment of ground motion effects in loess regions can be approximately estimated, and the amplification mechanism of the far-field ground motion mechanism can be further explained. In addition to the refraction and reflection theory of seismic waves, the resonance phenomenon may help explain the slope topographic effect through spectrum analysis.

Parametric Study of Local Site Response for Bedrock Ground Motion to Earthquake in Phuentsholing, Bhutan

Earthquakes, when it comes to natural calamities, are characteristically devastating and pose serious threats to buildings in urban areas. Out of multiple seismic regions in the Himalayas, Bhutan Himalaya is one that reigns prominent. Bhutan has seen several moderate-sized earthquakes in the past century and various recent works show that a major earthquake like the 2015 Nepal earthquake is impending. The southwestern city of Bhutan, Phuentsholing is one of the most populated regions in the country and the present study aims to explore the area using geophysical methods (Multispectral Analysis of Surface Waves (MASW)) for understanding possibilities pertaining to infrastructural development. The work involved a geophysical study on eight different sites in the study region which fall under the local area plan of Phuentsholing City. The geophysical study helps to discern shear wave velocity which indicates the soil profile of a region along with possible seismic hazard during an earthquake event, essential for understanding the withstanding power of the infrastructure foundation. The acquired shear wave velocity by MASW indicates visco-elastic soil profile down to a depth of 22.2 m, and it ranged from 350 to 600 m/s. A site response analysis to understand the correlation of bedrock rigidness to the corresponding depth was conducted using EERA (Equivalent-linear Earthquake Site Response Analysis) software. The amplification factors are presented for each site and maximum amplification factors are highlighted. These results have led to a clear indication of how the bedrock characteristics influence the surface ground motion parameters for the corresponding structure period. The results infer that the future constructional activity in the city should not be limited to two- to five-story buildings as per present practice. Apart from it, a parametric study was initiated to uncover whatever effects rigid bedrock has upon hazard parameters for various depths of soil profile up to 30 m, 40 m, 60 m, 80 m, 100 m, 120 m, 140 m, 160 m, 180 m and 200 m from the ground surface. The overriding purpose of doing said parametric study is centered upon helping the stack holders who can use the data for future development. Such a study is the first of its kind for the Bhutan region, which suffers from the unavailability of national seismic code, and this is a preliminary step towards achieving it.

Basic Study on the Causes of Cut-off Frequency <i>f<sub>max</sub></i> of Earthquakes (Part 3): Evaluation of Bedrock Ground Motion Using the Stochastic Green's Function Method Considering the Bedrock Characteristics of the Observation Site

Basic Study on the Causes of Cut-off Frequency <i>f<sub>max</sub></i> of Earthquakes (Part 3): Evaluation of Bedrock Ground Motion Using the Stochastic Green's Function Method Considering the Bedrock Characteristics of the Observation Site

Identification of Shear Modulus Reduction and Damping Curve for Deep and Shallow Sites: Kik-Net Data

ABSTRACT Study aims at selecting the suitable shear modulus reduction and damping curve for broad classification of soil, i.e., rock, gravel, sand, and clay. For this purpose, surface and bedrock ground motion recordings from KiK-Net downhole array for both deep and shallow soil sites have been used. Total stress one-dimensional non-linear site-response analysis has been carried out by varying the available curves for a corresponding soil type. Input parameters for few curves have been determined using parametric study. Linear mixed-effect models on residuals from predicted and recorded surface spectra have been used in suggesting the suitable curve for corresponding soil.

Study on Influence Factors of Site Ground Motion Parameters Coupling Model

The calculation of ground motion parameters involves many factors such as the acceleration time history of bedrock ground motion, the input interface of bedrock, the shear wave velocity of soil layer and the dynamic parameters of soil mass. And each factor has a certain degree of influence on the final calculation results. Therefore, it is necessary to conduct a scientific and effective sensitivity analysis of the factors affecting the ground motion parameters. And it is also necessary to analyze the influence degree of each factor on the result value, so as to give reasonable calculation results of site ground motion parameters. In this paper, starting with the calculation process of ground motion parameters, combined with the calculation input parameters of bedrock spectrum and the influence parameters of site spectrum, the main factors influencing the calculation results of site ground motions parameters are obtained. Aiming at the influence factors of bedrock seismic time history, bedrock input interface, soil shear wave velocity and soil dynamic parameters, the influence degree of different input parameters on the calculation results of site seismic parameters in analyzed. Finally, it is determined that the soil shear wave velocity, the dynamic triaxial parameters of sand layer and the layered thickness of soil are the key factors affecting the ground motion parameters.

Earthquake induced liquefaction hazard, probability and risk assessment in the city of Kolkata, India: its historical perspective and deterministic scenario

Liquefaction-induced ground failure is one amongst the leading causes of infrastructure damage due to the impact of large earthquakes in unconsolidated, non-cohesive, water saturated alluvial terrains. The city of Kolkata is located on the potentially liquefiable alluvial fan deposits of Ganga-Bramhaputra-Meghna Delta system with subsurface litho-stratigraphic sequence comprising of varying percentages of clay, cohesionless silt, sand, and gravel interbedded with decomposed wood and peat. Additionally, the region has moderately shallow groundwater condition especially in the post-monsoon seasons. In view of burgeoning population, there had been unplanned expansion of settlements in the hazardous geological, geomorphological, and hydrological conditions exposing the city to severe liquefaction hazard. The 1897 Shillong and 1934 Bihar-Nepal earthquakes both of M w 8.1 reportedly induced Modified Mercalli Intensity of IV–V and VI–VII respectively in the city reportedly triggering widespread to sporadic liquefaction condition with surface manifestation of sand boils, lateral spreading, ground subsidence, etc., thus posing a strong case for liquefaction potential analysis in the terrain. With the motivation of assessing seismic hazard, vulnerability, and risk of the city of Kolkata through a consorted federal funding stipulated for all the metros and upstart urban centers in India located in BIS seismic zones III, IV, and V with population more than one million, an attempt has been made here to understand the liquefaction susceptibility condition of Kolkata under the impact of earthquake loading employing modern multivariate techniques and also to predict deterministic liquefaction scenario of the city in the event of a probabilistic seismic hazard condition with 10% probability of exceedance in 50 years and a return period of 475 years. We conducted in-depth geophysical and geotechnical investigations in the city encompassing 435 km2 area. The stochastically synthesized bedrock ground motion for both the 1897 and 1934 earthquakes on non-linear analysis of local site conditions through DEEPSOIL Geotechnical analysis package present surface level peak ground acceleration of the order of 0.05–0.14 g for the 1934 Bihar-Nepal earthquake while for the 1897 Shillong earthquake it is found to be in the range of 0.03–0.11 g. The factor of safety (FOS) against liquefaction, the probability of liquefaction (P L), the liquefaction potential index (LPI), and the liquefaction risk index are estimated under the influence of these two earthquakes wherein the city is classified into severe (LPI > 15), high (5 < LPI ≤ 15), moderate (0 < LPI ≤ 5), and non-liquefiable (LPI = 0) susceptibility zones. While the 1934 Bihar-Nepal earthquake induced moderate to severe liquefaction hazard condition in the city in mostly the deltaic plain and interdistributary marsh geomorphologic units with 13.5% sites exhibiting moderate hazard with a median LPI of 1.8, 8.5% sites depicting high with a median LPI of 9.1 and 4% sites with a median LPI of 18.9 exhibiting severe hazard condition, 1897 Shillong earthquake induced mostly non-liquefaction condition with very few sites depicting moderate and high liquefaction hazard. A conservative liquefaction hazard scenario of the city on the other hand estimated through deterministic approach for 10% probability of exceedance in 50 years predicts a high hazard zone in the 3.5–19 m depth region with FOS < 1 and P L > 65% comprising of coarse-grained sediments of sand, silty sand, and clayey silty sand in mostly the deltaic plain geomorphologic unit with 39.1% sites depicting severe liquefaction hazard with a median LPI of 28.3. A non-linear regression analysis on both the historical and deterministic liquefaction scenarios in P L versus LPI domain with ± 1 standard deviation confidence bound generated a cubic polynomial relationship between the two liquefaction hazard proxies. This study considered a bench mark for other cities in the country and elsewhere forms an integral part of the mega-seismic microzonation endeavors undertaken in all the earthquake-prone counties in the world.

A Simple Approach to Site-Response Modeling: The Overlay Concept

Site‐response analyses are used to estimate the ground motion at the surface, as a function of the properties of the soil profile and the bedrock ground motion at the base of the soil profile. The most frequently employed site‐response model in engineering practice is the equivalent‐linear site‐response model, a frequency‐domain program that is coded in the computer program SHAKE (Schnabel et al. , 1972; Idriss and Sun, 1992; Ordonez, 2010). Using a large database of ground motions in the Kiban–Kyoshin network (KiK‐net) of downhole arrays in Japan, Kaklamanos et al. (2013, 2015) found the equivalent‐linear site‐response model becomes inaccurate at shear strains of approximately 0.1%–0.4%. At larger strains, to model fully nonlinear behavior, site‐response calculations should be performed in the time domain, which allows for a more accurate representation of the true cyclic stress–strain path. Finite‐element methods are a powerful technique for modeling nonlinear site response, allowing for the representation of advanced constitutive behavior, 3D wave propagation, soil–structure interaction, and complex geometry. The purpose of this paper is to present a simple methodology for modeling earthquake site response in any finite‐element program, using an overlay model to represent nonlinear soil behavior. This paper presents a literature review of the overlay approach, the advantages of the overlay approach, practical details on its implementation in any finite‐element program, how this approach may be easily extended to capture more complex behavior, and an analysis of example results using recent KiK‐net data. There has been much focus in the literature on improving soil models to advance our ability to predict site response (e.g., Hartzell et al. , 2004; Kwok et al. , 2007; Phillips and Hashash, 2009). Numerous nonlinear site‐response programs have been …

Earthquake scenario in West Bengal with emphasis on seismic hazard microzonation of the city of Kolkata, India

Abstract. Seismic microzonation is a process of estimating site-specific effects due to an earthquake on urban centers for its disaster mitigation and management. The state of West Bengal, located in the western foreland of the Assam–Arakan Orogenic Belt, the Himalayan foothills and Surma Valley, has been struck by several devastating earthquakes in the past, indicating the need for a seismotectonic review of the province, especially in light of probable seismic threat to its capital city of Kolkata, which is a major industrial and commercial hub in the eastern and northeastern region of India. A synoptic probabilistic seismic hazard model of Kolkata is initially generated at engineering bedrock (Vs30 ~ 760 m s−1) considering 33 polygonal seismogenic sources at two hypocentral depth ranges, 0–25 and 25–70 km; 158 tectonic sources; appropriate seismicity modeling; 14 ground motion prediction equations for three seismotectonic provinces, viz. the east-central Himalaya, the Bengal Basin and Northeast India selected through suitability testing; and appropriate weighting in a logic tree framework. Site classification of Kolkata performed following in-depth geophysical and geotechnical investigations places the city in D1, D2, D3 and E classes. Probabilistic seismic hazard assessment at a surface-consistent level – i.e., the local seismic hazard related to site amplification performed by propagating the bedrock ground motion with 10% probability of exceedance in 50 years through a 1-D sediment column using an equivalent linear analysis – predicts a peak ground acceleration (PGA) range from 0.176 to 0.253 g in the city. A deterministic liquefaction scenario in terms of spatial distribution of liquefaction potential index corresponding to surface PGA distribution places 50% of the city in the possible liquefiable zone. A multicriteria seismic hazard microzonation framework is proposed for judicious integration of multiple themes, namely PGA at the surface, liquefaction potential index, NEHRP soil site class, sediment class, geomorphology and ground water table in a fuzzy protocol in the geographical information system by adopting an analytical hierarchal process. The resulting high-resolution surface consistent hazard, liquefaction and microzonation maps are expected to play vital roles in earthquake-related disaster mitigation and management of the city of Kolkata.

Investigating the influence of topographic irregularities and two-dimensional effects on surface ground motion intensity with one- and two-dimensional analyses

Abstract. In this work, the surface ground motion that occurs during an earthquake in ground sections having different topographic forms has been examined with one and two dynamic site response analyses. One-dimensional analyses were undertaken using the Equivalent-Linear Earthquake Response Analysis (EERA) program based on the equivalent linear analysis principle and the Deepsoil program which is able to make both equivalent linear and nonlinear analyses and two-dimensional analyses using the Plaxis 8.2 software. The viscous damping parameters used in the dynamic site response analyses undertaken with the Plaxis 8.2 software were obtained using the DeepSoil program. In the dynamic site response analyses, the synthetic acceleration over a 475-year return period representing the earthquakes in Istanbul was used as the basis of the bedrock ground motion. The peak ground acceleration obtained different depths of soils and acceleration spectrum values have been compared. The surface topography and layer boundaries in the 5-5' cross section which cuts across the study area west to east were selected in order to examine the effect of the land topography and layer boundaries on the analysis results, and were flattened and compared with the actual status. The analysis results showed that the characteristics of the surface ground motion change in relation to the varying local soil conditions and land topography.

Site effect evaluation for Yanbu City urban expansion zones, western Saudi Arabia, using microtremor analysis

Recently, Yanbu City has been greatly growing; great developmental projects, urban community settlements, and petrochemical industries have been established. Historical and instrumental earthquakes were felt by this city, such as the earthquake on 19 May 2009 which was a moderate earthquake (moment magnitude (M w) 5.7) whose ground motions have affected some building structures. Yanbu City has been divided by a grid of points separated by about 500 m, and microtremor measurements have been conducted at 141 measuring sites. The acquired data have been processed using worldwide Geopsy software package to calculate the fundamental frequency peaks and their corresponding amplification factors. The natural origin of these peaks has been confirmed. Based on fundamental frequency (f 0), Yanbu City has been classified into three zones—from 0.13 to 1.0 Hz in the first zone, from 1.0 to 4.0 Hz in the second zone, and from 4.0 to 7.6 Hz in the third zone. The coastal zone of Yanbu illustrates smaller values of f 0 that reflect great thickness of soft sediments while the eastern zone presents an opposite phenomenon. Accordingly, high-rise buildings in the coastal zone will be affected greatly by low frequencies originating from distant earthquakes. Most of Yanbu City has f 0 values in the range of 1.0–4.0 Hz. Furthermore, bedrock ground motion could amplify as much as three times. This indicates that one- to three-story buildings in Yanbu City are vulnerable to hazardous resonant shaking from local and near earthquakes.

Large-Scale Two-Dimensional Nonlinear Analysis on Seismic Effect of Fuzhou Basin

So far, nonlinear analyses on seismic effect of large-scale basin model have rarely been reported. Based on the explicit finite element method and 32 CPU parallel computing cluster platform of ABAQUS, a two-dimensional（2D） large-scale refined model for Fuzhou Basin is established using a nonlinear analytical method in the time domain. Both the peak ground acceleration (PGA) and acceleration response spectra of ground surface are emphatically analyzed. Meanwhile, the results by one-dimensional equivalent linearization method in frequency domain are added for the sake of contrast. The study results show that the PGA of ground surface will be obviously amplified when compared with the peak acceleration of bedrock ground motion; and the ground acceleration response spectra of Fuzhou Basin may appear a double-peak or multi-peak phenomenon. Moreover, the 2D large-scale model can reflect amplification effect of Fuzhou Basin soil to bedrock ground motion in particular periods; In addition, the acceleration responses of ground surface right above the bedrock valleys and crests appear larger than the other positions adjacent to them, these phenomenon are generally manifested as focusing effect and amplification effect, which are thought to be produced by the recently deposited soils in the basins as well as the special soil layer such as lens. In general, the PGA of ground surface obtained by 2D model present to be greater than that obtained by the one-dimensional model.

Ground-Motion Amplification in Nonlinear Soil Sites with Uncertain Properties

This work presents a statistical study on the effect of soil layers with uncertain properties on ground-motion intensity at the soil surface. Surface motion is obtained by applying multiple real rock earthquake records at the base of different characterizations of the soil column, each one generated via Monte Carlo simulation. The effect of the soil is studied in terms of a site-specific, frequency-dependent amplification function, AF( f ), where f is a generic oscillator frequency. The goal here is the identification of ground-motion parameters that allow an efficient prediction of AF( f ). We investigated magnitude, M, source-to-site distance, R, of the input bedrock accelerogram along with bedrock ground-motion parameters such as peak ground acceleration, PGAr, and spectral acceleration values, and , both rr S ( f ) S ( f ) aa sc at the generic frequency f and at the specific initial fundamental frequency of vibra- tion, fsc of the soil column. This work includes two case studies: a saturated sandy site and a saturated soft clayey site. In the former, loss of shear strength owing to cyclic mobility is anticipated for severe levels of ground shaking, while in the latter, significant amplification is expected at long oscillator periods. The results show that of the input record is the single most helpful parameter for the prediction of r S ( f ) a AF( f ) at the same oscillator frequency, f. is more informative than PGAr and/ r S ( f ) a or the pair of M and R values of the event that generated the bedrock motion. A sufficiently accurate estimate of the median AF( f ) can be obtained by using 10 or fewer records, which may be selected without undue attention to the specific scenario events (i.e., M and R pairs) that control the hazard at the site. Finally, the effect of the uncertainty in the soil parameters on the prediction error of AF( f ) is of secondary importance compared to that from record-to-record variability. These findings will be used to estimate the hazard at the soil surface in a companion article in this issue (Bazzurro and Cornell, 2004).

Seismic Zonation of the Delhi Region for Bedrock Ground Motion

An endeavor is made to compute peak ground horizontal accelerations at bedrock level in the Delhi region due to the seismogenic sources present around Delhi. The entire area is divided into six seismogenic sources for which seismic hazard analysis is carried out using the complete and extreme part of the seismicity data. Maximum likelihood estimates of hazard parameters viz., seismic activity rate λ, b value and maximum probable earthquake Mmax are made for each zone. The return periods and the probabilities of occurrence of various magnitudes for return periods of 50, 100 and 1000 years are also computed for each zone. The peak ground acceleration (PGA) values for 20% exceedance in 50 years are then computed for the Delhi region from each zone. The maximum PGA value considering all the zones is 0.34 g, which is due to the Mathura fault zone. The seismogenic zones V and VI, i.e., Mathura fault zone and the Sohna fault zone are observed to be contributing maximum PGA values in the Delhi region governing the isoacceleration contours computed for the region. The seismic zonation map for the PGA values at the bedrock level is obtained for the Delhi region. This can be used directly as input for the microzonation of ground motion at the surface by incorporating the local site conditions.