Characteristics Of Earthquake Ground Motions Research Articles

Ground-Motion Intensity Measures for the Seismic Response of the Roof-Isolated Large-Span Structure

Ground-motion intensity measures (IMs), which quantify and describe the characteristics of earthquake ground motion, are of utmost importance in the assessment of seismic risk and the design of resilient structures with large spans. The appropriate selection of a ground-motion IM is crucial in establishing a reliable and robust correlation between seismic hazards and structural demands. The current study presents a novel ground-motion IM that incorporates the influence of multiple vibration modes and period elongation resulting from isolation based on the velocity spectrum. A comprehensive study has been conducted to examine the efficiency of 37 different ground-motion IMs on a roof-isolated large-span structure with engineering demand parameters (EDPs), using far-field ground-motion data. The initial examination of the proposed intensity measure involves a planar lumped-mass model. Subsequently, a numerical model of a large-span roof-isolated structure, specifically the Beijing Workers’ Stadium, is constructed and examined. The results suggest that the proposed intensity measure (IM) demonstrates satisfactory adequacy and achieves optimal efficiency when considering three different engineering demand parameters among 37 other ground-motion intensity measures.

Effects of important characteristics of earthquake ground motions on probabilistic seismic demand assessment of long-span cable-stayed bridges

The uncertainties of earthquake ground motions have the most important effects on seismic responses of bridge structures, especially long-span bridges, because of complex and special dynamic properties. The nonlinear dynamic time-history analysis is conducted for a two-pylon long-span cable-stayed highway bridge by using real earthquake ground motions rationally selected. The correlation between the important characteristics of earthquake ground motions and the probabilistic seismic demand assessment of the cable-stayed bridge reveals that the geometric means and dispersions of response spectra from selected ground motions have very significant effects on mean values, dispersions and probabilistic distributions of seismic demands of long-span bridges. The spectral shape of geometric mean spectra in the period ranges with large cumulative modal mass participation factors that should be well matched to the target spectrum to improve the precision and computational efficiency of probabilistic seismic demand assessment. If earthquake ground motions are rationally selected, response spectral values at the periods with comparatively large modal participation mass ratios or PGA can be used as intensity measures and even provide more precise probabilistic seismic demand assessment than response spectral values at the fundamental periods.

Characteristics of earthquake ground motions requiring extended dynamic analysis

ABSTRACT The absolute maximum response (AMR) of structures might occur posterior to the end of earthquake excitations, hence, extending the dynamic analysis to the post-shaking phase is required. This phenomenon was found to be more common for long-period and low-damping structures, which are prevailing in regions of low-to-moderate seismicity like Australia. In this article, the mechanism of the occurrence of AMR in the post-shaking phase is first explained using single-degree-of-freedom (SDOF) systems subjected to a simple sine wave. Further investigation on real earthquake ground motions reveals that whether an extended analysis is required depends greatly on the characteristics of the post-significant duration phase. Finally, suitable metrics for describing ground motions that require extended analysis (EAGMs) are identified based on the Kolmogorov–Smirnov test. Regression analyses are then conducted to predict the probability of EAGMs.

Characteristics of earthquake ground motions governing the damage potential for Delhi and the surrounding region of India

The proximity of Delhi and the surrounding region to the active faults along with its geographical settings is a subject of discussion to comprehend the seismic resilience of the capital region of India. The region may be affected by the far-field earthquakes from the Himalayas as well as the near-field earthquakes associated with the local seismic activity. Considering the ordinary settings of this region, the present study is an insight to differentiate the damage potential of ground motion associated with near and far-field conditions to further see their consequences to understand the comprehensive seismic hazard of the region. The acceleration and velocity response analysis of recorded strong ground motions from far-field and near-field earthquakes exhibit a clear distinct behavior in the form of amplification and corresponding predominant period. The comparison of estimated normalized spectral accelerations with that of the seismic design code of the Bureau of Indian Standards (BIS), shows that the current Indian building design code is within the structural limits proposed for the seismic forces of long periods, however, exceeded amplitude of the normalized Spectral Acceleration for far-field earthquakes may be attributed towards the damage potential for the high rise buildings in the capital region of India. On the other hand, near-field earthquakes do not meet the criteria with the design code of BIS at lower periods from 0.02s to 0.09s along with the amplified Spectral acceleration. It also suggests that the structural heterogeneities within the subsurface of Delhi and the surrounding region have a strong bearing in contributing to the impact of seismic waves from near-field earthquakes producing short-period waves that may be disastrous for low-rise buildings. Based on the results, the study region affected by the distinct seismicity patterns is important to understand the shaking behavior of the different kinds of infrastructures/buildings in case of near-field and far-field earthquakes to appropriately utilize the information for constructing new buildings and strengthening the existing infrastructures in Delhi and the surrounding region of India.

A numerical study of 3D topographic site effects considering wavefield incident direction and geomorphometric parameters

The topographic site effect plays a vital role in controlling the characteristics of earthquake ground motions. Due to its complexity, the factors affecting topographic amplification have not been fully identified. In this study, 100 ground motion simulations generated by double-couple point sources in the homogeneous linear elastic half-space are performed based on the 3D (three-dimensional) Spectral Element Method, taking the Menyuan area of Qinghai Province, China as a local testbed site. A relationship between incident direction and the strength of topographic amplification has been observed. The horizontal ground motion is affected by the back-azimuth, which is typically chosen to be the direction from seismic station to seismic source measured clockwise from north. Specifically, the east-west PGA (Peak Ground-motion Acceleration) is significantly amplified when back-azimuth is about 90° or 270°, and the north-south PGA is significantly amplified when back-azimuth is around 0° or 180°. The vertical ground motion is affected by the dipping angle, which is the angle from vertical at which an incoming seismic wave arrives. The vertical PGA is strongly amplified when the seismic wave is almost horizontally incident (e.g., dipping angle = 78°). A correlation study between geomorphometric parameters and frequency-dependent topographic amplification indicates that relative elevation and smoothed curvature contain similar information, both of which are closely related to the topographic amplification of horizontal components, but not the vertical component. Our study reveals the influence of source and propagation path on topographic amplification and provides a reference for considering the topographic site effect in real engineering sites.

Two-dimensional simulation of the seismic response of the Santiago Basin, Chile

Results from an ambient noise tomography, gravimetric studies, and local geophysical surveys are combined to develop three representative cross-sections of the Santiago Basin, Chile, that include the main geological features and the wave velocities of the materials. One-dimensional (1D) and two-dimensional (2D) seismic responses were simulated for each cross-section, considering input seismic signals polarized in different directions. Amplification factors calculated in the surface of the cross-sections highlight the larger amplitudes and durations of the strong-ground motions in the softer deposits of the Basin due to surface waves generated at the boundaries of the rock outcrops. 2D over 1D aggravation factors for several earthquake ground motion characteristics indicate that 2D effects are negligible in the stiff gravel deposits and that the 1D simulations cannot capture the basin edge effects in the fine-grained soils. Numerical predictions are confirmed using the analyses of earthquake records of the 2015 Mw 8.3 Illapel Earthquake.

Seismic response of large-span spatial structures under multi-support and multidimensional excitations including rotational components

To achieve rational and precise seismic response predictions of large span spatial structures (LSSSs), the inherent non-uniformity and multidimensionality characteristics of earthquake ground motions should be properly taken into consideration. However, due to the limitations of available earthquake stations to record seismic rotational components, the effects of rocking and torsional earthquake components are commonly neglected in the seismic analyses of LSSSs. In this study, a newly developed method to extract the rocking and torsion components at any point along the area of a deployed dense array from the translational earthquake recordings is applied to obtain the rotational seismic inputs for a LSSS. The numerical model of an actual LSSS, the Dalian International Conference Center (DICC), is developed to study the influences of multi-support and multidimensional excitations on the seismic responses of LSSSs. The numerical results reveal that the non-uniformity and multidimensionality of ground motion input can considerably affect the dynamic response of the DICC. The specific degree of influence on the overall and local structural displacements, deformations and forces are comprehensively investigated and discussed.

A spectral-acceleration-based combination-type earthquake intensity measure for high-rise stack-like structures

Ground motion intensity measures are of great importance for the seismic design of structures. A well-chosen intensity measure will reduce the detailed ground motion record selection effort for the nonlinear dynamic structural analyses. In this article, a spectral-acceleration-based combination-type earthquake intensity measure is presented. This intensity measure considers the higher modes effect and period elongation effect due to nonlinear deformation at the same time. The modal mass participation factors are determined to take weighting coefficients and the product of elastic first-mode period T1 and a constant C is expressed to represent the elongated period. Therefore, the proposed intensity measure is a combination of earthquake ground motion characteristics, elastic structural responses, higher modes participation, and the period elongation effect due to inelastic structural behaviors. Four three-dimensional models of reinforced concrete stack-like structures including a 240 m-high chimney, a 180 m-high chimney, a 120 m-high chimney, and a 42.3 m-high water tower are established and analyzed in ABAQUS to investigate the correlation between the intensity measure and the maximum curvatures under 44 far-field ground motions and 28 near-fault ground motions with a pulse-like effect. With the optimal vibration modes and the proper period elongation coefficient, the efficiency of the introduced intensity measure is compared with the other 15 intensity measures. The results indicate that the proposed intensity measure is believed to be a good choice for high-rise stack-like structures, especially under the near-fault ground motions with pulse-like effect.

Modeling of Subsurface Velocity Structures from Seismic Bedrock to Ground Surface in the Tokai Region, Japan, for Broadband Strong Ground Motion Prediction

For sophistication of strong ground motion prediction in terms of disaster mitigation, one of the principal issues is to model subsurface velocity structures so that characteristics of earthquake ground motions can be reproduced in the broadband range 0.1 Hz to 10 Hz. In recent years, subsurface structures have been modeled in sedimentary layers on seismic bedrock for a few regions of Japan, in a national project. In this study, subsurface velocity structures were modeled from seismic bedrock to the ground surface for the Tokai region. These models were constructed in accordance with the subsurface velocity structure modeling scheme published by the Headquarters for Earthquake Research Promotion. To begin with, initial models were constructed based on existing bore-hole data, geological information, etc. Next, they were improved based on results of microtremor explorations which had been conducted in recent years. It was found that the new model had different characteristics to the conventional model. This paper will present the modeling process and characteristics of distribution maps for velocity structures and amplification index.

Seismic Fragility Analysis of Steel Liquid Storage Tanks Using Earthquake Ground Motions Recorded in Korea

Liquid-containing storage tanks are important structures in industrial complexes. Because earthquake damages to liquid storage tanks can cause structural collapse, fires, and hazardous material leaks, there have been continuous efforts to mitigate earthquake damages using seismic fragility analysis. In this regard, this study focuses on the seismic responses and fragility of liquid storage tanks. First, the characteristics of earthquake ground motions are a critical factor influencing the seismic fragility of structures; thus, this study employs real earthquake records observed in the target area, southeastern Korea, with the earthquake characteristics estimated based on the ratio of peak ground acceleration to peak ground velocity. When a liquid storage tank oscillates during an earthquake, additional forces can impact the tank wall owing to hydrodynamic pressures. Therefore, this study presents a sophisticated finite element (FE) model that reflects the hydrodynamic effect of an oscillating liquid. Another advantage of such an FE model is that detailed structural responses of the entire wall shells can be estimated; this is not possible in simplified lumped mass or surrogate models. Lastly, probabilistic seismic demand models are derived for three critical limit states: elastic buckling, elephant’s foot buckling, and steel yielding. Using the real earthquake ground motion records, constructed FE model, and limit states, a seismic fragility analysis is performed for a typical anchored steel liquid storage tank in Korea. In addition, for comparison purposes, a ring-stiffened model is investigated to derive a seismic fragility curve. The results of the seismic fragility assessment show that elastic buckling is the most vulnerable damage state. In contrast, elephant’s foot buckling and steel yielding indicate relatively severe damage levels. Furthermore, it is observed that ring stiffeners decrease the elastic buckling damage, although there is no practical effect on elephant’s foot buckling and steel yielding in all ground motion intensities.

Computation of amplification factor of earthquake ground motion for a local sedimentary structure

We present methodology of calculating acceleration and corresponding earthquake ground motion characteristics at a site of interest assuming acceleration at a reference site for two basic configurations. In one configuration we assume that the reference ground motion is not affected by the local structure beneath the site of interest. In the other configuration we assume that the reference ground motion is affected by the local structure. Consequently, the two configurations differ from each other by the presence of the reference site within the computational model. For each of the two configurations we assume two wavefield excitations: a vertical plane-wave incidence and a point double-couple source. We illustrate the methodology on the example of the Grenoble valley. The extensive investigation of effects of local surface sedimentary structures based on the developed methodology is presented in the accompanying article by Moczo et al. (Bull Earthq Eng, 2018) in this volume.

Key structural parameters affecting earthquake ground motion in 2D and 3D sedimentary structures

Alluvial valleys generate strong effects on earthquake ground motion (EGM). These effects are rarely accounted for even in site-specific studies because of (a) the cost of the required geophysical surveys to constrain the site model, (b) lack of data for empirical prediction, and (c) poor knowledge of the key controlling parameters. We performed 3D, 2D and 1D simulations for six typical sedimentary valleys of various width and depth, and for a variety of modifications of these 6 “nominal models” to investigate sensitivity of EGM characteristics to impedance contrast, attenuation, velocity gradient and geometry. We calculated amplification factors, and 2D/1D and 3D/2D aggravation factors for 10 EGM characteristics, using a representative set of recorded accelerograms to account for input motion variability. The largest values of the amplification and aggravation factors are found for the Arias intensity and cumulative absolute velocity, the lowest for the root-mean-square acceleration. The aggravation factors are largest for the vertical component. For each model, at least one EGM characteristic exhibits a significant 2D/1D aggravation factor, while all EGM characteristics exhibit significant 2D/1D aggravation factor on the vertical component. For all investigated sites, there is always an area in the valley for which 1D estimates are not sufficient. 2D estimates are insufficient at several sites. The key structural parameters are the shape ratio and overall geometry of the sediment-bedrock interface, impedance contrast at the sediment-bedrock interface, and attenuation in sediments. The amplification factors may largely exceed the values that are usually considered in GMPEs between soft soils and rock sites.

Effects of ground motion frequency content on performance of isolated bridges with SSI

The present study considers a multi-span continuous bridge, isolated by lead rubber bearing (LRB). Dynamic soil-structure interaction (SSI) is modelled with the help of a simplified, sway-rocking model for different types of soil. It is well understood from the literature that SSI influences the structural responses and the isolator performance. However, the above-mentioned effect of SSI also depends on the earthquake ground motion properties. It is very important to understand how the interaction between soil and structure varies with the earthquake ground motion characteristics but, as far as the knowledge of the authors go, no study has been carried out to investigate this effect. Therefore, the objectives of the present study are to investigate the influence of earthquake ground motion characteristics on: (a) the responses of a multi span bridge (isolated and non-isolated), (b) the performance of the isolator and, most importantly, (c) the soil-structure interaction. Statistical analyses are conducted by considering 14 earthquakes which are selected in such a way that they can be categorized into three frequency content groups according to their peak ground acceleration to peak ground velocity (PGA/PGV) ratio. Lumped mass model of the bridge is developed and time history analyses are carried out by solving the governing equations of motion in the state space form. The performance of the isolator is studied by comparing the responses of the bridge with those of the corresponding uncontrolled bridge (i.e., non-isolated bridge). On studying the effect of earthquake motions, it is observed that the earthquake ground motion characteristics affect the interaction between soil and structure in such a way that the responses decrease with increase in frequency content of the earthquake for all the types of soil considered. The reverse phenomenon is observed in case of the isolator performance where the control efficiencies increase with frequency content of earthquake.

2008年岩手・宮城内陸地震の震源域を対象とした計測震度トモグラフィ解析による減衰構造不均質分布の推定

In the 2008 Iwate-Miyagi Nairiku Earthquake occurred at volcanic area, large amplitude strong ground motions were recorded at observatories near fault region. Because subsurface structure is very complicated in the volcanic area, it is pointed out that considering heterogeneous attenuation structure is important to investigate characteristics of earthquake ground motions. In this study, we conducted tomographic analysis using seismic intensity data of aftershock records for the purpose of estimating heterogeneous attenuation structure. We also tried to reproduce seismic intensity of aftershocks which occurred near and on the asperity of the main shock. By comparing estimated and observed seismic intensity, we indicated how the heterogeneous attenuation structure influenced amplitudes of earthquake ground motions. The tomographic analysis using seismic intensity was conducted in three phases of procedures. First, we obtained an attenuation relationship of seismic intensity using strong motion records of inter-plate and intra-plate earthquakes, and evaluated a site factor of each station. Next, we obtained the attenuation relationship for crustal earthquakes using records of aftershocks assuming the same site factor. We then calculated residuals of each observed and estimated seismic intensity data. Finally, supposing residuals to be attributed to the heterogeneous attenuation structure, a block inversion was conducted dividing the focal area of the main shock to strip form with horizontal sizes of approximately 3km. An attenuation level of each block was expressed as increase/decrease ratio of seismic intensity (we called “G” value). In this block inversion weighting by the ray length which passes through the block, 326 records of Aratozawa Dam, Kurikoma Dam and KiK-net stations IWTH25, IWTH26, MYGH02, IWTH28 and AKTH04 which located near and on the fault plane of main shock were used. To confirm a resolution of G value estimated at each block, we conducted a numerical experiment using checkerboard. Although large errors were found where the number of ray was sparse, errors were less than 10% in most domains on the fault plane of the main shock. We conducted the same inversion using observed records, and found that estimated heterogeneous attenuation structure was very complicated. As indicated in the previous studies, high attenuation zone was distributed around Mt. Kurikoma. Around IWTH25 which located just above side of the main shock hypocenter, low attenuation zone was distributed. Around Aratozawa Dam and Kurikoma Dam, attenuation structure was very complicated too. We tried to reproduce seismic intensity at IWTH25, Aratozawa Dam and Kurikoma Dam for 3 aftershocks which occurred near and on the asperity of the main shock. When uniform attenuation structure was used, the observed seismic intensity of IWTH25 and Aratozawa Dam were larger than estimated. Observed seismic intensities at Kurikoma Dam were slightly smaller than estimated. In the case of considering heterogeneous attenuation, reproducibility of observed records was greatly improved. In case that azimuths of hypocenters are different, amplitudes of the earthquake ground motions are different even if magnitude and hypocentral distance are the same. We concluded that the propagation path greatly influences amplitudes of earthquake ground motions. It is thought that this complicated attenuation structure was one of factors that large amplitude strong ground motions were recorded at particularly IWTH25 and Aratozawa Dam in the main shock.

Site, local, and regional earthquake ground motion characterization and application

A systematic overview and comparison is presented on the seismic hazard assessment based on the observations and seismic hazard model and the characteristics of earthquake ground motions for a site, a local area, and a region. It shows that the seismic hazard estimated by directly using the observations could differ from that evaluated using an adopted seismic hazard model. It indicates the importance to judiciously select the seismic hazard model, as well as to understand that historical records for a limited period may not fully reflect the seismic hazard. The comparison of the ground motion characteristics is focused on the variability of the ground motion measures, the coherency for record components in two orthogonal orientations, and the spatial correlation and spatial coherency. Procedures for simulating bidirectional excitations at a site and record components at multiple sites for a scenario seismic event are given. The application of the simulated record components to estimate the seismic loss for a portfolio of hypothetical buildings by considering a scenario event is shown, indicating the effectiveness of the presented methodology for seismic loss estimation.

Effect of Uncertainties in Development of Fragility Curves for URBM Building

Seismic fragility analysis of Un-reinforced Brick Masonry (URBM) Buildings is useful for determining the possible extent of damage in the event of an earthquake. It is known that there are uncertainties with respect to both the mechanical properties of masonry and the characteristics of earthquake ground motion. This paper presents sensitivity studies: by considering Weibull distribution as candidate distribution for compressive strengths of brick and mortar and varying the Coefficient of Variation (COV); and by using 12 different earthquakes time histories available in the strong motion atlas of India separately. For this purpose a one room, single storey URBM building has been considered and the fragility curves have been obtained by integrating Improved storey shear modelling, Monte Carlo simulation and Incremental dynamic analysis. From the study, it has been observed that while using the Weibull distribution as candidate distribution for mechanical properties of masonry, fragility curves of URBM buildings are not significantly affected by the COV whereas they are sensitive to the choice of earthquake ground motion. Fragility curves obtained using different earthquake time histories show three bands. For the suite of earthquake time histories considered in this study, the observed fragilities are generally increasing with the epicentral distance. In other words observed fragilities corresponding to the far-field earthquake time histories are generally higher than those corresponding to the near-field earthquake time histories. Thus, for vulnerability analysis, selection of suite of earthquake time histories judiciously is more important than modelling the statistical variations in mechanical properties of masonry. Keywords: Coefficient of Variation, Incremental Dynamic Analysis, Seismic Fagility Curves, Sensitivity, Unreinforced Brick Masonry, Weibull Distribution

Effect of Uncertainties in Development of Fragility Curves for URBM Building

Seismic fragility analysis of Un-reinforced Brick Masonry (URBM) Buildings is useful for determining the possible extent of damage in the event of an earthquake. It is known that there are uncertainties with respect to both the mechanical properties of masonry and the characteristics of earthquake ground motion. This paper presents sensitivity studies: by considering Weibull distribution as candidate distribution for compressive strengths of brick and mortar and varying the Coefficient of Variation (COV); and by using 12 different earthquakes time histories available in the strong motion atlas of India separately. For this purpose a one room, single storey URBM building has been considered and the fragility curves have been obtained by integrating Improved storey shear modelling, Monte Carlo simulation and Incremental dynamic analysis. From the study, it has been observed that while using the Weibull distribution as candidate distribution for mechanical properties of masonry, fragility curves of URBM buildings are not significantly affected by the COV whereas they are sensitive to the choice of earthquake ground motion. Fragility curves obtained using different earthquake time histories show three bands. For the suite of earthquake time histories considered in this study, the observed fragilities are generally increasing with the epicentral distance. In other words observed fragilities corresponding to the far-field earthquake time histories are generally higher than those corresponding to the near-field earthquake time histories. Thus, for vulnerability analysis, selection of suite of earthquake time histories judiciously is more important than modelling the statistical variations in mechanical properties of masonry.

Numerical Investigation of Seismically Induced Rock Mass Fatigue as a Mechanism Contributing to the Progressive Failure of Deep-Seated Landslides

The importance of earthquakes in triggering catastrophic failure of deep-seated landslides has long been recognized and is well documented in the literature. However, seismic waves do not only act as a trigger mechanism. They also contribute to the progressive failure of large rock slopes as a fatigue process that is highly efficient in deforming and damaging rock slopes. Given the typically long recurrence time and unpredictability of earthquakes, field-based investigations of co-seismic rock slope deformations are difficult. We present here a conceptual numerical study that demonstrates how repeated earthquake activity over time can destabilize a relatively strong rock slope by creating and propagating new fractures until the rock mass is sufficiently weakened to initiate catastrophic failure. Our results further show that the damage and displacement induced by a certain earthquake strongly depends on pre-existing damage. In fact, the damage history of the slope influences the earthquake-induced displacement as much as earthquake ground motion characteristics such as the peak ground acceleration. Because seismically induced fatigue is: (1) characterized by low repeat frequency, (2) represents a large amplitude damage event, and (3) weakens the entire rock mass, it differs from other fatigue processes. Hydro-mechanical cycles, for instance, occur at higher repeat frequencies (i.e., annual cycles), lower amplitude, and only affect limited parts of the rock mass. Thus, we also compare seismically induced fatigue to seasonal hydro-mechanical fatigue. While earthquakes can progressively weaken even a strong, competent rock mass, hydro-mechanical fatigue requires a higher degree of pre-existing damage to be effective. We conclude that displacement rates induced by hydro-mechanical cycling are indicative of the degree of pre-existing damage in the rock mass. Another indicator of pre-existing damage is the seismic amplification pattern of a slope; frequency-dependent amplification factors are highly sensitive to changes in the fracture network within the slope. Our study demonstrates the importance of including fatigue-related damage history—in particular, seismically induced fatigue—into landslide stability and hazard assessments.

ESTIMATION OF SUBSURFACE STRUCTURE BASED ON MICROTREMOR, BORE HOLE OBSERVATIONS AND STOCHASTIC STRONG GROUND MOTION SIMULATIONS IN PALU CITY, CENTRAL SULAWESI, INDONESIA: A VALIDATION AND SENSITIVITY STUDY ON THE 23 JANUARY 2005 (PALU) EARTHQUAKE

In this study, we investigated the subsurface structure and strong ground motion parameters for Palu City. One of the major structures in Central Sulawesi is the Palu-Koro Fault system. Several powerful earthquakes have struck along the Palu-Koro Fault during recent years, one of the largest of which was an M 6.3 event that occurred on January 23, 2005 and caused several casualties. Following the event, we conducted a microtremor survey to estimate the shaking intensity distribution during the earthquake. From this survey we produced a map of the peak ground acceleration, velocity and ground shear strain in Palu City. We performed single observations of microtremors at 151 sites in Palu City. The results enabled us to estimate the site-dependent shaking characteristics of earthquake ground motion. We also conducted 8-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Palu. From the dispersion curve of array observations, the central business district of Palu corresponds to relatively soil condition with Vs ≤ 300 m/s, the predominant periods due to horizontal vertical ratios (HVSRs) are in the range of 0.4 to 1.8 s and the resonant frequency are in the range of 0.7 to 3.3 Hz. Three boreholes were throughout the basin especially in Palu area to evaluate the geotechnical properties of subsurface soil layers. The depths are varying from 1 m to 30 m. Strong ground motions of the Palu area were predicted based on the empirical stochastic green’s function method. Peak ground acceleration and peak ground velocity becomes more than 0.04 g and 30 kine in some areas, which causes severe damage for buildings in high probability. Keywords: Palu-Koro fault, microtremor, bore holes, peak ground acceleration and velocity.

Characteristics of Earthquake Ground Motion on the Seafloor

The differences between onshore and offshore ground motions were studied based on the records from the K-net and SEMS. The horizontal response spectrum and vertical-to-horizontal response spectral ratio for offshore and adjacent onshore ground motions were analyzed. The results indicate that the characteristic period of the response spectra for offshore ground motions is obviously larger. The epicentral distance can influence the horizontal response spectra for offshore ground motions in a moderate earthquake, while the influence of water depth is not noticeable. Moreover, the vertical-to-horizontal response spectral ratio for offshore ground motions is significantly smaller for periods less than 1 s.