Nonstationary Stochastic Simulation of Strong Ground-Motion Time Histories : Application to the Japanese Database

For earthquake-resistant design, engineering seismologists employ time-history analysis for nonlinear simulations. The nonstationary stochastic method previously developed by Pousse et al. (2006) has been updated. This method has the advantage of being both simple, fast and taking into account the basic concepts of seismology (Brune's source, realistic time envelope function, nonstationarity and ground-motion variability). Time-domain simulations are derived from the signal spectrogram and depend on few ground-motion parameters: Arias intensity, significant relative duration and central frequency. These indicators are obtained from empirical attenuation equations that relate them to the magnitude of the event, the source-receiver distance, and the site conditions. We improve the nonstationary stochastic method by using new functional forms (new surface rock dataset, analysis of both intra-event and inter-event residuals, consideration of the scaling relations and VS30), by assessing the central frequency with S-transform and by better considering the stress drop variability.Comment: 10 pages; 15th World Conference on Earthquake Engineering, Lisbon : Portugal (2012

On the Testing of Ground--Motion Prediction Equations against Small--Magnitude Data

Ground-motion prediction equations (GMPE) are essential in probabilistic seismic hazard studies for estimating the ground motions generated by the seismic sources. In low seismicity regions, only weak motions are available in the lifetime of accelerometric networks, and the equations selected for the probabilistic studies are usually models established from foreign data. Although most ground-motion prediction equations have been developed for magnitudes 5 and above, the minimum magnitude often used in probabilistic studies in low seismicity regions is smaller. Desaggregations have shown that, at return periods of engineering interest, magnitudes lower than 5 can be contributing to the hazard. This paper presents the testing of several GMPEs selected in current international and national probabilistic projects against weak motions recorded in France (191 recordings with source-site distances up to 300km, 3.8\leqMw\leq4.5). The method is based on the loglikelihood value proposed by Scherbaum et al. (2009). The best fitting models (approximately 2.5\leqLLH\leq3.5) over the whole frequency range are the Cauzzi and Faccioli (2008), Akkar and Bommer (2010) and Abrahamson and Silva (2008) models. No significant regional variation of ground motions is highlighted, and the magnitude scaling could be predominant in the control of ground-motion amplitudes. Furthermore, we take advantage of a rich Japanese dataset to run tests on randomly selected low-magnitude subsets, and check that a dataset of ~190 observations, same size as the French dataset, is large enough to obtain stable LLH estimates. Additionally we perform the tests against larger magnitudes (5-7) from the Japanese dataset. The ranking of models is partially modified, indicating a magnitude scaling effect for some of the models, and showing that extrapolating testing results obtained from low magnitude ranges to higher magnitude ranges is not straightforward

The 2000 Tottori (Japan) earthquake: triggering of the largest aftershock and constraints on Dc.

The goal of this study is to investigate the effect of the static and dynamic stress changes on the triggering of faults under slip-dependent friction law. We specifically focus on the 2000 Western Tottori (Japan) earthquake and on the triggering of its largest aftershock. To this end we compute the dynamic and static stress changes caused by the 2000 Western Tottori (Japan) earthquake for which a good knowledge of the rupture history and aftershock sequence exists. We compute the coseismic stress evolution caused by the mainshock fault, on the fault plane of the largest aftershock located 20 km SW of the mainshock. The static stress changes cannot explain the occurrence of the largest aftershock, located in a stress shadow whatever the friction coefficient that we use. Hence we propose that dynamic stresses have promoted the triggering of the largest aftershock. Using the discrete wavenumber and the reflectivity methods we compute the complete time-dependent coulomb failure function CFF(t). We investigate the influence of the adopted coefficient of friction μ, the depth and the location of the hypocenter on the shape of the CFF(t). Finally, using a non-linear slip dependent friction law with a stability/instability transition, we constrain the frictional properties of the largest aftershock fault plane knowing the state of stress on the fault and the time delay of 48 hours. We propose that Dc must be greater than 0.3 m

Taxonomy of κ: A review of definitions and estimation approaches targeted to applications

In a way perhaps not dissimilar to stress drop (Atkinson and Beresnev, 1997), the high-frequency attenuation parameter κ (kappa), introduced by Anderson and Hough (1984), is one of the most used yet least understood or agreed-upon parameters in engineering seismology. It describes the deviation at high frequencies between observed Fourier amplitude spectra calculated from seismograms and an ω−2 source model, such as the Brune (1970) model. Almost 30 years after its introduction, κ is used by seismologists and engineers alike and constitutes an important input parameter for several applications. Perhaps because of its importance, it is estimated, physically explained, and used in many different ways. This note aims to illustrate the multiple approaches to its estimation, and to suggest that, in order to reduce ambiguities, the parameter should always be given a notation consistent with its measurement and application to help avoid inconsistencies in its application of κ scaling to ground-motion models. Hanks (1982) observed that above a given frequency the acceleration spectrum decays sharply. He termed this frequency fmax (e.g., Fig. 1a) and attributed it mainly to local site conditions. Soon after, Anderson and Hough (1984) introduced an alternative parameter to model this decay, which is the one most commonly used today: κ. They measured κ directly from the high‐frequency part of the acceleration Fourier amplitude spectrum of a record. Above a certain frequency (which they named fe but we will call here f1), the overall shape of the spectrum generally decays exponentially with frequency; the decay constant is most easily measured by finding a linear approximation to the spectrum plotted in log–linear space. The slope of the linear approximation is −πκ (e.g., Fig. 1b). In this note we use the notation κr for individual observations of κ, for example, the κ value corresponding to the slope of a particular record; this record may be at any epicentral distance Re≥0. Anderson and Hough (1984) also observed that κr at individual stations increases with distance and concluded that it includes components related both to the local geology of the top few km of crust beneath the station and to the regional structure. They then suggested that the site component of κ (denoted κ0) could be computed by extrapolating the κr values to zero epicentral distance, thus correcting for the regional effect of anelastic Q. In this note, we discuss the use of κ0 in various engineering seismology applications today and why interest in this parameter has been revived. We briefly discuss its possible physical interpretations, and detail the known approaches to estimate κ0 from seismic records. We group these approaches into families according to basic features, such as the range of frequencies over which κ0 is computed and the trade‐off with path effects. We then discuss the alternative option for estimating κ0 when site‐specific records are not available, based on empirical correlations with VS30. We collect previously published correlations and demonstrate the scatter observed across different studies. Finally, we make suggestions as to how κ0 estimation can be made in a more consistent way with the applications that use it, and how existing correlations can be made more consistent to improve both the inference of κ0 in the absence of site‐specific data and the physical understanding of κ0

Effects of finite source rupture on landslide triggering: the 2016 Mw 7.1 Kumamoto earthquake

The propagation of a seismic rupture on a fault introduces spatial variations in the seismic wave field surrounding the fault. This directivity effect results in larger shaking amplitudes in the rupture propagation direction. Its seismic radiation pattern also causes amplitude variations between the strike-normal and strike-parallel components of horizontal ground motion. We investigated the landslide response to these effects during the 2016 Kumamoto earthquake (Mw 7.1) in central Kyushu (Japan). Although the distribution of some 1500 earthquake-triggered landslides as a function of rupture distance is consistent with the observed Arias intensity, the landslides were more concentrated to the northeast of the southwest–northeast striking rupture. We examined several landslide susceptibility factors: hillslope inclination, the median amplification factor (MAF) of ground shaking, lithology, land cover, and topographic wetness. None of these factors sufficiently explains the landslide distribution or orientation (aspect), although the landslide head scarps have an elevated hillslope inclination and MAF. We propose a new physics-based ground-motion model (GMM) that accounts for the seismic rupture effects, and we demonstrate that the low-frequency seismic radiation pattern is consistent with the overall landslide distribution. Its spatial pattern is influenced by the rupture directivity effect, whereas landslide aspect is influenced by amplitude variations between the fault-normal and fault-parallel motion at frequencies <2 Hz. This azimuth dependence implies that comparable landslide concentrations can occur at different distances from the rupture. This quantitative link between the prevalent landslide aspect and the low-frequency seismic radiation pattern can improve coseismic landslide hazard assessment

Testing the applicability of correlations between topographic slope and VS30 for Europe

In the past few years a series of articles have been published concerning the use of topographic slope from digital elevation models (DEMs) constructed through remote sensing (satellite imaging) to give first-order estimates of National Earthquake Hazards Reduction Program (NEHRP) site classes based on the average shear-wave velocity in the top 30 m, VS30 (Wald and Allen, 2007). We evaluate the potential applicability of these methods taking advantage of a large (706 sites) new database of measured and estimated VS30 values and their topographic slopes for locations in Europe and the Middle East. Novel statistical tests are performed to evaluate the predictive power of the procedure in this region. We evaluate the percentage of sites correctly classified/misclassified for each site class for active and stable regimes. We also analyze the marginal distributions of the input VS30 and slope values and their impact on the VS30-slope correlations and we evaluate whether the method performs better than does chance. We also consider the surface geology of sites and investigate whether differences in geology can help explain why certain sites are poorly classified by the method. Finally, we use the city of Thessaloniki, Greece, as a test case for comparison between the results of a recent microzonation and the site classes predicted by VS30-slope correlations. Our results show that the method does a better job than blind chance for all site classes in active regions, but only for class B (rock) and to a lesser extent class C (stiff soil) sites located in stable areas, although the conclusions for stable areas are based on limited data. We recommend that site classifications based on the VS30-slope correlations proposed by Wald and Allen (2007) be used only for regional or national (and not local or site-specific) first-order studies in active parts of Europe and only in the absence of other more detailed information, excluding sites inside small basins or those with special geological conditions that may affect results (e.g., flat-lying volcanic plateaus, carbonate rocks, continental glaciated terrain, or a coastal location if slope is not calculated using bathymetric data)

Compilation and critical review of GMPEs for the GEM-PEER Global GMPEs Project

International audienceGround-motion prediction equations (GMPEs) relate a ground-motion parameter (e.g. peak ground acceleration, PGA) to a set of explanatory variables describing the source, wave propagation path and site conditions. In the past five decades many hundreds of GMPEs for the prediction of PGA and linear elastic response spectral ordinates have been published. We discuss the pre-selection of GMPEs undertaken within the framework of the GEM-PEER Global GMPEs Project. The pre-selection criteria adopted were consistent with the current state-of-the-art in ground-motion characterization and sought to retain only the most robust GMPEs. Consideration of broad tectonic regionalization (e.g. shallow crustal seismicity in tectonically-active areas, stable continental regions and subduction zones) was made but it was assumed (based on previous studies) that strong regional differences were not present within these tectonic classes. In total about thirty GMPEs were pre-selected for closer inspection and testing to obtain a final set of ground-motion models

H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto

Defining a consistent strategy to model ground-motion parameters for the GEM-PEER Global GMPEs project

International audienceThe project entitled Global Ground Motion Prediction Equations is funded by the Global Earthquake Model (GEM) Foundation and has the objective of recommending a harmonized suite of ground motion prediction equations (GMPEs) that can be used at the global and regional scales for seismic hazard analysis and loss estimation studies. As part of this project, Task 1a experts were commissioned to make recommendations on the critical aspects of seismological predictor parameters that are used by predictive model developers to estimate ground motions for different earthquake scenarios. It is hoped that these recommendations would lead to the optimum description of ground-motion models that can be used efficiently for reliable seismic hazard assessment studies