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Quito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin's seismic response and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito's urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve interstations surface-wave Green's functions in the frequency range of 0.1–2 Hz. We find that Love waves travelling in the basin's longitudinal direction (NNE–SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them in conjunction with the HVSR curves to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin's structure between its northern and southern parts. In the centre and northern areas, the estimated basin depth and mean shear-wave velocity are about 200 m and 1800 ms−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement's depth and shear-wave velocity in the southern part are about 900 m and 2500 ms−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the centre-north and the south of the basin explains the spatial distribution of low-frequency seismic amplifications observed during the M w 7.8 Pedernales earthquake in April 2016 in Quito. [ABSTRACT FROM AUTHOR]

Some geological configurations, like sedimentary basins, are prone to site effects. Basins are often composed of different geological layers whose properties are generally considered as spatially homogeneous or smoothly varying. In this study, we address the influence of small-scale velocity fluctuations on seismic response. For this purpose, we use the spectral element method to model the 2-D SH wave propagation on a basin of 1.1 km long and ≈ 60 m deep, representing a 2-D profile in the city of Nice, France. The velocity fluctuations are modelled statistically as a random process characterized by a Von Karman autocorrelation function and are superimposed to the deterministic model. We assess the influence of the amplitude and correlation length of the random velocities on the surface ground motion. We vary the autocorrelation function's parameters and compute seismic wavefields in 10 random realizations of the stochastic models. The analyses of our results focus on the envelope and phase differences between the waveforms computed in the random and deterministic models; on the variability of ground motion intensity measures, such as the peak ground velocity, the pseudo-spectral acceleration response; and the 2-D basin response (transfer function). We find that the amplitude of fluctuations has a greater effect on the ground motion variability than the correlation length. Depending on the random medium realization, the ground motion in one stochastic model can be locally amplified or deamplified with respect to the reference model due to the presence of high or low velocity contrasts, respectively. When computing the mean amplification of different random realizations, the results may be smaller than those of the reference media due to the smoothing effect of the average. This study highlights the importance of knowing the site properties at different scales, particularly at small scales, for proper seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Seismic waves generated by rockfalls contain valuable information on the properties of these events. However, as rockfalls mainly occur in mountainous regions, the generated seismic waves can be affected by strong surface topography variations. We present a methodology for investigating the influence of topography using a Spectral‐Element‐based simulation of 3‐D wave propagation in various geological media. This methodology is applied here to Dolomieu crater on the Piton de la Fournaise volcano, Reunion Island, but it can be used for other sites, taking into account local topography and medium properties. The complexity of wave fields generated by single‐point forces is analyzed for different velocity models and topographies. Ground‐motion amplification is studied relative to flat reference models, showing that Peak Ground Velocity (PGV) and total kinetic energy can be amplified by factors of up to 10 and 20, respectively. Simulations with Dolomieu‐like crater shapes suggest that curvature variations are more influential than depth variations. Topographic effects on seismic signals from rockfalls at Dolomieu crater are revealed by interstation spectral ratios. Results suggest that propagation along the topography rather than source direction dominates the spectral ratios and that resulting radiation patterns can be neglected. The seismic signature of single rockfall impacts is studied. Using Hertz contact theory, impact force and duration are estimated and then used to scale simulations, achieving order‐of‐magnitude agreement with observed signal amplitudes and frequency thresholds. Our study shows that combining Hertz theory with high‐frequency seismic wave simulations on real topography improves the quantitative analysis of rockfall seismic signals. Key Points: First‐ever simulation of high‐frequency rockfall seismic waves using the 3D Spectral Element MethodGround‐motion amplification induced by volcano topography found to be dependent on soil properties and rockfall positionSimulations and observations successfully compared by means of inter‐station spectral ratios and Hertz theory [ABSTRACT FROM AUTHOR]

The influence of lateral heterogeneities on alluvial deposits is a topic of particular interest in the field of urban planning and engineering design of structures and infrastructures. This work focuses on the effects of such heterogeneities on the shear strains produced within the recent alluvial deposits of the Tiber River in the historical center of Rome in case of the worst expected earthquake scenario. To this aim, a 3D engineering-geology model of the subsoil is used to derive four geological sections across the Tiber River valley as well as 48 soil columns to perform numerical simulations. Various models are considered: a viscoelastic equivalent linear rheology in a ID finite-difference model for one-component horizontal input, a nonlinear elastoplastic model in a ID finite-element scheme for three-component input, and a nonlinear viscoelasto- plastic rheology in a 2D finite-difference model under one-component horizontal input. After comparing these different simulations, results have shown that lateral heterogeneities play a key role with respect to the expected shear strains within multilayered soils. To this aim, some specific indexes are introduced to estimate the maximum shear strain (MSS) concentration within the soil layers as well as to highlight their effect due to the stratigraphic position of the layers, within the soil column, independently from its depth. A final differential index leads to the evaluation of the lateral heterogeneity effect on the estimated MSS, demonstrating their prevalent role with respect to the bedrock shape (i.e., the angle of inclination of the buried valley slopes). From these results, an MSS zoning map is obtained for the historical center of Rome, showing that the local seismic response should be modeled by assuming ID or 2D conditions depending on the location considered. [ABSTRACT FROM AUTHOR]

High-frequency acceleration pulses recorded during recent damaging earthquakes show that the evolution of pore water pressure in liquefiable soils may have a significant effect on earthquake ground motions. Such observations suggest that advanced constitutive soil models capable of treating the phase transformation behavior of liquefiable soils should be used for reliable predictions of earthquake site response. Advanced constitutive models require knowledge of the dilatancy parameters that describe the potential of soils to generate excess pore water pressure. We demonstrate that these dilatancy parameters can be determined directly from field observations by inverting strong motions recorded on vertical arrays (i.e., installation of surface and borehole accelerometers). We analyze the records of the 1987 M 6.6 Superstition Hills earthquake, the 1993 M 7.8 Kushiro-Oki, Japan, earthquake, and the 2011 M 9.0 Tohoku, Japan, earthquake to quantify the dilatancy parameters at the Wildlife liquefaction array (WLA), at Kushiro port (KP), and the KiK-net site FKSH14, respectively. Synthetic acceleration time series obtained from the minimum misfit models are describing the time and frequency evolution of the observations more precisely than previously published models. Dilatancy parameters obtained for WLA and KP suggest that soils at these sites were more resistant to liquefaction than predicted from field and laboratory tests. We also infer a high liquefaction resistance (CRR7.5 = 0.5) for the site FKSH14, which exhibited dilation pulses of up to 0.4g during the Tohoku earthquake. These findings indicate that even soils with a strong liquefaction resistance may exhibit cyclic mobility effects during strong and prolonged ground motions. [ABSTRACT FROM AUTHOR]

In this study, information carried by the ambient seismic field is exploited to extract impulse response functions between two seismic stations using one as a 'virtual' source. Interferometry by deconvolution method is used and validated by comparing the extracted ambient noise impulse response waveforms with records of moderate magnitude earthquakes (from Mw 4 to 5.8) that occurred close to the virtual source station in Japan. As the information is only available at low frequencies (less than 0.25 Hz), the ambient seismic field approach is coupled to a non-stationary stochastic model to simulate time domain accelerograms up to 50 Hz. This coupling allows the predicted ground motion to have both the deterministic part at low frequencies coming from the source and the crust structure and the high-frequency random contribution from the seismic waves scattering. The resulting combined accelerograms for an Mw 5.8 event show a good agreement with observed ground motions from a real earthquake. [ABSTRACT FROM AUTHOR]

A ground-motion prediction equation (GMPE) specific to rock and stiff-soil sites is derived using seismic motion recorded on high VS30 sites in Japan. This GMPE applies to events with 4.5 ≤ Mw ≤ 6.9 and VS30 ranging from 500 to 1500 m/s (stiff-soil to rock sites). The empirical site coefficients obtained and the comparison with the simulated site functions show that seismic motion on rock and stiff-soil sites does not depend only on VS30, but also on the high-frequency attenuation site properties (K0). The effects of the site-specific K0 on site amplification are analyzed using stochastic simulations, with the need to take into account both of these parameters for rock-site adjustments. Adding the site-specific K0 into the GMPEs thus appears to be essential in future work. The rock-site stochastic ground-motion simulations show that the site-specific K0 controls the frequency corresponding to the maximum response spectral ac-celeration (famp1)- This observation is used to link the peak of the response spectral shape to K0 in this specific Japanese dataset and then to add the effects of high-frequency attenuation into the previous GMPE from the peak ground acceleration and up to periods of 0.2 s. The inclusion of K0 allows the observed bias to be corrected for the intraevent residuals and thus reduces sigma. However, this k0 determination is limited to a minimum number of rock-site records with Mw ≥4.5 and to distances of less than 50 km. [ABSTRACT FROM AUTHOR]

Ground motions of the 2011 Tohoku earthquake recorded at Onahama port (Iwaki, Fukushima prefecture) rank among the highest accelerations ever observed, with the peak amplitude of the 3-D acceleration vector approaching 2g. The response of the site was distinctively non-linear, as indicated by the presence of horizontal acceleration spikes which have been linked to cyclic mobility during similar observations. Compared to records of weak ground motions, the response of the site during the Mw 9.1 earthquake was characterized by increased amplification at frequencies above 10 Hz and in peak ground acceleration. This behaviour contrasts with the more common non-linear response encountered at non-liquefiable sites, which results in deamplification at higher frequencies. We simulate propagation of SH waves through the dense sand deposit using a non-linear finite difference code that is capable of modelling the development of excess pore water pressure. Dynamic soil parameters are calibrated using a direct search method that minimizes the difference between observed and simulated acceleration envelopes and response spectra. The finite difference simulations yield surface acceleration time-series that are consistent with the observations in shape and amplitude, pointing towards soil dilatancy as a likely explanation for the high-frequency pulses recorded at Onahama port. The simulations also suggest that the occurrence of high-frequency spikes coincided with a rapid increase in pore water pressure in the upper part of the sand deposit between 145 and 170 s. This sudden increase is possibly linked to a burst of high-frequency energy from a large slip patch below the Iwaki region. [ABSTRACT FROM AUTHOR]

Knowledge of the acceleration spectral shape is crucial to various applications in engineering seismology. Spectral amplitude decays rapidly at high frequencies. Anderson and Hough (1984) introduced the empirical factor κ to model this attenuation. This is the first time κ is studied in a vertical array consisting of more than two stations. We use 180 earthquakes recorded at a downhole array with five stations in soils and rock to investigate the effect of soil conditions on κ. Given that κ computation processes vary across literature when following the classic AndersonHough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of κ0 range between 0.0 17 and 0.031 s at the surface and between 0.004 and 0.024 s at rock. This variability due to the assumptions made is larger than the error of each estimate and larger than the average difference in values between sediment and rock. For this data set, part of it can be attributed to the type of distance used. Given this variability, κ0 values across literature may not always be comparable; this may bias the results of applications using κ0 as an input parameter, such as ground-motion prediction equations. We suggest ways to render the process more homogeneous. We also find that κ at rock level is not well approximated by surface records from which we deconvolved the geotechnical transfer function. Finally, we compute κ on the vertical component and find a dependence of the vertical-to-horizontal κ ratio on site conditions. [ABSTRACT FROM AUTHOR]

We simulate non-linear behaviour of soils during strong ground motion in the Rhône valley in southern Switzerland. Previous studies of the site response using weak ground motion, ambient noise and linear 3-D FD simulations suggest that the 2-D structure of the basin will lead to amplification factors of up to 12 in the frequency band between 0.5 and 10 Hz. To estimate the importance of non-linear soil behaviour during strong ground motion in the Rhône valley we simulate the response of a superficial soft layer with a fully non-linear 1-D finite difference code. The non-linear wave propagator is based on an effective stress constitutive soil model capable of predicting pore pressure evolution due to shear. We determine the required dilatancy parameters from laboratory analysis of soil samples using cyclic triaxial tests. In order to include the effect of the strong 2-D structure in our non-linear analysis synthetic seismograms are convolved with the transfer function of the basin and then propagated through a 1-D non-linear layer. We find that reduced amplification due to soil non-linearity can be expected at rock accelerations above 0.5 ms−2, and that de-amplification occurs at ground motion levels of approximately 2 ms−2. Nevertheless, the spectral accelerations simulated for the valley centre are still exceeding the design spectra at about 0.5 Hz for magnitudes above 6.0, which reflects the strong amplification of ground motion by the deep 2-D resonance of the basin. For frequencies above 1 Hz the design spectra are generally in agreement with the strongest simulated accelerations. We evaluate the occurrence of soil failure using the 5 per cent strain criterion as a function of hypocentral distance and magnitude. Results confirm observations of liquefaction reported after the 6.4 earthquake of Visp, and they suggest that soil liquefaction may occur at distances beyond those predicted by empirical relations in the valley. Near the basin edge, however, the simulated liquefaction occurrence agrees with the empirical relations. These results suggest that the response of the whole structure needs to be simulated in order to estimate the non-linear seismic response of complex basins like the Rhône valley. [ABSTRACT FROM AUTHOR]

Mathematical models of soil nonlinearity in common use and recently developed nonlinear codes are compared to investigate the range of their predictions. We consider equivalent linear formulations with and without frequency-dependent moduli and damping ratios and nonlinear formulations for total and effective stress. Average velocity profiles to 150 m depth with midrange National Earthquake Hazards Reduction Program site classifications (B, BC, C, D, and E) in the top 30 m are used to compare the response of a wide range of site conditions from rock to soft soil. Nonlinear soil models are compared using the amplification spectrum, calculated as the ratio of surface ground motion to the input motion at the base of the velocity profile. Peak input motions from 0.1g to 0.9g are considered. For site class B, no significant differences exist between the models considered in this article. For site classes BC and C, differences are small at low input motions (0.1g to 0.2g), but become significant at higher input levels. For site classes D and E the overdamping of frequencies above about 4 Hz by the equivalent linear solution with frequency-independent parameters is apparent for the entire range of input motions considered. The equivalent linear formulation with frequency-dependent moduli and damping ratios under damps relative to the nonlinear models considered for site class C with larger input motions and most input levels for site classes D and E. At larger input motions the underdamping for site classes D and E is not as severe as the overdamping with the frequency-independent formulation, but there are still significant differences in the time domain. A nonlinear formulation is recommended for site classes D and E and for site classes BC and C with input motions greater than a few tenths of the acceleration of gravity. The type of nonlinear formulation to use is driven by considerations of the importance of water content and the availability of laboratory soils... [ABSTRACT FROM AUTHOR]