ALBERT

Description: This study reviews seismograms from 10 rock-fall events recorded between 1992 and 2001 by the permanent seismological network Sismalp in the French Alps. A new seismic-magnitude scale was defined that allowed us to compare and classify ground-motion vibrations generated by these Alpine rock falls. Each rock fall has also been characterized by its ground-motion duration t (sub 30) at an epicentral distance of 30 km. No relation was found between rock-fall parameters (fall height, runout distance, volume, potential energy) and rock-fall seismic magnitudes derived from seismogram amplitudes. On the other hand, the signal duration t (sub 30) shows a rough correlation with the potential energy and the runout distance, highlighting the control of the propagation phase on the signal length. The signal analysis suggests the existence of at least two distinct seismic sources: one corresponding to the initial rupture associated with an elastic rebound during the detachment and the other one generated by the rock impact on the slope. Although the fall phenomenon includes other complex processes (fragmentation of the block, interaction with topography, plastic deformation during and after impact) 2D finite-element simulations of these two seismic sources are able to retrieve the main seismogram characteristics.

Description: Physical models that can be used to obtain realistic accelerograms usually require a thorough knowledge of the source, path, and site effects. In addition, the computational resources needed might be expensive. Thus, empirical models still represent a good alternative for simulating strong ground motion. In this work, we modify and improve the model developed by Sabetta and Pugliese (1996). This new method models the time-domain accelerogram based on the assumption that the phase is random and that the time envelope can be represented by the so-called average instantaneous power. This is, in turn, described as a lognormal distribution for P and S waves combined with an algebro-exponential function representing the envelope of coda waves. In addition, the frequency content of the signal is nonstationary and follows a modified omega -square model. The method depends on four common indicators in earthquake engineering: peak ground acceleration, strong-motion duration, Arias intensity, and central frequency. These indicators are empirically connected to a given database by means of ground-motion prediction equations. In this study we calibrate the model using Japanese data recorded by the K-net array, which has high-quality digital accelerograms and station-site conditions characterized by geotechnical measurements. In addition, this technique permits the inclusion of the uncertainty of the model parameters to take into account the ground-motion natural variability in the stochastic generation of the time histories. The main goal of this work is to provide the earthquake engineering community with a flexible tool to generate realistic accelerograms for dynamic studies.

Description: The empirical Green's functions (EGF) technique is used to investigate two methods for predicting ground motion in a sedimentary basin for a future earthquake, including variability assessment. This study focuses on the Grenoble basin (French Alps). The basic principle of both methods is to generate a variety of source parameter sets based on a grid testing approach. Next, these sets are used to compute a population of ground motions by means of a kinematic EGF method and to estimate ground-motion variability. The first method tried, called the direct-parameter-input approach, selects input parameter combinations from assumed source parameter probability density functions. It is demonstrated that this approach leads to overestimated variability. Moreover, these simulation results are not calibrated. A new (screened-parameter-input) procedure is therefore proposed: (1) reference rock site response spectra are simulated for fractiles of several orders using empirical ground-motion prediction equations; (2) a large population of rock site response spectra is generated by means of the EGF method with varying rupture parameter combinations; (3) the spectra that do not fit the empirical motion for the chosen fractiles are screened out and sets of permissible source parameter combinations are thus obtained; (4) sediment site response spectra are computed with this EGF procedure and with these permissible parameter combinations; and (5) for each frequency median spectral acceleration and standard-deviation values are derived.

Description: Ground-motion amplitudes from recent European earthquakes of moderate magnitudes have been observed to be systematically smaller than the values expected from a number of popular empirical ground-motion prediction equations used for seismic hazard analysis. It has been suggested that these discrepancies are caused by regional variations in seismotectonic character and as a consequence that seismic hazard estimates using these ground-motion models would be too high. In the present article, we explore the hypothesis that the discrepancy can simply be explained by the fact that these models adopt magnitude-independent functional forms (letting Y designate the response spectral acceleration M designate the magnitude, then dlog Y/dM is assumed independent of both magnitude and distance). The data collected by the KiK-net array (see the Data and Resources section) provides a unique opportunity to test this hypothesis and to analyze some of the pitfalls of deriving magnitude-independent functional form models and applying them for predictions of ground motion from smaller events (and vice versa). Borehole rock KiK-net ground-motion data (337 events, 3894 records) have been used to derive empirical ground-motion models with magnitude-independent functional forms for various magnitude ranges. By using these new ground-motion models and stochastic simulations, we discuss the ground-motion distance decays and magnitude effects for ground-motion models obtained with different magnitude range datasets. This analysis clearly indicates that response spectral amplitudes of ground motions from large earthquakes decay slower with distance than those from small earthquakes and confirms that the magnitude scaling of ground motion decreases as earthquake magnitude increases. Using stochastic simulations, we demonstrate that the observed decay in scaling could be a mixture of geometrical decay from extended sources and the fact that response spectral values instead of Fourier spectral values are considered. New ground-motion models (with functional forms including coefficients to model the observed magnitude-dependent scaling and decay rate) have finally been calculated for both surface and borehole site conditions of the analyzed dataset. These models show similar decays for intermediate period or moderate magnitude earthquakes. Our site classifications remove most of the statistical trend of the site effect and suggest that source and path effects could dominate the aleatory variability.

Description: Displacement spectra of earthquakes recorded by the French accelerometric network at regional scale are modeled as the product of source, propagation (including geometric and anelastic attenuation), and site effects. We use an iterative Gauss-Newton inversion to solve the nonlinear problem and retrieve these different terms. This method is easy to implement because the partial derivatives of the amplitude spectrum with respect to the different parameters have simple analytic forms. After convergence, we linearize the problem around the solution to compute the correlation matrix, which allows us to identify the parameters which are poorly resolved. We analyze data from two tectonically active regions: the Alps and the Pyrenees. Eighty-three earthquakes with local magnitudes between 3.0 and 5.3 are analyzed, with epicentral distances in the range 15-200 km. S-wave displacement spectra are computed using a fast Fourier transform and integration in the 0.5-15-Hz frequency domain. We assume a Brune-type source, with a geometric attenuation of the form R (super -gamma ) , gamma being constant, and a frequency-dependent quality factor of the form Q = Q (sub 0) Xf (super alpha ) . The results reveal that the attenuation parameters are correlated to each other and to the seismic moments. The two regions have different attenuation patterns. The geometrical spreading factor is equal to 1 for the Alps and 1.2 for the Pyrenees. The anelastic attenuation exhibits low Q (sub 0) values (322 and 376 for the Alps and the Pyrenees, respectively) with regional variations for alpha (0.21 in the Alps and 0.46 in the Pyrenees). Computed moment magnitudes are generally 0.5 unit smaller than local magnitudes, and the logarithms of the corner frequencies decrease linearly with magnitude according to log (sub 10) (f (sub c) ) = 1.72-0.32XM (sub w) . Stress drops range from 10 (super 5) to 10 (super 7) Pa (i.e., 1-100 bars), with a slight dependence to magnitude (large stress drops for large magnitudes). Finally, robust site responses relative to an average rock-site response are derived, allowing us to identify good reference rock sites.

Description: Little work has been undertaken to examine the role of specific long-term fault properties on earthquake ground motions. Here, we empirically examine the influence of the structural maturity of faults on the strong ground motions generated by the rupture of these faults, and we compare the influence of fault maturity to that of other source properties (slip mode, and blind versus surface rupturing). We analyze the near-field ground motions recorded at rock sites for 28 large (M (sub w) 5.6-7.8) crustal earthquakes of various slip modes. The structural maturity of the faults broken by those earthquakes is classified into three classes (mature, intermediate, and immature) based on the combined knowledge of the age, slip rate, cumulative slip, and length of the faults. We compare the recorded ground motions to the empirical prediction equation of Boore et al. (1997). At all frequencies, earthquakes on immature faults produce ground motions 1.5 times larger than those generated by earthquakes on mature faults. The fault maturity appears to be associated with larger differences in ground-motion amplitude than the style of faulting (factor of 1.35 between reverse and strike-slip earthquakes) and the surface rupture occurrence (factor of 1.2 between blind and surface-rupturing earthquakes). However, the slip mode and the fault maturity are dependent parameters, and we suggest that the effect of slip mode may only be apparent, actually resulting from the maturity control. We conclude that the structural maturity of faults is an important parameter that should be considered in seismic hazard assessment.