Search

The relationship between the Green's function and the average correlations of the elastic displacements within a diffuse field has been firmly established. The energies of horizontal and vertical motion in such a field are proportional to the imaginary part of the corresponding diagonal components of the Green's tensor for coincident source and receiver. Given this remarkable connection, we examine the partitions of elastic waves due to dynamic horizontal and vertical surface loads in a layered elastic media. The elastic radiation characteristics for horizontally layered media are much more complicated than for the homogeneous half-space. While the corresponding power partitions of the different types of waves in a half-space do not vary with respect to frequency, even in an elastic medium consisting of layers over a half-space, the modal structure shows strong variations in the frequency, mainly around the main peak and the minimum of the horizontal-to-vertical spectral ratio (HVSR). Using a test model with a moderately high impedance contrast, we find that most of the energy density of the horizontal displacements is concentrated in the fundamental mode of the Love waves, reaching ∼80 per cent of the power in the horizontal components. The vertical displacement of the P - SV body waves reaches a maximum of ∼85 per cent, while the fundamental mode of Rayleigh waves reaches a maximum of ∼99 per cent of the available power in the vertical motion. With this analysis, it is possible to analyse the multimodal effects in the dispersion diagrams and explicitly relate the partition of the energy density with the dispersion curves and the HVSR under the diffuse field assumption. [ABSTRACT FROM AUTHOR]

The assessment of the local site amplification during an earthquake requires, among other input information, a reliable estimate of the shear-wave velocity profile, including the contact with engineering and seismic bedrocks. We determine the shear-wave velocity profiles at two Swiss strong-motion borehole-station sites at Visp (Valais) and Buochs (Nidwalden) by inversion of microtremor horizontal-to-vertical spectral ratio [H/V(z,f)] curves measured at the surface and at different depths. These borehole stations were built to monitor not only the seismic activity in Switzerland and the surrounding areas but also the nonlinear site response, especially liquefaction processes during strong local and regional earthquakes. The boreholes are equipped with accelerometers at various depths, with the deepest borehole located at 102 m below the surface. In the first part, we review the forward modeling algorithm of the full-microtremor H/V(z,f), with a focus on the computational cost and accuracy. In the second part, we perform a temporal analysis of the H/V(z,f) curves obtained from the accelerometers. The results show seasonal variabilities in H/V between summer and winter. The third part presents the inversions of the H/V curves for a single day in summer and winter at both sites. From the full H/V(z,f) inversion, we obtain shear-wave velocities in the upper 30 m of 216 and 209 m/s at Visp in winter and summer, respectively. At Buochs, the corresponding are 269 and 345 m/s. The depths of the seismic bedrock are at 219 and 210 m at Visp, and at Buochs they are at 293 and 213 m. The estimated velocity profiles compare well with independent estimates from array measurements of ambient seismic vibrations, gravimetry, and geological logging information. Finally, we use the obtained seismic velocity profile information to model the theoretical 1D shear wave transfer function. The latter result compares well with amplification function results obtained using earthquake recordings. [ABSTRACT FROM AUTHOR]

Since the eighties, several subduction earthquakes on the Mexican Pacific coast have been recorded at accelerometric networks in the Valley of Mexico (VM). Such are the cases of the 19 September 1985 M 8.1 Michoacán earthquake and the recent 7 September 2021 M 7.1 Guerrero earthquake. These events have caused from minor damages to widespread structural collapses in the VM. The vertical ground motion recorded at several sites in the VM clearly show recurrent Rayleigh wavetrains, which present similar characteristics in period, amplitude, and duration among stations. These characteristics are very stable in space and time. On the other hand, the corresponding horizontal ground motion are strongly affected by the surface geology of the valley because of the shear wave-energy trapped at the uppermost soft layers. In addition, the horizontal response at several soft-ground sites displays significant azimuthal effects. Based on these observations, we analyze thrust-faulting subduction earthquakes that occurred along the Mexican Pacific coast from Michoacán to Chiapas states, all recorded at the VM accelerometric networks. We identified similarities in the vertical and horizontal ground motion not only among stations but also between seismic events recorded at the same station. The local generation of surface waves is studied using a kinematic analysis in the frequency-wavenumber domain (f-k), and sundry techniques in frequency and time domains. As a preliminary explanation, our analysis suggests that azimuthal effects related to thrust faulting subduction earthquakes strongly influence the local generation of surface waves affecting the 3D response of the valley. Moreover, for all earthquakes in all stations, our results show that while the vertical ground motion is similar and its amplifications are small and governed by earthquake size, the two horizontal components reveal strong site effects. [ABSTRACT FROM AUTHOR]

Dynamic site characterization was performed at 25 sites located on the western portion of the Mexico City Basin that were severely damaged during the Mw7.1 2017 Puebla-Morelos, Mexico, earthquake. Testing was conducted using active and passive seismic surface wave methods and the microtremor horizontal-to-vertical spectral ratio method to determine site periods and develop one-dimensional (1D) shear wave velocity (Vs) profiles for the first 60 m of the subsoil. The measured site periods were compared to site period maps developed in 2004 and 2020 along with values computed using the Design Seismic Actions System (SASID) software following the 2020 version of the Complementary Technical Norms for Seismic Design (NTCDS). The most noticeable biases in the predictions from the 2004 site period map were observed between the boundary of Zone II and Zone IIIa, at which site periods are overestimated. These estimates were improved upon in the 2020 site period map and showed a close similarity with SASID computed site period values. The Vs, depth, and thickness of the lacustrine clay layer were also determined to be quite variable within the basin. The softest sites are located between the lakebeds with a Vs between 45 and 57 m/s. Sites located toward the outer rim of the North lakebed have a higher Vs between 80 and 100 m/s. The thickness of the clay layer varies significantly in the western side of the Basin with values ranging between approximately 3 and 34 m. Overall, the results of this study indicate good agreement with the model embedded in the SASID software. The results (1) emphasize the need to regularly monitor changes that occur over time in the lacustrine clay layer, (2) complement the development of models that improve our understanding of wave propagation within the Basin, and (3) update and improve Mexico City's Norms. [ABSTRACT FROM AUTHOR]

The scattering and diffraction of waves by irregular surface profiles is of interest in seismology and in many other areas. Diverse techniques have been proposed to quantitatively study the problem. Among them, domain approaches such as finite differences, spectral elements and finite elements have been used. Because the reduction of dimensionality boundary formulations is widely used. Recently, the direct boundary-element method has been applied using some series approximations for surface scattering, including the preconditioned splitting series, for the numerical description of rough surface scattering. Extending further and simplifying this approach, we use the indirect boundary-element method. The ensuing Fredholm integral equation of the second kind that arises in IBEM leads to a very efficient iterative scheme based on the classical Jacobi method. A discussion of direct and indirect approaches is presented. Assuming incident SH waves, results are obtained with the various approaches and compared among them for both a canyon and a hill, both of semicircular shape. Besides, an example is presented of a surface profile that produces strong scattering. This was inspired by the diverse problems that arise in the emerging field of metamaterials. [ABSTRACT FROM AUTHOR]