Antonio García-Jerez

Characterization of the Sedimentary Cover of the Zafarraya Basin, Southern Spain, by Means of Ambient Noise

The sedimentary structure in the Zafarraya basin, located in the south of Spain, is studied by using the horizontal-to-vertical spectral ratio (HVSR) for ambient noise. To improve the reliability of the results, the stability of the HVSR measurement was checked by using a time-dependent analysis of the noise records taking into account only clear peaks. The resonant frequency varies through the basin and does not ever coincide with the maximum of the horizontal power spectrum. Several fits between the frequency of the main peak in the HVSR for microtremors and the depth to the bedrock are obtained for data from 17 sites, where geotechnical information was available. The relation derived using a scheme based on surface waves was considered to be the most reliable one. Thus, a function describing the average velocity versus depth for the sedimentary cover of the basin is also derived under assumptions of lateral homogeneity and smooth vertical variations of velocity due to age and/or confining pressure. The measurement of microtremors at 86 uniformly distributed points provides a map of the sedimentary thickness based on the previously calculated relationships, showing values greater than 200 m at the deeper zones.

A computer code for forward calculation and inversion of the H/V spectral ratio under the diffuse field assumption

During a quarter of a century, the main characteristics of the horizontal-to-vertical spectral ratio of ambient noise HVSRN have been extensively used for site effect assessment. In spite of the uncertainties about the optimum theoretical model to describe these observations, over the last decade several schemes for inversion of the full HVSRN curve for near surface surveying have been developed.In this work, a computer code for forward calculation of H/V spectra based on the diffuse field assumption (DFA) is presented and tested. It takes advantage of the recently stated connection between the HVSRN and the elastodynamic Greens function which arises from the ambient noise interferometry theory.The algorithm allows for (1) a natural calculation of the Greens functions imaginary parts by using suitable contour integrals in the complex wavenumber plane, and (2) separate calculation of the contributions of Rayleigh, Love, P-SV and SH waves as well. The stability of the algorithm at high frequencies is preserved by means of an adaptation of the Wangs orthonormalization method to the calculation of dispersion curves, surface-waves medium responses and contributions of body waves.This code has been combined with a variety of inversion methods to make up a powerful tool for passive seismic surveying. H/V spectral ratios of ambient seismic noise are modeled by using full wavefield.The theoretical framework is consistent with ambient noise interferometry.The method provides separate calculations of different wave modes.The software is also suitable for coda waves and any other diffuse-like wavefields.It supports joint inversion of seismic velocity models from H/V and dispersion curves.

Local site effect microzonation of Lorca town (SE Spain)

Local site effect assessment based on subsurface ground conditions is often the key to evaluate urban seismic hazard. The site effect evaluation in Lorca town (south-eastern Spain) started with a classification of urban geology through the geological mapping at scale 1:10,000 and the use of geotechnical data and geophysical surveys. The 17 geological formations identified were classified into 5 geological/seismic formations according to their seismic amplification capacity obtained from ambient vibration measurements as well as from simultaneous strong motion records. The shear-wave velocity structure of each geological/seismic formation was evaluated by means of inversion of Rayleigh wave dispersion data obtained from vertical-component array records of ambient noise. Nakamura’s method was applied to determine a predominant period distribution map. The spectral amplification factors were fourfold the values recorded in a reference hard-rock site. Finally, the capability of this study for explaining the damage distribution caused by the May 11th, 2011 Lorca destructive earthquake (Mw

Assessing the Reliability of the Single Circular-Array Method for Love-Wave Ambient-Noise Surveying

=

Shear-wave velocity structure from MASW and SPAC methods: The case of Adra town, SE Spain

= 5.2) was examined. The methods used in this work are of assistance to evaluate ground amplification phenomena in urban areas of complex geology as Lorca town due to future earthquakes with applicability on urban seismic risk management.

Shear Wave Velocity Structure for Seismic Microzonation of Lorca town (SE Spain) from MASW Analysis

In this article, we present a method to simulate wave propagation in a 2D medium with an irregular free surface by using a finite-difference method. The free-surface conditions are developed through an explicit scheme in displacement. In our technique, a conventional grid is used to define the zone where there is material and the zone where there is not material. In this method a fictitious line of material above the free surface is used to compute the displacement at the free surface. A classification of the points that shape the fictitious line is presented. Then displacements in the internal points of the material are computed together with the displacements at the points of the free surface, and subsequently the displacement at the points of the fictitious line are updated applying the boundary conditions by using an explicit finite-difference scheme. We present some results of the application of this technique by means of the simulation of the seismic response of a canyon and a mountain using an explosive source and a vertical force, respectively. We compared the results with the synthetics calculated by the indirect boundary element method (ibem). First, we tested the method simulating the wave propagation at a half-space with a planar surface. The comparison with the results of the ibem gave us confidence to deal with other models with topography features. These topographical models provided results that were in very good agreement with the results obtained by the ibem.

Comparative Study of SPAC and MASW Methods to Obtain the Vs30 for Seismic Site Effect Evaluation in Lorca town, SE Spain

Abstract The single circular array (SCA) method is a spatial autocorrelation (SPAC)–like technique for ambient noise exploration. Its main feature is the possibility of calculation of Love-wave dispersion curves by using centerless circular arrays of 3-component seismometers, allowing independent processing of each circle. Situations in which Rayleigh-wave and Love-wave arrivals or waves coming from different azimuths are mutually correlated are also correctly dealt with in this method. An algorithm for practical calculation of the SCA coefficient B is described. The algorithm includes averaging over a set of time windows and minimizes the number of spectral ratios to be computed for the purposes of stability. Numerical tests show that SCA coefficients estimated in this way have quite a robust behavior. Bias due to use of a finite number of sensors, as well as to effects of nonpropagating incoherent noise, has been theoretically studied in both the deterministic and the stationary random-field formulations. Using a finite number of stations is a cause of bias even under isotropic illumination conditions. Nevertheless, its effect can be neglected for wavelengths-to-radius ratios above a threshold that depends on the number of evenly distributed sensors. By contrast, uncorrelated noise may affect the whole frequency band and is behind the limitations of the method at low frequencies. Finally, we present the first real data test of this method, consisting of a comparison between theoretical and experimental Love-wave dispersion curves for a site where the structure is known. In practice, the minimum wavelength for direct velocity retrieval for a pentagonal array with radius r was approximately , although this value depends on the signal-to-noise ratio. Experiments demonstrate that the usable range can be extended, mainly toward shorter wavelengths, if the effects of noise and of the finite number of sensors are included in the analysis.

A theory for microtremor H/V spectral ratio: application for a layered medium

Many populated areas located in moderate seismic risk regions have been hit by earthquakes of moderate magnitude which, surprisingly, have caused very serious damage to buildings and led to the loss of human life. The city of Lorca (SE Spain) is a clear example. On 11 May 2011, two main shocks occurred in the vicinity of Lorca city with a maximum magnitude of 5.1 Mw causing some casualties and serious widespread damage in the city and its surroundings. Most of the damage was concentrated in certain districts in the city, while other parts remained intact. Actually, in Lorca, investigators detected both a clear example of seismic site effect and a seismic wave amplification occurrence due to the type of geological materials on which the city is located. For this reason, several studies are being carried out in Lorca city to assess the actual contribution of the soil conditions to the seismic amplification phenomena. In these studies, the shear wave velocity plays a key role as a parameter for evaluating the dynamic behaviour of the shallowest geological materials. Consequently, site characterization applied to calculating seismic hazard is usually based on the near surface shear-wave velocity distribution. This study looked at the average shear-wave velocity for the uppermost 30 m of ground, which is referred to as Vs30. The Vs30 values obtained from multichannel analysis of surface waves (MASW) were used to create a new soil classification map of Lorca city. Thus, the derived Vs30 map was transformed into the NEHRP and Eurocode 8 (EC8) standards. In Lorca city, the softness and the thickness of shallow geological formations have been observed as two important factors that affected the level of ground shaking and the degree of damage. The results show that there is a significant correlation between the Vs30 values and the damage distribution within the city.