Dicelis, G. E., Estudo Integrado da Camada de Basalto em Bebedouro SP, Bacia do Paraná: Relocalização dos sismos induzidos por poços tubulares, 2011. Dissertação de Mestrado Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, SP. A partir de 2004, a população do distrito de Andes, Bebedouro (SP), passou a sentir pequenos tremores de terra. Uma rede sismográfica instalada em 2005 mostrou que os sismos locais são induzidos por poços tubulares profundos usados para irrigação de lavouras (Assumpção et al., 2007). Neste projeto de pesquisa foi melhorada a localização desses sismos, usando análise da forma de onda e os tempos de chegada das ondas P e S, com o propósito de efetuar uma melhor análise da sismicidade induzida. Para isto foram aproveitados resultados de estudos anteriores como sísmica de refração, dispersão de ondas de superfície, sondagem elétrica vertical, função do receptor e TDEM. Para obter o melhor modelo de velocidades 1D, foram testados diferentes algoritmos de localização e técnicas de relocalização de hipocentros, para aproximadamente 3000 microtremores gravados pela rede sismográfica no período 2005 2010. Os resultados mostraram que as profundidades focais dos sismos relocalizados ficaram em sua maioria dentro da camada de basalto...
Along his history the Lower Tagus Valley (LTV) region was shaken by several earthquakes, some of them produced in large ruptures of offshore structures located southwest of the Portuguese coastline. Among these is the Lisbon earthquake of 1 November 1755 (M∼8.5-8.7), and other moderates earthquakes that were produced by local sources such as the 1344 (M6.0), 1531 (M7.1) and 1909 (M6.0) earthquakes.
Previous simulations  have shown high velocity amplification in the region. The model used in the simulations was updated from low to high resolution using all the new available geophysical and geotechnical data on the area (seismic reflection, aeromagnetic, gravimetric, deep wells and geological outcrops) .
To confirm this model in the areas where it was derived by potential field methods we use broadband ambient noise measurements collected in about 200 points along seven profiles on the LTV basin, six perpendicular and one parallel to the basin axis. We applied the horizontal to vertical (H/V) spectral ratio method  to the seismic noise profiles in order to estimate the distribution of amplification in the basin. The H/V curves obtained reveals the existence of two low frequency peaks centered on 0.2 and 1 Hz . These peaks are strongly related with the thickness of Cenozoic and alluvial sediments. The velocity model obtained by inversion of the H/V curves is in good agreement with borehole data...
Due the infrastructure construction, several instability problems have been presented in cutting slopes over clay bearing rocks around the world. The principal cause is associated with the highly susceptibility to environmental changes of these materials. Their physical and mechanical characteristics change in a short period of time after the cutting process, thus, their performance change between rock and soil. This makes very complicated the classification process and the selection of suitable parameters for design and construction. Different classification proposals have been developed on fracture patterns for basis and laboratory tests, but the subjectivity of the observer and the process for obtaining good samples affect the final result. Geophysical tests are a good alternative for the characterization of these materials in the field. This paper investigates the disintegration of these materials in cutting slopes using the microtremor H/V technique. The results show good correlation with disintegration classification systems.; Due the infrastructure construction, several instability problems have been presented in cutting slopes over clay bearing rocks around the world. The principal cause is associated with the highly susceptibility to environmental changes of these materials. Their physical and mechanical characteristics change in a short period of time after the cutting process...
1 - 1; En esta investigación se realizaron mediciones de microtremors en un trayecto de 50 km de la concesión Briceño Tunja Sogamoso, en el sector correspondiente entre el Siga y Tuta. Adicionalmente se realizó una revisión de la geología y geomorfología en los puntos de medición seleccionados. A partir de este proceso se estableció la naturaleza de los procesos modeladores del paisaje, y las condiciones diagenéticas importantes que tiene influencia en el desempeño de los materiales en estudio. Se utilizó el método Nakamura (relación espectral H/V), el cual que permite obtener períodos fundamentales de los depósitos, los cuales fueron asociados posteriormente con los niveles de degradación establecidos con la escala de degradación Slake Durability Rating (SDR). Los resultados obtenidos indican una correlación favorable, y permitieron verificar las hipótesis planteadas sobre la bondad de la técnica en detección de meteorización en estudios anteriores.; In this research, microtremors were registered along 50km of Highway Bogotá-Tunja – Sogamoso, in the zone between el Sisga and Tuta. The geology and geomorphology of every point was reviewed. It was key in the analysis of the nature of the landscape evolution and the determination of the diagenetic conditions that have effect in the performance of the materials in study. The Nakamura technic was used (H/V ratio). It is useful for the predominant period determination of the soil deposits. This results were correlated with the disintegration levels from classification chart of Slake durability Rating. A good correlation was obtained...
La concesión Briceño – Tunja - Sogamoso surge ante la necesidad de la población de expandirse económicamente y potenciar el desarrollo de las actividades que dan sustento, como agricultura, ganadería y minería. Esta obra de infraestructura se encuentra delimitada, a lo largo de su extensión por importantes estructuras geológicas cuyas características permiten clasificarla, de acuerdo con la NSR-10, como una zona de amenaza sísmica intermedia.
El área de estudio se centró en la zona axial de la cuenca Cordillera Oriental, conocida como La Sabana de Bogotá, específicamente entre las poblaciones de Chocontá y Tunja. Desde finales del Cretácico tardío hasta el Mioceno medio, la Cordillera Oriental fue sometida principalmente, a eventos tectónicos compresivos que permitieron su evolución, generando extensos sinclinales y anticlinales con una alta intensidad de fallamiento. La concesión Briceño – Tunja – Sogamoso, presente en al área de estudio, esta bordeada por grandes sinclinales al sur y una zona de estrechos y abruptos anticlinales en la parte central, que se extienden hasta la población de Tunja.
En el presente trabajo se llevó a cabo una campaña de medición de vibraciones ambientales en un trayecto de 50 km de la concesión. Con esta información se caracterizó el índice de degradación del área de estudio...
This paper presents the results from a series of experimental tests carried out to determine the damping
characteristics of a section of a 375 m long pile-supported wharf structure under forced excitation. The
test program was designed with two primary objectives: (1) to identify the fundamental damping of the
structure by using structural microvibration signals produced by tides, wind and microtremors, and (2) to
evaluate the variation of the dynamic properties as a function of response amplitude by applying initial
displacements of varying amplitude to the deck using a pull mechanism.
Although the wharf was designed as a series of independent deck sections, the study revealed that the
non-structural frames and piping supported on top of the wharf tie adjacent sections together and have
a significant influence on the dynamic behaviour, particularly in the longitudinal direction. Care must
be taken in to provide sliding connections for wharf supported structures or to include the influence of
these elements in the original design. From our review of the properties identified under the different
excitation levels, it was determined that the wharf has relatively linear behaviour with an equivalent
viscous damping of about 3%. This is a good reference damping value to be used for the analysis of the
pile-supported wharf structures under operational loads and low magnitude seismic events.; This
publication has been supported by Fondecyt project # 1070319.
Artículo de publicación ISI; Nowadays, microtremors' horizontal-to-vertical spectral ratio (HVSR) has been extensively used in the estimation
of the predominant vibration frequency of soils, mainly for microzonation purposes. In the present
study, we show results from extensive microtremor measurements performed at the cities of Talca and
Curicó, Central Chile. We found a strong correlation between surface geology and microtremors' HVSR,
even in complex geological settings. Considering the damage produced by the 2010 Maule earthquake
(Mw 8.8), we also estimated high-density macroseismic intensities at these cities, with values ranging
from VI up to VIII on the MSK64 scale. We believe that themain responsible of these variations are the surface
geological conditions, also reflected in the variations of the resonance frequencies of the soils. The evidence
presented here supports the use of microtremors' HVSR in combination with surface geology to improve seismic
microzonation studies, particularly in an Andean context.
The analysis of site effects is very important since the amplification of
seismic motion in some specific areas can be very strong. In this paper, the
site considered is located in the centre of Nice on the French Riviera. Site
effects are investigated considering a numerical approach (Boundary Element
Method) and are compared to experimental results (weak motion and
microtremors). The investigation of seismic site effects through numerical
approaches is interesting because it shows the dependency of the amplification
level on such parameters as wave velocity in surface soil layers, velocity
contrast with deep layers, seismic wave type, incidence and damping. In this
specific area of Nice, a one-dimensional (1D) analytical analysis of
amplification does not give a satisfactory estimation of the maximum reached
levels. A boundary element model is then proposed considering different wave
types (SH, P, SV) as the seismic loading. The alluvial basin is successively
assumed as an isotropic linear elastic medium and an isotropic linear
viscoelastic solid (standard solid). The thickness of the surface layer, its
mechanical properties, its general shape as well as the seismic wave type
involved have a great influence on the maximum amplification and the frequency
for which it occurs. For real earthquakes...
Strong earthquake ground shaking has been investigated by the study of 15 events recorded in El Centro, California. The strong-motion records analyzed show that no simple features (e.g., local site conditions) govern the details of local ground shaking. Any effects of local subsoil conditions at this site appear to be overshadowed by the source mechanism and the transmission path, there being no distinctly identifiable site periodicities.
Microtremor measurements have been taken in the area surrounding the strong-motion site. The objective was an investigation of possible correlations with strong ground motions and the analysis of site-response characteristics. Basic difficulties in ascertaining local site conditions through such low-amplitude ground motions are illustrated. It has been found that in this area microtremor and earthquake processes are widely different in character, there being little to no correlation between the ground's response to earthquakes and to microtremor excitations. Microtremors have been found to be nonstationary over periods of about a day or so, introducing further uncertainties into inferences from such measurements.
A frequency-domain formulation of the Aki (1957, 1965) autocorrelation method has been applied to seismic noise recorded by a 100-m wide circular array deployed on soft Holocene sediments in the Garigliano river valley, where a large amplification of ground motion during earthquakes was experienced (Rovelli et al., 1988). The application of this method to ambient noise recordings demonstrates that microtremors in the valley are dispersive and dominated by surface waves. By assuming that the vertical component reflects Rayleigh wave motion, we obtain a dispersion curve that is interpreted in terms of a layered shear-wave velocity structure. Layer thicknesses are constrained by the stratigraphic information provided by a deep hole drilled in the area, and shear velocities are estimated by means of a trial-and-error approach to achieve a satisfactory fit of the ambient noise dispersion. The best-fit velocity model is used to compute a theoretical transfer function, which is then compared with an average spectral ratio obtained from earthquake weak ground motions recorded at two stations, one in the valley and the other on a limestone reference site. An overall agreement is found between the theoretical curve and the observed spectral ratios. The discrepancies that do exist may be ascribed to the assumption of 1-D inhomogeneity which considerably simplifies the theoretical transfer function.
Our results show that the spatial-correlation method can be useful to infer velocity structure down to depths of hundreds of meters...
We employed ambient-noise measurements to assess the potential for seismic site response in sediment-filled valleys that intersect beneath downtown Providence, Rhode Island. At eight valley stations and at two sites on an adjacent bedrock highland, we recorded ground motion from two types of sources: pile drivers at a local construction site and ambient microtremors. At all valley sites where sediment thicknesses exceed 10 m, spectral ratios contain amplitude peaks at frequencies of 1.5 to 3.0 Hz. In contrast, spectral ratios from the two sites on the bedrock highland where sediment cover is less than 4-m thick are relatively flat within this frequency range. A variety of borehole logs identified two fundamental sediment types (soft sediment and a consolidated glacial till) and were used to map layer thicknesses over the entire study region. Refraction data constrained P-wave velocity in the bedrock to be 3680 ± 160 m/sec and indicated two soft-sediment layers with P-wave velocities of 300 ± 50 and 1580 ± 120 m/sec. Using a one-dimensional reflection matrix technique, we matched the spectral-ratio peak observed at each valley site with the frequency of fundamental resonance predicted for local layer thicknesses and velocities. A positive correlation between the best-fitting soft-sediment velocities and bedrock depth may reflect greater compaction in the deepest sediments or a locally two-dimensional sediment resonance at the deepest sediment sites. We conclude that unconsolidated sediment layers under downtown Providence have the potential to amplify earthquake ground motion at frequencies damaging to engineered structures.
Recording and analysis of microtremors is used in seismic risk evaluation. We discuss the applicability and results for the Spatial Autocorrelation Method (SPAC), which estimates a site velocity model for ID propagation wave modeling and more complete site response estimation. We present results for four Mexican cities with different spatial, urban and geological conditions. The purpose and requirements are different for each particular study. This affects the spatial array settings and the depth resolution for the SPAC method. We discuss these studies and their corresponding results. We conclude that it is possible to characterize site effects and site response using small linear arrays. Depending on the geological conditions, it is possible to reach deep layers.
Station LNIG is part of the new generation of seismological observatories that have been recently installed in Mexico by the Servicio Sismológico Nacional (Mexico). This station started operations in January 2006 in the Facultad de Ciencias de la Tierra from the Universidad Autónoma de Nuevo León, in Linares, Nuevo León, Mexico. We evaluate the site response at LNIG according to the definition of standard reference rock site. We obtained (a) the average shear wave velocity for the first 30 m depth, V S30, and (b) the resonance frequency. We got V S30 = 1870 m/s from seismic refraction profiles. This corresponds to a hard rock site type A in accordance to NEHRP. On the other hand, the spectral ratios were obtained from microtremors and local weak-motion earthquakes (3.1 < M < 4.5), they show a flat response (unitary amplitude) for the frequency range from 0.2 to 20 Hz. According to the definition proposed by Steidl et al. (1996) and Cadet et al. (2010), the LNIG site, located on shale of the Mendez Formation of the Upper Cretaceous, corresponds to a reference site.
We present results of the exploration of the subsoil in the lake bed zone of Mexico City using correlation of microtremors. We recorded microtremors using broad band stations and recording windows between a few minutes and one hour. Vertical components were analyzed using both SPAC and time interferometry to recover Rayleigh wave dispersion. Our measurements allow us to compute correlation of microtremors for a very wide range of distances between stations, from 10 m to almost 2 km. At short distances, we obtained significant correlation and the shallow velocity structure could be well determined. We show that lateral heterogeneities of the clay layer are important even over short distances. At larger distances, it was not possible to obtain good correlation. Correlation between two stations requires that the medium, at the wavelength scale of distance between stations, be able to sustain Rayleigh waves. Our results suggest that Rayleigh waves cannot be retrieved from correlation analyses for wavelengths between 1 and 9 km.
In this work we evaluate the cortical structure derived of dynamical properties of the cities Cumaná and Cumanacoa, Suﾭcre state, Venezuela. With this goal, spectral curves of frequency (H/V) were obtained using the JSESAME (ECU. 2000) program in the plots of environmental noise. We tried to fit with method of least squares, these curves to those obtained by applying the Nakamura model for ground microtremors, and the used equation was a corrected expression for S wave multireflection in parallel layers, which depends on the shallow crust model parameters (silt thickness, silt and basement S wave velocities, substrate and basement densities quotient, frequency, background noise, and high frequencies extinction). The families of curves linked to the different shallow crust models were obtained by a random approach, the phenotypic genetic algorithms. The solutions have two possible ranks of depth, a shallow one and a deep one, and this fact looks to depend on the frequency rank where the resonance picks appear. According to the results, we can state that the substrate thickness is significantly smaller at Cumanacoa than at Cumaná, which can be appreciated in both regimes average as the quantity of shallow solutions confronted with the deep ones: 95% to 35% in Cumanacoa and 86% to 82% in Cumaná. On the other hand...