Shallow Soil Layers Fundamental Vibration Mode: A Comparison of Estimations Obtained With the Random Decrement Method and H/V Spectral Ratios Using Weak and Moderate Seismic Ground Motions
This article presents the results of the fundamental vibration frequency estimations at experimental sites in Puerto Rico as part of a long term project for seismic zonation of the urban area of the city of Mayaguez in the western part of the island. On those sites ambient noise measurements and small magnitude earthquake signals were recorded. The collected time series of weak ground motions (microtremors, ambient noise) and moderate seismic ground motions (moderate magnitude earthquakes) were processed with either both H/V spectral ratios (HVSPR) and the Random Decrement Method (RDM). The site fundamental period was obtained with both methods and the results compared. In addition to the fundamental vibration study, the article includes preliminary results of a damping estimation study where the RDM method applied directly to the surface signal is compared with the inverse Fourier transform of the H/V spectral ratios. The interpretation of results in terms of the physical properties, and the geometry of the layers at depth were carried out using the modified 1-D wave propagation Thomson-Haskel method. When geotechnical information was available at the site, the results obtained from these geophysical methods were compared in terms of the degree of the resolution of the estimations and uncertainty of the estimated values.
Random Decrement Method and Modeling H/V Spectral Ratios: An Application for Soft Shallow Layers Characterization
Results of an ongoing study to estimate the ground response upon weak and moderate earthquake excitations are presented. A reliable site characterization in terms of its soil properties and sub-soil layer configuration are parameters required in order to do a trustworthy estimation of the ground response upon dynamic loads. This study can be described by the following four steps: (1) Ambient noise measurements were collected at the study site where a bridge was under construction between the cities of Tijuana and Ensenada in Mexico. The time series were collected using a six channels recorder with an ADC converter of 16 bits within a maximum voltage range of ± 2.5 V, the recorder has an optional settings of: Butterworth/Bessel filters, gain and sampling rate. The sensors were a three orthogonal component (X, Y, Z) accelerometers with a sensitivity of 20 V/g, flat frequency response between DC to 200 Hz, and total full range of ±0.25 of g, (2) experimental H/V Spectral Ratios were computed to estimate the fundamental vibration frequency at the site, (3) using the time domain experimental H/V spectral ratios as well as the original recorded time series, the random decrement method was applied to estimate the fundamental frequency and damping of the site (system), and (4) finally the theoretical H/V spectral ratios were obtained by means of the stiffness matrix wave propagation method.. The interpretation of the obtained results was then finally compared with a geotechnical study available at the site.
Site Characterization in the Urban Area of Tijuana, B. C., Mexico by Means of: H/V Spectral Ratios, Spectral Analysis of Surface Waves, and Random Decrement Method
Results of site characterization for an experimental site in the metropolitan area of Tijuana, B. C., Mexico are presented as part of the on-going research in which time series of earthquakes, ambient noise, and induced vibrations were processed with three different methods: H/V spectral ratios, Spectral Analysis of Surface Waves (SASW), and the Random Decrement Method, (RDM). Forward modeling using the wave propagation stiffness matrix method (Roësset and Kausel, 1981) was used to compute the theoretical SH/P, SV/P spectral ratios, and the experimental H/V spectral ratios were computed following the conventional concepts of Fourier analysis. The modeling/comparison between the theoretical and experimental H/V spectral ratios was carried out. For the SASW method the theoretical dispersion curves were also computed and compared with the experimental one, and finally the theoretical free vibration decay curve was compared with the experimental one obtained with the RDM. All three methods were tested with ambient noise, induced vibrations, and earthquake signals. Both experimental spectral ratios obtained with ambient noise as well as earthquake signals agree quite well with the theoretical spectral ratios, particularly at the fundamental vibration frequency of the recording site. Differences between the fundamental vibration frequencies are evident for sites located at alluvial fill (~0.6 Hz) and at sites located at conglomerate/sandstones fill (0.75 Hz). Shear wave velocities for the soft soil layers of the 4-layer discrete soil model ranges as low as 100 m/s and up to 280 m/s. The results with the SASW provided information that allows to identify low velocity layers, not seen before with the traditional seismic methods. The damping estimations obtained with the RDM are within the expected values, and the dominant frequency of the system also obtained with the RDM correlates within the range of plus-minus 20 % with the one obtained by means of the H/V spectral ratio.
Soil Characterization and Site Response of Marine and Continental Environments
An in situ soil properties study was conducted to characterize both site and shallow layer sediments under marine and continental environments. Data from the SCoOBA (Sea of Cortez Ocean Bottom Array) seismic experiment and in land ambient vibration measurements on the urban areas of Tijuana, B. C., and Ensenada, B. C., Mexico were used in the analysis. The goal of this investigation is to identify and to analyze the effect of the physical/geotechnical properties of the ground on the site response upon seismic excitations in both marine and continental environments. The time series were earthquakes and background noise recorded within interval of 10/2005 to 10/2006 in the Gulf of California (GoC) with very-broadband Ocean Bottom Seismographs (OBS), and ambient vibration measurements collected during different time periods on Tijuana and Ensenada urban areas. The data processing and analysis was conducted by means of the H/V Spectral Ratios (HVSPR) of multi component data, the Random Decrement Method (RDM), and Blind Deconvolution (BD). This study presents ongoing results of a long term project to characterize the local site response of soil layers upon dynamic excitations using digital signal processing algorithms on time series, as well as the comparison between the results these methodologies are providing.