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Soil damping and site (system) dominant vibration frequency estimations
were obtained by means of the Random Decrement Method (RDM) using
numerically simulated time series of soil model responses upon random
excitations and real earthquake records. Highly reliable estimations
were obtained when the system response was dominated by a distinctive or...
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Citations
... For instance, d31 is the electrical field applied in direction 1 d is the piezoelectric constant, c is the effective elastic constant, ε is the dielectric constant and s is the elastic compliance. It is important to mention that the elastic compliance (s = strain/stress) is at a constant electric field. ...
... The main advantage of this method is that only the system response is needed and this can be obtained from records of ground motion of only one station. The numerical implementation of the RDM was developed by Huerta [19] and modified for this work. The program allows a first iteration, making a sweeping of certain thresholds of amplitude and time, previously fixed by us, and offers a matrix of order N (amplitude thresholds) by M (time windows). ...
We use the random decrement method (RDM) to determine the damping (ξ) and dominant period (Td) of soils using earthquake records and ambient noise, and compare the values of ξ with the spectral decay parameter kappa (κ0) previously estimated at several sites in the San Bernardino Valley, northeastern Sonora, Mexico.The dominant period take values from 0.12 s at sites on igneous rocks, to 0.17–0.20 s at sites on continental deposits (Tertiary conglomerates). The damping varies between 2.8 and 5.7%.We find the empirical relationship: ξ=(0.50±0.178)×κ0, which predominate in the study area, and is comparable with that obtained for stations on conglomerates (ξ=(0.43±0.130)×κ0). We also find a theoretical equation ξ=(Vs/2H)κ0 useful for any region, that depend on the shear wave velocity (VS), the decay parameter (κ0) and the thickness of the soil-layer considered (H).We calculate the thickness of the surface layer at each site and find that it varies from 80- to −150-m. We conclude that: it is possible to obtain ξ and Td simultaneously with the RDM; consistent estimates of ξ and Td can be obtained using the whole record or parts of it; the RDM avoids the calculation of spectral ratios to estimate Td and ξ as well as any other pre-processing of the signal; the vertical component of ground motion reflects the properties of shallow layers at the analyzed sites.
We conducted experimental work to explain the large peak ground accelerations observed at the Cerro Prieto volcano in Mexicali Valley, Mexico. Using ambient noise and earthquake data, we compared horizontal-to-vertical spectral ratios (HVSRs) computed for sites on the volcano against those calculated for locations outside it. High-HVSR values (∼11 at ∼2 Hz) were obtained on the top of the volcano at 183 m of altitude, decreasing for sites located at lower elevations. We calculated a median HVSR of ∼1 at 2 Hz from HVSRs computed for nine sites located along an N18°E transect and at an average elevation of ∼25 m. The earlier comparison suggests a relative amplification on the volcano. In addition, we calculated HVSRs from accelerograms generated by 62 earthquakes (2.6≤ML≤5.4; 4.6≤Mw≤7.2) recorded at four locations: two on the volcano (at 194 and 110 m of elevation) and two outside it. These last two sites, located up to 6 km away in a north-northwest and south-southwest direction relative to the volcano, are at an average altitude of 22 m. For the four locations, we also computed the HVSRs from ambient noise data. Although the HVSR results derived from both types of data are slightly different, we also found high HVSRs for the two sites on the volcano and low HVSRs for the two sites outside it, corroborating the relative amplification on the volcano. Using the 1D wave propagation modeling, based on the stiffness matrix method, we modeled the experimental HVSRs to analyze the local site effects. Therefore, we propose that the ground-motion amplification at the Cerro Prieto volcano may be due to a combination of its topography and shallow site effects.
In situ evaluation of the response of seafloor sediments to passive dynamic loads, as well
as spectral analyses of earthquakes are presented in this investigation. The overall goal of
this work was to develop a cost-effective method of characterizing offshore geotechnical
sites in deep water. The generic approach was to place an ocean bottom seismograph on
the seafloor and record ambient noise and distant earthquakes over periods of a month or
more. Horizontal-to-vertical (H/V) spectral ratios are used to characterize the local sediment
response in terms of the distribution of ground motions with their respective resonant
frequencies. Both ambient noise and distant earthquakes are used as generators of
passive dynamic loads. One-dimensional (1D) wave propagation modeling using the stiffness
matrix method is used to estimate sediment properties (mainly shear stiffness, density,
and material damping) and theoretical amplification factors of the shallow sediment
layers. The objectives in this study were fourfold: First, to characterize the spectral
characteristics of earthquake signals recorded in the seafloor at an experimental site in
the Gulf of Mexico (GOM); second, to characterize the local site effect produced by
shallow marine sediments at the GOM experimental site; third, to characterize the site in
terms of its physical properties (layering and sediment properties); and fourth, to estimate
the transfer functions of the top 50 m (164 ft) of soil and of each layer in the discrete soil
model. The resulting sediment properties fall well within the expected range, indicating
the potential of the proposed exploration approach for characterizing deep-water sites.
