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.
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.
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