Site Amplification at Permanent Stations in Northeastern Italy

ABSTRACT The application of earthquake recordings to the estimation of an event’s magnitude and the construction of rapid-response ground-motion maps requires an adequate classification of the recording stations in terms of their site response. For permanent stations, this information can be obtained from a sufficiently large database of past recordings. In this work, we analyze more than 7300 three-component recordings collected between 1996 and 2017 by 67 permanent stations in northeastern Italy to assess their site amplification. The signals come from 368 earthquakes with a magnitude range of M 3.2–5.8 and a distance range of 10–300 km. We evaluate the frequency-dependent amplification function with respect to a reference station with a flat seismic noise horizontal-to-vertical spectral ratio. The evaluation relies on the decomposition of the S-wave amplitude spectra in terms of source, propagation, and site response. We solve the decomposition with a nonparametric, single-step generalized inversion in the frequency band 0.5–20 Hz. In addition, we compute the amplification factors for peak ground acceleration and velocity with respect to a well-established ground-motion prediction equation. The results highlight that only 11 stations show a relatively flat unitary response with respect to the reference site, whereas the frequency-averaged amplification function at 23 out of 67 stations exhibits a value larger than 2. We classified the sites according to their surface geology and geomorphological scenario and found that amplification affects not only stations installed on the alluvial soil but also several stations installed on what are assumed to be rock sites. Sites in caves and mines exhibit deamplification, whereas the stations with sensors in boreholes exhibit the typical interference pattern. A good correlation between the amplification factors and the frequency-averaged amplification functions suggests the possibility of predicting time-domain peak ground-motion values from amplification functions estimated by generalized inversion.

Determination of path attenuation and site characteristics of the North-west Himalayan region and adjoining regions within the Indian Territory using Generalized inversion method

North-west Himalayas and its adjoining regions have been experiencing deadly earthqaukes from time to time and are home for a large portion of population of Indian subcontinent. Knowledge of regional path attenuation and site parameters are prerequisite while attempting seismic hazard studies towards minimizing damages during future earthqaukes for a region. Present work focuses on the determination of path attenuation and site characteristics of earthqaukes recording stations, located in the north-west Himalayas and its adjoining regions, within India. It is done using two- step generalized inversion technique. In the first step of inversion, non-parametric attenuation curves are developed by constraining attenuation to be a smooth decaying function with hypocentral distance. Qs = (105 ± 11)f (0.94 ± 0.08) as S wave quality factor is obtained indicating that the region is seismically active having high degree of heterogeneities in the crustal medium. In the second step of generalized inversion, site amplification curve, at each recording station, is computed as the ratio of site spectral amplitude of horizontal and vertical components. In addition, based on Horizontal to vertical spectral ratio (HVSR) method, predominant frequency of each recording station is calculated. Values of predominant frequencies based on HVSR and generalized inversion are found matching for each of the recording station. Based on obtained predominant frequency, site class of 101 recording stations, which at present are absent, are determined in this work. Determined path attenuation as well as site parameters can be collectively used for developing regional ground motion models and subsequently for seismic hazard studies for the selected region.

A Generalized Stress Inversion Approach With Application to Residual Stress Estimation

We develop a generalized stress inversion technique (or the generalized inversion method) capable of recovering stresses in linear elastic bodies subjected to arbitrary cuts. Specifically, given a set of displacement measurements found experimentally from digital image correlation (DIC), we formulate a stress estimation inverse problem as a partial differential equation-constrained optimization problem. We use gradient-based optimization methods, and we accordingly derive the necessary gradient and Hessian information in a matrix-free form to allow for parallel, large-scale operations. By using a combination of finite elements, DIC, and a matrix-free optimization framework, the generalized inversion method can be used on any arbitrary geometry, provided that the DIC camera can view a sufficient part of the surface. We present numerical simulations and experiments, and we demonstrate that the generalized inversion method can be applied to estimate residual stress.

Spectral Characteristics of Ground Motion from Induced Earthquakes in the Fort Worth Basin, Texas, Using the Generalized Inversion Technique

ABSTRACT A generalized inversion technique (GIT) is applied to local seismic data from 90 induced earthquakes (ML 2.0–3.9) in the Fort Worth Basin (FWB) of north Texas to separate path, site, and source characteristics and to improve local seismic hazard assessment. Seismograms from three earthquake sequences on spatially separated basement faults are recorded on 66 temporary stations. Because of the lack of hard-rock recording sites within the sedimentary basin, we developed a site correction method for the appropriate GIT process. At about 30 km distance from the hypocenters, we observed a change in spectral attenuation and thus focus data analysis within this distance range. The estimated quality factors for S and P waves result in a QS that is larger than QP which we interpret as a result of concentrations of crustal pore fluids or partial fluid-saturated material along the path; an interpretation consistent with fluid-rich sedimentary rocks in the FWB. Strong site amplifications as much as five times on horizontal components reflect the thick sediments in the basin. A limited number of sites exhibit amplification or deamplification on the vertical component that limits the use of horizontal-to-vertical spectral ratio methods for characterizing the site effect relative to the site effects estimated by GIT. Stress drops for all earthquakes range from 1.18 and 21.73 MPa with a mean of 4.46 MPa, similar to values reported for tectonic intraplate events. The stress-drop values suggest that strong motion and seismic hazard from the injection-induced earthquake in the FWB are comparable to those for tectonic earthquakes. The strong site amplification and fluid effects on propagation attenuation may be crucial factors to take into account for estimating seismic hazards of induced earthquakes in sedimentary basins.