Maufroy E.,French National Center for Scientific Research |
Chaljub E.,French National Center for Scientific Research |
Theodoulidis N.P.,Institute of Engineering Seismology and Earthquake Engineering ITSAKEPPO |
Roumelioti Z.,Aristotle University of Thessaloniki |
And 6 more authors.
Bulletin of the Seismological Society of America | Year: 2017
We compare different methods to estimate frequency-domain amplification and duration lengthening of earthquake ground motion in the Mygdonian basin (Greece). Amplification is measured by standard spectral ratios (SSRs) of horizontal component or by single-station earthquake horizontal-to-vertical ratios (EHVRs). Duration lengthening is measured either by the group delay method (Beauval et al., 2003) and labeled GDDL, or based on the significant duration (Trifunac and Brady, 1975) and labeled TBDL. The methods are applied both to high-quality recordings of the European experimental site EUROSEISTEST array and to a large set of 3D synthetics computed in a new basin model for 1260 sources regularly distributed in depth, distance, and azimuth from the center of the array. The analysis of the recordings in the center of the basin shows an anticorrelation between amplification and duration lengthening, that is, maxima (resp. minima) of GDDL correspond to minima (resp. maxima) of SSR. The maxima of GDDL are also found to coincide with those of SSR variability. This is confirmed by the analysis of the synthetics, which also reveals a pronounced north-south asymmetry of both amplification and duration lengthening caused by nonisotropic excitation of surface waves at the basin edges. We find that all estimates of site response depend on source location and that EHVR is also strongly sensitive to energy partitioning in the analyzed wavefield. We quantify the source-related variability of each estimate, discuss the biases in site response estimation using incomplete source catalogs, and investigate whether the azimuthal dependence of site response can be identified in the recordings. © 2017, Seismological Society of America. All rights reserved.
Stewart J.P.,University of California at Los Angeles |
Klimis N.,Democritus University of Thrace |
Savvaidis A.,Institute of Engineering Seismology and Earthquake Engineering ITSAKEPPO |
Theodoulidis N.,Institute of Engineering Seismology and Earthquake Engineering ITSAKEPPO |
And 6 more authors.
Bulletin of the Seismological Society of America | Year: 2014
The time-averaged shear-wave velocity in the upper 30 m of a site (VS30) is commonly used for ground-motion prediction. When measured velocities are unavailable, VS30 is estimated from proxy-based relationships developed for application on global or local scales. We describe the development of a local relationship for Greece, which begins with compilation of a profile database (PDB) from published sources and engineering reports. The PDB contains 314 sites; 238 have profile depths ≥ 30 m and 59 are within 100 m of accelerographs. We find existing relations for extrapolating a time-averaged velocity for depths less than 30 m to VS30 to overpredict VS30. We present equations for these extrapolations. We then compile proxies for PDB sites, including terrain type, surface geology, and surface gradients at 30 and 3 arcsec resolution (from radar-derived digital elevation models [DEMs]). When checked against ground survey data, we find ground elevations from 3 arcsec DEMs to be more accurate relative to survey data than alternative 30, 9, and 1 arcsec DEMs. Drawing upon expert opinion, we develop geologic categories based on age, gradation, and depositional environment and assign such categories to PDB sites. We find an existing 30 arcsec gradient-based global model to be biased relative to local VS30 data for gradients >∼0:05 m=m. Bias relative to a California model is also found for four of the eight well-populated geomorphic categories, and new (local) values are provided.We find statistically significant effects of the 3 arcsec gradient on VS30 for Quaternary and Tertiary materials but no gradient effect for those from the Mesozoic. Among Quaternary sediments, Holocene, mapped Quaternary (age unspecified), and mixed/fine-gradation materials exhibit consistent VS30-gradient trends, whereas Pleistocene and coarse-gradation sediments have faster velocities. For the study region, we recommend use of the modified terrain- and geology- based methods in combination for proxy-based VS30 estimation. © 2014 Bulletin of the Seismological Society of America. All Rights Reserved.