Search

Many devastating earthquakes inflicted heavy casualties and property losses in the seismically active China Seismic Experimental Site area (CSES: 97.5° ~ 105.5°E, 21° ~ 32°N). We performed a first-order seismic zoning based on Neo-deterministic Seismic Hazard Assessment (NDSHA) in the study area delimited by 94° ~ 108°E and 19° ~ 35°N, containing the South-East margin of the Tibetan Plateau and the Sichuan-Yunnan region. The seismic hazard is expressed by maps of peak ground displacement (PGD), peak ground velocity (PGV) and design ground acceleration (DGA) values, extracted from synthetic seismograms computed at a regional scale and mapped on a regular grid of 0.2° × 0.2° over the study area. For the computation of synthetic seismograms, we considered and updated all the available geophysical-geological-tectonic information, including historical and instrumental earthquake catalogues, seismogenic zones, seismogenic nodes, focal mechanisms, and geophysical structural models. We tested the performance of our assessments with available data (i.e., after the Great Wenchuan (2008, May 12th, Ms = 8.0) and Lushan (2013, April 20th, Ms = 7.0) earthquakes) and verified the negligible influence of large events located “far” from the study area. The results indicate the high seismic hazard of the region, with a particular attention (i.e., where DGA > 0.6 g) to the areas located around the main fault zones, e.g., the Longmenshan, Anninghe and Zemuhe Fault Zones. These first-order NDSHA zoning findings may serve as a knowledge basis to support both large- to mid-range preparedness actions and (multi-scenario) site-specific studies.

Throughout time, the area of historical Cairo has been affected by several earthquakes from near and distant seismogenic zones. The maximum earthquake intensity reported in Cairo is VII, on the Modified Mercalli Intensity scale, due to the 1992 Cairo earthquake (Mw=5.9). About 212 Coptic and Islamic monuments were damaged. The spatial distribution of damage, suggests the occurrence of strong local site effects in combination with high vulnerability of the cultural heritage buildings. This study presents the application of a novel seismic hazard analysis approach at a local scale based on physics-based ground motion simulations. A site-specific multi-scenario seismic input (ground motion time histories and response spectra) is computed at a heritage building in Cairo, considering the local site effect. The seismic input is calculated in three steps: a regional scale analysis followed by a site-specific analysis and the combinations of the computed scenarios ground motion. In the regional scale analysis synthetic accelerograms are computed at the site bedrock. Then, the site-specific analysis is performed to calculate the site surface ground motions or response spectra considering the local site effect. Three earthquake scenarios have been considered, characterized by different locations, magnitudes, and fault configurations. The last step consists in the combination of the computed scenario ground motion into one single multi-scenario seismic input specific for the site of interest. This input is represented by spectral ordinates and their variability. For the engineering purpose of time history analysis, the method allows also to extract the computed site specific physics-based accelerograms, requiring no amplitude scaling nor filtering by magnitudes, distances, or site classifications, as usually done with real ground motion records.

The P-SV- and SH-wave field in the Talchir Basin is simulated along eight profiles: four profiles strike across the basin and the other four along the basin. The hybrid method, which combines two computational techniques, modal summation and finite differences, is used to produce multiphase synthetic seismograms. An M = 6 earthquake is considered, with hypocenter along the North Orissa Boundary Fault (NOBF) at a depth of 5 km and with the focal mechanism parameters: dip = 90°, strike = 160° and rake = 180°. The peak acceleration (A MAX ) along each profile is determined considering the maximum acceleration obtained at the horizontal components. The response spectra ratio (RSR) as a function of frequency is computed for the eight profiles and the higher amplification is seen to increase in correspondence with the thicker sedimentary cover, especially for the radial component. Higher site amplification for all the profiles is observed in the frequency range from 0.5 to 1.4 Hz. To validate the obtained site-effects, the sources, for the profiles across the basin, are placed near to the southern end of the profile and the site amplifications are recomputed. Even if the spatial distribution of A MAX is mainly controlled by the epicentral distance, i.e. the geometrical spreading prevails on local soil effects, the RSR shows a pattern that can be easily correlated with the local site conditions. The RSR at the intersection of the profiles is dependent not only upon the local lithology and sediment thickness but also upon the epicentral distance. The Talchir Basin is classified into three zones based on the RSR values: low RSR zone (1.6–1.9), intermediate RSR zone (2.0–2.8) and high RSR zone (2.9–5.2). The PGA estimated for the bedrock model by Bhatia et al. (1999) for the study region is around 0.05 to 0.10 g while the Indian seismic zonation map estimated it to be in the range from 0.1 to 0.2 g. In the present study, that considers the effects of lateral variations in the structure, the PGA is estimated to be as high as 0.55 g along the BA profile.

A hybrid method for the calculation of realistic synthetic seismograms in laterally heterogeneous, anelastic media has been used to model the ground motion in Tehran city. The synthetic records compare reasonably well with the observed ground motion due to the 2004 Firozabad Kojor earthquake located 60 km north of Tehran. The ratio between the response spectrum for the signals calculated along a laterally varying structure, Sa(2D), and that for the signals at the bedrock regional reference structure, Sa(1D), shows a high amplification of seismic waves in Tehran. The procedure can be readily implemented for the entire Tehran city, which experienced large historical earthquakes in the past, simply by extending the analysis, made so far, in space and to different scenario earthquakes, consistent with the local and regional tectonics.