Pure and Applied Geophysics

Roca, A.; Oliveira, C.

doi: 10.1007/PL00001170pmid: N/A

Sokolov, V. Yu.; Chernov, Y. K.

doi: 10.1007/PL00001171pmid: N/A

 — The problem of accounting for local soil effect on earthquake ground motion is especially urgent when assessing seismic hazard – recent needs of earthquake engineering require local site effects to be included into hazard maps. However, most recent works do not consider the variety of soil conditions or are performed for generalized site categories, such as “hard rock,”“soft soil” or “alluvium.” A technique of seismic hazard calculations on the basis of the Fourier Amplitude Spectra recently developed by the authors allows us to create hazard maps involving the influence of local soil conditions using soil/bedrock spectral ratios. Probabilistic microzoning maps may be constructed showing macroseismic intensity, peak ground acceleration, response and design spectra for various return periods (probability of exceedance), that allow optimization of engineering decisions. An application of this approach is presented which focused on the probabilistic microzoning of the Tashkent City.

Boccaletti, M.; Corti, G.; Gasperini, P.; Piccardi, L.; Vannucci, G.; Clemente, S.

doi: 10.1007/PL00001172pmid: N/A

 — The city of Florence possesses a concentration of cultural and artistic treasures which is unique in the world. In this sense it has a particularly high seismic exposure and a potentially high vulnerability. In order to better evaluate its seismic hazard and risk, we analyzed the seismic response of the urban area of Florence by performing a multidisciplinary study on the effects of earthquakes on the city. By a computer aided methodology we re-evaluated the seismic intensity reports of the May 18 and June 6, 1895 earthquakes in different parts of the city and compared these data with recent studies on surface geology, active tectonics and actual fault movements in the Florence basin. We concluded that more detailed studies of soil response are needed to form a basis for public policy.

Triantafyllidis, P.; Hatzidimitriou, P. M.; Suhadolc, P.

doi: 10.1007/PL00001173pmid: N/A

 — We apply an algorithm based on the modal summation method to theoretically estimate the site effect at selected locations underlain by different geological formations within the city of Thessaloniki (Greece). Complete strong motion synthetics are constructed for all components of motion at each site, for a maximum frequency of 10 Hz. The anelastic, local 1-D velocity models are based on cross-hole data. Four point sources with different azimuths and distances from the city are used to compute the input signals. The theoretical amplification is estimated through spectral ratios of accelerograms obtained by the local 1-D over those obtained by the regional 1-D velocity model. The results from the numerical modeling are compared with those derived from experimental techniques, such as of Standard Spectral Ratio and Horizontal-to-Vertical Spectral Ratio, which had been applied to acceleration data recorded at the same sites. The comparison demonstrates that the theoretical amplifications based on known and simple subsurface geology can be used as a first-order estimate, while for cases of more complex geometries the use of at least 2-D modeling in site effects estimation is mandatory.

Makra, K.; Raptakis, D.; Chávez-García, F. J.; Pitilakis, K.

doi: 10.1007/PL00001174pmid: N/A

 — Modern seismic codes usually include provisions for site effects by considering different coefficients chosen on the basis of soil properties at the surface and an estimate of the depth of bedrock. However, complex local geology may generate site amplification on soft soils significantly larger than what would be expected if we assume that the subsoil consists of plane soil layers overlaying a homogeneous half-space. This paper takes advantage of the large number of previous studies of site effects done at Euroseistest (northern Greece). Those studies have supplied a very detailed knowledge of the geometry and properties of the materials filling this shallow valley. In this paper we discuss the differences between site effects evaluated at the surface using simple 1-D computations and those evaluated using a very detailed 2-D model of the subsoil structure. The 2-D model produces an additional amplification in response spectra that cannot be accounted for without reference to the lateral heterogeneity of the valley structure. Our numerical results are extensively compared with observations, which show that the additional amplification computed from the 2-D model is real and affects by a significant factor response spectra, and thus suggests that some kind of aggravation factor due to the complexity of local geology is worthy of consideration in microzonation studies and seismic codes.

Marrara, F.; Suhadolc, P.

doi: 10.1007/PL00001175pmid: N/A

 — An estimation of local site effects in the Volvi basin as derived from observation and modeling is presented in this paper. The Volvi basin is located in the Mygdonian graben in northern Greece near the city of Thessaloniki. This test site has been studied and instrumented in the framework of the “EURO-SEISTEST” and “EURO-SEISMOD” projects funded by the European Union, aimed at improving knowledge of the influence of the local geology on the seismic response of a target area. In this context we calculate synthetic seismograms along a 2-D profile intersecting the graben, instrumented and accurately investigated with a geophysical survey and geotechnical tests. The seismic wavefield from the source to the target area has been computed with the modal summation method, while inside two of the investigated models representing the 2-D section, the wavefield has been numerically propagated with the finite-difference method. We compare the results of the two simulations, both in the time and frequency domain. We also compare the results with experimental data related to an event recorded by the Reftek network installed in the target area. This permits a better understanding of how the structural features of the 2-D models affect the seismic wavefield, especially in the frequency range between 2 Hz and 4 Hz, where one can observe differences between the simulations and the observations. This means that the general features of the models are able to reproduce observed amplification effects, apart from some discrepancies due to still unresolved structural features of the site.

Panza, G. F.; Vaccari, F.; Romanelli, F.

doi: 10.1007/PL00001176pmid: N/A

 — The estimation of realistic seismic input can be obtained from the computation of a wide set of time histories and spectral information, corresponding to possible seismotectonic scenarios for different source and structural models. Such a data set can be very constructively used by civil engineers in the design of new seismo-resistant structures and in the reinforcement of the existing built environment, therefore supplying a particularly powerful tool to the prevention efforts of Civil Defense. The availability of realistic numerical simulations enables us to estimate the amplification effects in complex geological structures exploiting the available geotechnical, lithological, geophysical parameters, topography of the medium, tectonic, historical, paleoseismological data, and seismotectonic models. The realistic modeling of the ground motion is a very important source of knowledge for the preparation of groundshaking scenarios which represent a valid and economical tool in seismic microzonation.

Moldoveanu, C. L.; Panza, G. F.

doi: 10.1007/PL00001177pmid: N/A

 — The mapping of the seismic ground motion in Bucharest, due to the strong Vrancea earthquakes, is carried out using a complex hybrid waveform modeling method that allows easy parametric tests. Starting from the actually available strong motion database, we can make realistic predictions for the possible ground motion. The basic information necessary for the modeling consists of: (a) The representative mechanisms for the strong subcrustal events, (b) the average regional structural model, and (c) the local structure for Bucharest. Two scenario earthquakes are considered and the source influence on the local response is analyzed in order to define generally valid ground motion parameters, to be used in the seismic hazard estimations. The source has its own (detectable) contribution on the ground motion and its effects on the local response in Bucharest are quite stable on the transversal component (T), while the radial (R) and vertical (V) components are sensitive to the scenario earthquake. Although the strongest local effects affect the T component, both observed and synthetic, a complete determination of the seismic input for the built environment requires the knowledge of all three components of motion (R, V, T). The damage observed in Bucharest for the March 4, 1977 Vrancea event, the strongest earthquake to strike the city in modern times, is in agreement with the synthetic signals and local response.

Sincraian, M. V.; Oliveira, C. S.

doi: 10.1007/PL00001178pmid: N/A

 — A 2-D and a 3-D finite element representation using Drucker-Prager cap model is employed in the study to determine the seismic response of a volcanic hill located in one of the islands in the Azores Archipelago. In order to test the applicability of these models we used the motion recorded at the base of the hill during an aftershock of the July 9, 1998 earthquake and compared the numerical response with the record obtained at the top of the hill. Several comparisons and sensitivity analyses were made to identify the most important dynamic parameters influencing the response. Even though the match is not yet adequate for any one of the representations, especially in time domain, the 3-D model showed a good fitting in terms of Fourier Spectrum. Up to a PGA of 0.24 g the behavior of the hill is approximately linear, with higher amplifications going upwards along a vertical interior column; beyond this limit, there is a clear nonlinear behavior.

Luzón, F.; Al Yuncha, Z.; Sánchez-Sesma, F. J.; Ortiz-Alemán, C.

doi: 10.1007/PL00001179pmid: N/A

 — In this paper we study the seismic response of flat sedimentary basins and carry out numerical experiments to determine the extent to which we could go using the Horizontal to Vertical Spectral Ratio (HVSR) for a given site. The HVSR has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil, and one of its variants, that proposed by NAKAMURA (1989) in which records of microtremors are used, is one of the most applied in recent years. We study the response of different configurations under incident waves coming from an explosive source using the Indirect Boundary Element Method (IBEM), and we investigate two cases: low- and high-velocity contrast. We compute the seismic response using the HVSR technique at various locations in the free surface of the basins, and compare it with the response calculated with the horizontal Sediment to Bedrock Spectral Ratio (SBSR) and with the Horizontal Component (HC) of the transfer function for the displacement at the same locations. The comparison shows that, in general, HVSR cannot provide the predominant period of a site due to the fact that this technique cannot predict accurately the Spectral amplification levels. On the other hand, the HVSR provides an erroneous response in the sedimentary basins which have a low-impedance contrast, with respect to bedrock, and with shape ratios like the one studied here, whereas it can reasonably well predict the fundamental local frequency when there is a high-impedance contrast, except in the center of the basin.