ریزپهنه بندی لرزه ای کاربردی در محیط های زمین شناسی
Abstract
1- Introduction
2- Proposed methodological approach
3- Study case and application
4- Validation of results
5- Conclusions
References
Abstract
The seismic design of buildings and infrastructure components requires the estimation of the hazard considering the dynamic response of the soil deposits, which substantially modifies the characteristics of the input motion at the rock basement. Seismic microzonation studies attempt to identify geologic zones of an area of interest with similar seismic hazard at a local scale. This paper presents a methodology to obtain seismic spectral amplification factors within each soil zone characterization considering the main sources of uncertainty. Results are presented in terms of spectral amplification factors for various seismic intensities and soil profile vibration periods. Design soil amplification factors can then be mapped using the measured vibration period of the soil profile at each location and the seismic intensity at bedrock for a given design return period. Response and design spectra may then be estimated at surface level for every location. Results can be easily integrated into probabilistic risk assessment platforms such as CAPRA (www.ecapra.org) for hazard and risk evaluations.
Introduction
The seismic design of buildings and infrastructure components requires the selection of a set of seismic records or a design spectra that adequately represent the seismic hazard at a certain location. The design spectrum represents the maximum seismic intensities for design in terms of ground acceleration, velocity or displacement. Seismic design parameters near the surface shall consider the hazard assessment at the bedrock level and the effects generated by the dynamic response of the soil deposits. Since the 1950s, increased research interest in seismic microzonation studies has been observed. After the occurrence of earthquakes such as those in San Francisco (1906), Mexico City (1985) and Kobe (1995), it was clear the need for more detailed assessment of the response of soft sedimentary deposits subjected to earthquakes. In the United States, Gutenberg [1,2] analyzed the differences between ground motions due to variations in geological conditions in Southern California. Richter [3] determined probabilistic seismic intensity variations due to geologic conditions for Los Angeles basin. Borcherdt [4] correlated seismograms measured on surface with the ones obtained at a nearby reference stations located on competent bedrock; this methodology assumed that the difference in the response was due to the local geological or topographical characteristics of the site and that epicentral distance and source radiation were similar for near sites. Aki [5] observed a dependency between the site amplification factor on the response spectra and the frequency of the ground motion. According to the author, soil sites showed higher amplifications than rock sites for periods longer than 0.2 s; this trend was opposed for periods lesser than 0.2 s. Between 1976 and 1994, U.S. seismic building codes used site categories and coefficients S1 to S3 that were defined based on statistical studies [6,7]. A fourth category and factor, S4, was later added after the observations made during the 1985 Mexico City earthquake [8]. In this approach, each site category was associated to a spectral shape and the S factor only scaled the long period part of the spectrum. Idriss [9,10] showed that peak ground accelerations and spectral level at short periods can be significantly amplified at soft sites. These observations were later used to define two important aspects that were incorporated into the NERPH [11] and the Uniform Building Code UBC [12]: (1) higher values of soil site coefficients for areas of lower shaking and (2) the addition of a hard rock category to better reflect geologic conditions in eastern United States. In addition, further studies indicated the importance of the shear wave velocity variation in the upper 30 m column of soil [13,14]. These findings were considered in the 1994 and 1997 NEHRP [15,16] provisions and 1997 UBC [12], which included five new site classes (A to E) in terms of the average shear wave velocity to a depth of 30 m (Vs30). In addition, the old site coefficient S (NEHRP versions prior 1994) were replaced by the site amplification factor Fv at long periods and a new coefficient Fa was introduced for short periods [17].