Application of ambient vibration recordings to seismic microzonation of Salta city – Argentina

LÍA OROSCO; JORGE TORRES ; MIKA HAARALA; JOSÉ VIRAMONTE; ROBERTO CARNIEL ; LUCA BARBUI; FERNANDO ALBARRACÍN; JOSÉ MORALES SOTO; DANIELA IBÁÑEZ; CARMEN QUISPE

Foz de Iguazu

Congreso; AGU Joint Meeting of the Americas; 2010

American Geophysical Association

Abstract Salta is located in the North of Argentina; its population and infrastructure have increased at a high rate during the last 10 years. The city lays on the northern sector of a sedimentary valley (Valle de Lerma), which is a depression filled with quaternary sediments with increasing depth from borders to center. It is surrounded by outcrops of the ordovicic basement. The National Agency of Seismic Prevention (INPRES) assigned to the city a seismicity level 3, in a national scale which ranges from 0 to 4. On February 27th the region was struck by an earthquake of magnitude Mw 6.1 – 6.4 and was assigned a degree III-IV in MMI scale. The hypocenter is not well defined yet, but it was located approximately 30 km from the city. The major damages happened in Campo Quijano, a village situated 30 Km W-SW from Salta. Two casualties were reported. The event confirmed the high seismic risk of the northern portion of the Valley. In some sites of the city measurements of ambient vibrations were carried out. The spectral analysis of the signals and the application of HVSR method yielded estimation of fundamental periods of the soils in these zones of city, with the aim of  the seismic microzoning of the Salta city just the one with the major population of the Province (700.000 inhabitants).  The method is reliable to get the fundamental frequencies of vibrations, but not the amplification factors. Nevertheless, in some points this parameter will be calculated relatively to a reference site located in Cerro San Bernardo (ordovicic outcrop). The fundamental periods of buildings were also determined in an empirical way, by ambient vibrations recordings and subsequent analysis of the signals in the frequency domain. By linear regression, a relation was found between period and number of floors (height) of buildings. This parameter was widely used in Latin America, and is the most appropriate in this case taking into account the available data; in that way the results can be compared with those obtained in another countries of Latin America. The results of both campaigns were mapped in an image of Salta city. Those sites where significant amplifications are expected owing to resonance phenomena (soil-building response coupling) are pointed out, thus contributing to the complex task of estimating seismic risk in urban areas.