SEISMIC AMPLIFICATION IN TACNA-PERU, USING SEISMIC MOTION DATA AND ARRANGEMENTS OF MICROTREMOR

Tacna city in Peru is located within the area that includes the Pacific Ring of Fire, a zone of high seismic activity. According to its seismic history, earthquakes have been occurring in Tacna for the past five hundred years, causing the total destruction of the city on more than one occasion such as the 1868 earthquake which had a magnitude of Mw=9.0 and an intensity of X MMI (Modified Mercalli Intensity Scale). The most recent major seismic event took place on June 23^rd, 2001 which recorded a magnitude of Mw=8.4. Most of the damage occurred in the districts of Ciudad Nueva and Alto de la Alianza which are built on loose sand deposits and volcanic deposits. This research analyzes the ambient noise (microtremor) by applying the Nogoshi and Igarashi (1971) and Nakamura (1989) methods. It also analyzes eighty seismic records from six seismic stations located in the city of Tacna. The S-wave velocity profile was estimated using the spatial autocorrelation (SPAC) method, and the bedrock is found at a depth of between 124 m and 306 m. The Vs30 values obtained were between 340 m/s to 600m/s and, according to the ASCE-7, the site is classified as Class C and Class D. The seismic records were analyzed through response spectra and were compared with response spectra obtained from non-linear site response analysis. In addition, the fundamental period was obtained by four methods: microtremor H/V spectral ratio (HV-M), spectral ratio of seismic record (HV-S), transfer function from Equivalent-Linear analysis (TF-EL) and spectral ratio between soil spectrum to rock spectrum from the non-linear analysis (Soil/Rock-NL). The results show that the fundamental period of soil increases with the intensity of the seismic event with respect to value of period obtained from microtremor. This is due to the effects of nonlinear soil behavior. According to the data, soil amplification values in Tacna, vary from 1.3 to 1.6. The fundamental period for site Class D varies from 0.34 s to 0.37 s, based on the EL method, and from 0.75 s to 0.80 s based on the NL method, while for site Class C the fundamental period varies from 0.15 s to 0.30 s, based on the EL method, and from 0.19 s to 0.34 s, based on the NL method. The acceleration response spectra show that the values of the amplification coefficients vary from 3.2 to 3.5. These results are greater than such values proposed by the Peruvian Seismic Design Code E.030.