Shear-wave velocity structure from MASW and SPAC methods. The case of Adra town, SE Spain

Abstract

The damage distribution in the town of Adra (south-eastern Spain) during the 1993 and 1994 Adra earthquakes, magnitude Mw 5.0 and maximum intensity degree of VII (European Macroseismic Scale), was mainly concentrated in the southeast sector, where low-diagenetic (soft) sediments outcrop. As new urbanizations are planned in that sector, a soil classification based on the shallow shear-wave velocity (Vs) structure is needed. For the purpose of earthquake disaster mitigation, the Spatial Autocorrelation (SPAC) and the Multichannel Analysis of Surface Waves (MASW) methods were used to propose integrated 2D Vs models for the seismic response characterization of shallow geology. Joint inversion of H/V spectral ratios of ambient noise, interpreted under the Diffuse Field Approach, and dispersion curves derived from the SPAC method allowed to obtain more constrained models. Both, SPAC and MASW methods provided similar results for the surveyed geological formations. From these models, a classification of the geological formations has been carried out in terms of Vs30 values and Eurocode 8 (EC8, 1998) classes. Lower Vs30 values in the 180-360 m/s range were found in the southeastern sector of the town, where soft sediments outcrop and some building damage was reported from the 1993-1994 earthquakes. The highest Vs30 values above 800 m/s appear at the northern sector, where the hardest rocks outcrop and no building damage was reported. The combination of the well-suited Vs database prepared for different geological formations with the 1:5,000 scale geological mapping was a step to obtain a detailed soil microzonation map of Adra. It gives a new predictive insight into the building damage distribution, which will contribute to the appropriate urban planning of the future growth of the town.