Shear‐wave velocity based seismic microzonation of Lorca city (SE Spain) from MASW analysis

Abstract

Many populated areas located in moderate seismic risk regions have been hit by earthquakes of moderate magnitude which, surprisingly, have caused very serious damage to buildings and led to the loss of human life. The city of Lorca (SE Spain) is a clear example. On 11 May 2011, two main shocks occurred in the vicinity of Lorca city with a maximum magnitude of 5.1 Mw causing some casualties and serious widespread damage in the city and its surroundings. Most of the damage was concentrated in certain districts in the city, while other parts remained intact. Actually, in Lorca, investigators detected both a clear example of seismic site effect and a seismic wave amplification occurrence due to the type of geological materials on which the city is located. For this reason, several studies are being carried out in Lorca city to assess the actual contribution of the soil conditions to the seismic amplification phenomena. In these studies, the shear wave velocity plays a key role as a parameter for evaluating the dynamic behaviour of the shallowest geological materials. Consequently, site characterization applied to calculating seismic hazard is usually based on the near surface shear‐wave velocity distribution. This study looked at the average shear‐wave velocity for the uppermost 30 m of ground, which is referred to as Vs30. The Vs30 values obtained from multichannel analysis of surface waves (MASW) were used to create a new soil classification map of Lorca city. Thus, the derived Vs30 map was transformed into the NEHRP and Eurocode 8 (EC8) standards. In Lorca city, the softness and the thickness of shallow geological formations have been observed as two important factors that affected the level of ground shaking and the degree of damage. The results show that there is a significant correlation between the Vs30 values and the damage distribution within the city.

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

“…In the study area, the active tectonics controls the basement uplift rate and the faulting style, the combination of the active tectonics and sea‐level changes control the accommodation space for the Late Miocene and Quaternary sedimentary record (Goy and Zazo ; Sanz de Galdeano et al ), and some human‐induced actions have determined the high erosion and sediment‐supply rates during the last 2,500 years (Alcalá et al ; Paz et al ). This combination of natural and human‐induced geological processes controls the spatial distribution and ranges of in the urban areas of southeastern Spain (Benito et al ; García‐Jerez et al ; Navarro et al , ; Martínez‐Pagán et al ).…”

“…Within this framework, the shallow shear‐wave velocity was considered a good quantitative indicator for geological material stiffness, useful for soil microzonation of urban areas for earthquake disaster mitigation (Martínez‐Pagán et al ). is a key parameter in dealing with mechanical properties of subsurface materials (e.g., Olona et al ; Duffy et al ; Lorenzo, Hicks and Vera ; Yilmaz ), ground amplification, and liquefaction potential phenomena in earthquake events (e.g., Chang et al ; Tarque et al ; Vella, Galea and D’Amico ).…”

“…In particular, the National Earthquake Hazard Reduction Program (National Earthquake Hazards Reduction Program, Building Seismic Safety Council ) in the USA and the Eurocode 8 (European Committee for Standardization ) in Europe adopted the average in the top 30 m (usually denoted by ) as the fundamental ground parameter to be considered for the structural design of buildings against earthquake events (Mahajan ). The International Building Code (IBC) published the same classes in 2000 (Martínez‐Pagán et al ). This is based on the premise established from empirical studies that the top 30 m comprise the most influential layers for the shaking ground response (Park ).…”

“…García‐Jerez et al ), and the Multichannel Analysis of Surface Waves (MASW) (Park, Miller and Xia ) are different methods that are widely used to provide 1D and 2D models of subsurface geological materials which can be integrated for subsurface classification maps. The MASW method (Park et al ; Xia, Miller and Park ) has been applied to a wide range of geo‐technical problems for mapping the 2D structure (Suto ), such as detecting cavities (Miller, Xia and Park ), searching for bedrock structure (Carnevale et al ), examining water seepage (Ivanov et al ), monitoring ground improvement (Burke and Schofield ), and recently, in earthquake hazard studies (e.g., Sairam et al ; Eker, Akgün and Koçkar ; Kolat, Ulusay and Suzen ; Panzera and Lombardo ; Martínez‐Pagán et al ).…”

“…The MASW method depends on the Rg‐wave analysis for retrieving the near‐surface models. Usually based on the inversion of surface‐wave dispersion curves by local 1D forward modelling, the MASW method has often been applied to laterally varying sites (Boiero et al ) as well as on long and continuous seismic lines or profiles (Martínez‐Pagán et al ). The SPAC is a passive‐source method which takes advantage of the energy in the elastic waves which make up the ambient noise wavefield.…”