Ambient Noise Measurements in the City of Lorca (Se Spain) for Seismic Microzonation


Abstract eng:
A M w 5.2 earthquake shook the city of Lorca, in Murcia region (SE-Spain) on 11 May 2011 at 18:47 local time. The main shock was preceded by a large Mw 4.5 foreshock, about two hours before. The strongest aftershock, around four hours later, reached M w 3.9. The event caused nine fatalities and more than 300 people were injured in a town with a population of around 60000 in an area of 7 km2. Around 1000 buildings, including residential, cultural heritage, schools, government buildings, healthcare, security facilities, etc., were affected with different degrees of severity. Damage was concentrated in several areas of the town where around 40% of buildings were affected. In the historical center 16% of buildings were damaged. Historical heritage was severely affected including old churches and medieval wall towers while nearby towns and provinces were not seriously affected. Within the two years following the earthquake in Lorca, several ambient noise measurement campaigns including both single-station and multi-station array configurations were performed in order to constrain seismic properties of the subsoil in the city and its surroundings. Single-station ambient-noise records were obtained at 86 sites in Lorca, mainly in its urban area. Additionally, at 13 sites, nine three-directional digital tromographs were deployed in 2-D array configurations, characterized by dimensions of the order of 100 m. Ambient noise measurements in the vertical component were used to retrieve the relevant effective dispersion curve of Rayleigh waves by considering ESAC and f-k approaches. At each measurement point within the array, horizontal-to-vertical spectral ratios (HVSR) were also computed. Furthermore, polarization analysis was carried out, which allowed detecting possible anisotropies in the ambient vibration wavefield. The dispersion and HVSR curves obtained at each site were jointly inverted by considering a genetic algorithm (GA) approach, assuming that the monitored ambient vibration wavefield is dominated by surface waves including both Love and Rayleigh waves with relevant higher modes. By this procedure, S-wave velocity profiles, along with the relevant uncertainties, were estimated at each site up to depths of the order of hundreds of meters. The obtained results, including HVSR amplitude mapping, reveal that possible ground motion amplification by local soils is not significant in most of the Lorca urban area and they also suggest the presence of significant lateral heterogeneities in the subsoil structure, resulting from past tectonic activity of major faults present in the region. The few very local resonance shows no apparent correlation with observed damage. The lateral heterogeneities in observed damages probably rely on different vulnerability levels of buildings

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Conference Title:
Conference Title:
16th World Conference on Earthquake Engineering
Conference Venue:
Santiago (CL)
Conference Dates:
2017-01-09 / 2017-01-13
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 Record created 2017-01-18, last modified 2017-01-18


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