Nieves Lantada

Seismic vulnerability and risk evaluation methods for urban areas. A review with application to a pilot area

The most relevant seismic vulnerability and risk analysis methods are discussed and compared in this article using, as a pilot urban area, the city of Barcelona, Spain, where risk studies have been carried out over the last 15 years in the framework of different research projects. Most of the buildings in Barcelona, which have unreinforced masonry structure or reinforced concrete structure with waffled slab floors, show a high degree of vulnerability to earthquakes. The physical seismic risk but also the socio-economic implications of risk are considered in the article. The robustness of the innovative holistic approach, based on indicators related to the physical exposure, the social fragilities and the lack of resilience of urban area, is also proved. Using a geographic information system (GIS), the seismic risk results are described by means of scenarios of expected losses, but also as scenarios of probabilities of occurrence of predefined damage states.

Geological structures evaluated by means of scattering noise in ground penetrating radar images

Coastal geology in Barcelona City is formed by Quaternary sediments overlying Tertiary strata. The Quaternary plane is crossed by paleochannels and streams, all of them under the city structures and constructions. Boreholes demonstrate differences in the grain size distribution of the materials from the different geological structures. In this work, Ground-penetrating radar was proved as a tool to obtain quick information about these changes. The results from more than 20 km of GPR profiles in Barcelona city, seems to indicate that, depending on the relationship between the wavelength and grain size, GPR signal scattering increases significantly. Therefore, the analysis of the noise in GPR signals supplies information to determine the possible existence of geological changes in the quaternary deposits. Several tests and measures in well-known emplacements, as well as models and simulations, were used to determine the ability of this method in the study of the Barcelona plain shallow geology. Correlation between paleochannels and infilled streams and higher scattering noise is observed in this study. The analysis of the scattering effect on the amplitude of the radar data could help to locate paleochannels and subterranean infilled streams, being a valuable tool to improve the knowledge of the city subsoil.

Earthquake Risk Scenarios in Urban Areas: A Review with Applications to the Ciutat Vella District in Barcelona, Spain

ABSTRACT The vulnerability index and capacity spectrum-based methods used to assess the expected physical damage in dwellings are revisited, and their main advantages and shortcomings are analyzed and discussed. Then, a vulnerability index-based method is used to assess the expected damage in the oldest district of Barcelona, Spain. The risk analysis is performed for earthquake scenarios defined by macroseismic intensities of V, V-VI, VI, VI-VII, and VII to design seismic emergency plans. According to the Spanish Seismic Code, the basic peak ground acceleration for a 475-year return period is 0.04 g, which corresponds to an intensity of VI. Thus, the expected physical damage is quantified and its impact on population and other quantities, such as debris and economic cost, are studied. In spite of the low-to-moderate seismic hazard in Barcelona, the results show that the risk is high, due to the high exposure and vulnerability of the built environment.

Simulation of rockfall fragmentation mechanism in a GIS-based tool

Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The attitude and persistency of joints within the rock mass define the size of kinematically unstable rock volumes. Furthermore, the rock block will eventually split in several fragments during its propagation downhill due its impact with the ground surface. Knowledge of the size, energy and trajectory of each block resulting from fragmentation is critical in determining the hazard of buildings and protection structures. The objective of this study is to simulate stochastically the fragmentation mechanism in rockfall propagation trajectories and in the calculation of impact energies in a GIS-Based tool which includes common modes of motion for falling boulders. A stochastic fragmentation model is proposed and tested to determine if it can simulate the fragmentation phenomena properly.

Probabilistic Assessment of Seismic Risk of Dwelling Buildings of Barcelona. Implication for the City Resilience

The knowledge of seismic risk of buildings can contribute to increase the resilience of cities. In the present work a new assessment of the seismic risk of dwelling buildings of Barcelona was done. This assessment was performed according to a probabilistic methodology, which is summarized in the following steps: (1) performing a probabilistic seismic hazard assessment (PSHA) to obtain exceedance rates of macroseismic intensities; (2) performing a probabilistic seismic vulnerability assessment (PSVA) of each building in order to determine probability density functions that describe the variation of a vulnerability index; and (3) performing a probabilistic seismic risk assessment (PSRA) to generate seismic risk curves in terms of frequencies of exceedance of damage states. In the present work 69,982 dwelling buildings of Barcelona were assessed. According to the results the percentage of dwelling buildings of Barcelona that have a probability equal or greater than 1% of suffer partial collapse in the next 50 years is a value between 0% and 34.29%. A value of 0% corresponds to the results of seismic risk obtained for the case where regional vulnerability modifiers were not considered during the procedure to assess the seismic vulnerability of buildings and 34.29% correspond to the case where regional vulnerability modifiers were considered. For the same two options, the losses due to the physical damage of the dwelling buildings of Barcelona assessed for an exposure time of 50 years, could vary from 807.3 to 1739.4 millions of euros, respectively. Finally, possible uses of the seismic risk results computed in the present work are mentioned.

Seismic energy analysis as generated by impact and fragmentation of single-block experimental rockfalls

The analysis of seismic signals obtained from near-source triaxial accelerometer recordings of two sets of single-block rockfall experiments is presented. The tests were carried out under controlled conditions in two quarries in northeastern Spain; in the first test (Foj limestone quarry, Barcelona), 30 blocks were released with masses ranging between 475 and 11,480 kg. The second test (Ponderosa andesite quarry, Tarragona) consisted of the release of 44 blocks with masses from 466 to 13,581 kg. An accelerometer and three high-speed video cameras were deployed, so that the trajectories, velocities, and block fragmentation could be tracked precisely. These data were used to explore the relationship between seismic energy and rockfall kinetics (the latter obtained from video analysis). We determined absolute and relative values of seismic energy and used them to estimate rockfall volumes. Finally, the seismic signature of block fragmentation was assessed in both the frequency and time domains. The ratios of seismic energy after impact to kinetic energy before impact ranged between 10−7 and 10−4. These variables were weakly correlated. The use of seismic energy relative to impacting kinetic energy was preferred for the estimation of volumes. Block fragmentation impacts were dominated by higher acceleration spectrum centroid frequencies than those of nonfragmentation impacts: 56.62 ± 2.88 and 48.46 ± 4.39 Hz at Foj and 52.84 ± 12.73 and 38.14 ± 4.73 Hz at Ponderosa. Plain Language Summary Gravitational instability movements on mountain slopes are spontaneous in nature. In most cases, continuous seismic recordings are the only available data; thus, inference techniques are used to approximate the initial characteristics of the detached mass. Here we perform two sets of individual rock block release experiments in a highly monitored setting in northeastern Spain; in addition to the seismic instrumentation, a detailed study of block properties and the deployment of video cameras allow an accurate determination of the input parameters (volumes and impacting speeds), which are usually unknown in natural settings. This is used to relate seismic observations to known characteristics of the performed block releases. We find that (1) radiated elastic energies from the point of impact are significantly smaller than the impact energy, regardless of block breakage, (2) the usage of kinetic energy is preferred for the estimation of rockfall volumes, and (3) block fragmentation can be characterized by means of individual parameters associated with the radiated wave frequencies. These results should shed further light on how efficient the conversion of impact to radiated elastic energy is and on the level of inference that can be achieved based on seismic signals.

Comparison of Photogrammetric Techniques for Rockfalls Monitoring

The use of Unmanned Aerial Vehicles, UAVs to image capture for monitoring natural hazards has had a major boost for its wide possibilities in the last decade. These are, for example, the studying and monitoring of unstable slopes, glaciers and rocky escarpments. Moreover, to evaluate the risk after a rockfall or debris flow event, for example measuring volume of displaced material, trajectories of blocks or building and/or infrastructure damaged. But the use of these devices requires a specific treatment regarding the studied case and geomatic techniques suitable to get the adequate precision of the movement, size of items or events to study. For each application it is necessary to determine what kind of capture is the most appropriate to obtain an optimal benefit-cost ratio. A comparison of the use of terrestrial photogrammetry, UAV photogrammetry and video from UAV has been done. The best result has been obtained combining techniques aerial and terrestrial since ground points with a best quality can be identified and measured and all the surface has a best image coverage.