Earthquake Scenarios for School Buildings in the City of Basel (Switzerland)

Abstract eng:
Developing earthquake scenarios for cities in areas with a moderate seismicity is a challenge due to the limited amount of available seismological and engineering data, which is a source of large uncertainties. This concerns both the seismic hazard, for which only recordings for small earthquakes are available and the unknown earthquake resistance of pre-code structures that constitute the vast majority of the building stock. Within the framework of the risk mitigation project of the city of Basel, a pilot study has been performed to estimate the impact and consequences of strong earthquakes of different sizes and return periods on school buildings. The hazard analysis benefitted from the new Swiss Probabilistic Seismic Hazard Model of 2015 computed for a welldefined reference rock velocity profile. The city of Basel is located in the Rhinegraben, where sedimentary deposits of several hundred meters thickness substantially influence the ground motion. This ground motion amplification has been estimated at high spatial resolution using different methods, including recordings of small events on a dense strong motion network. This estimation benefitted from 20 years of geophysical investigations and a large number of numerical simulations of earthquake ground motion from strong events. A selection of 121 cantonal school buildings in Basel have been classified according to a specifically developed typology. Most of them are unreinforced masonry or reinforced concrete shear-wall buildings. Displacement-based analyses have been performed to calculate capacity curves representing the behavior of the different types. Based on these capacity curves, fragility and vulnerability curves were derived with new methods with a special attention paid to the uncertainties and their propagation. The fragility curves have been checked against empirical curves to ensure that the analysis method yielded realistic results and further improved. The scenarios considered in the study are based on historical events, in particular the 1356 Basel event (Mw=6.6), and on the de-aggregation of the Swiss Seismic Hazard Model for 475 years return period. The computations were run with the Openquake software, propagating all the recognized uncertainties. The scenarios allow us to quantify the number of casualties, the number of pupils that cannot go to school and financial losses that such events would cause. Moreover, a long-term retrofitting project is currently implemented for the school buildings, not only related to earthquake safety. A building type is assigned to the building before and after the retrofit, and a cost-benefit analysis is performed to determine the impact of the retrofitting on the earthquake safety. A framework for real-time assessment is also developed, and a prototype Shakemap at high spatial resolution was implemented as test version.

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