Earthquake ground-motion scenario : case study for the Xemxija Bay area, Malta

Abstract

It is well known that the estimation of strong ground motion in regions of high seismicity is an extremely important first step in seismic design, hazard analysis, and risk mitigation. The evaluation of the expected peak ground motion caused by an earthquake is an important problem in earthquake seismology, and seismic risk assessment is an important step for loss reduction. Knowing the possible loss allows for appropriate mitigation disaster plans and improves public awareness. In this study we present the preliminary results on earthquake ground-motion scenario for the Xemxija bay area (Malta). The Maltese islands are perceived to have a low seismicity because in the last years the earthquake activity was characterized by low magnitude events in the surrounding areas. However the islands have experienced several earthquakes in the past centuries and this indicates that it is necessary to adequately address the seismic risk for Malta. Stochastic simulations are generated for extended-fault ruptures using the Extended-Fault Model Simulation code. The simulations were carried out taking into account results on region-specific crustal attenuation, source scaling, fault geometry and local site conditions. The absolute peak ground accelerations (PGA) and peak ground velocities (PGV) were computed, for several magnitude and different sources. In particular, stochastic strong ground motion simulations were applied to estimate the spatial distribution of ground parameter from the destructive 1693 Hyblean earthquake. Several other factors must be considered in as- sessing the seismic risk of an area. For example, it is well known that seismically-induced landslide or cliff collapse can contribute significantly to the damage potential. The seismic vulnerability of local building typologies must also be methodically considered in an urban-scale risk assessment. In this study some preliminary observations and considerations of the holistic seismic risk to a case-study area (Xemxija) were made, by considering the various building types and locations and mapping the geological features, such as surface fractures and topographically unstable features that may be susceptible to ground shaking. In conclusion, stochastic finite-fault modeling based on a dynamic frequency approach, coupled with field investigations, confirms to be a reliable and practical method to simulate ground motion records of moderate and large earthquakes especially in regions prone to widespread structural damage.