Focal Mechanism Influence on Probabilistic Estimation of Strong Ground Motion

One major challenge of the modern seismology is the mitigation and the reduction of the consequences on structures and society due to the occurrence of future earthquakes. Probabilistic seismic hazard analysis (PSHA), in the last years has assumed a fundamental role in this research field. In fact, using the past seismicity and information regarding the tectonic environment, it allows to estimate the most probable strong ground motion level at a particular site for a given time period. Moreover, from an engineering point of view, using de-aggregation techniques, it permits to infer the design earthquake. The aim of the present work is to illustrate a new methodology for improving the probabilistic peak ground motion estimation and, consequently, the seismic hazard through the modification of attenuation laws. In fact, attenuation laws are usually used in tectonic environment different from that they have been retrieved. This may conduct to unrealistic estimates of the strong ground motion thus influencing the PSHA result. The proposed modifications are carried out by means of theoretical and empirical considerations and, in particular, regard the source focal mechanism. A corrective multiplicative coefficient is formulated in order to use a-priori information on the most probable focal mechanism expected in a given seismogenic source region. This information can be inferred, for example, by the actual tectonic stress regime or by historical seismicity, and are often available in the present-day catalogues. The proposed methodology is used also to extend the design earthquake to the focal mechanism. In fact, a specific probability density function of the focal mechanism parameters can be formulated and introduced into the general formulation of the PSHA. This probability density function allows the analytical formulation of available a-priori information. An application to a site located in the southern Apennines (Italy) along with the formal treatment of the proposed methodology is shown.