The 2009 April 6, Mw 6.3, L’Aquila (central Italy) earthquake: finite-fault effects on intensity data

Abstract

We analyse the spatial distribution of the intensity data points surveyed after the Mw 6.3, 2009 L’Aquila (central Italy) earthquake, with the aim to recognize and quantify finite-fault and directivity effects. The study is based on the analysis of the residuals, evaluated with respect to attenuation-with-distance models, calibrated for L’Aquila earthquake. We apply a non-parametric approach considering both the epicentral and the rupture distance, which accounts for the finite extension of the source. Then, starting from a simplified kinematic rupture model of the L’Aquila fault, we compute four directivity predictors proposed in literature, and assess their correlation with intensity residuals. We derive a so-called Intensity Directivity Factor by the correlation between theoretical predictors and observed residuals that allows us to identify and quantify the intensity data points affected by forward and backward directivity during L’Aquila earthquake. We find that the effects are more pronounced in the forward directivity direction and increments up to 1 MCS intensity unit are expected. Moreover, the directivity predictor that accounts for radiation pattern poorly correlates with residuals. These results show that the spatial distribution of the L’Aquila macroseismic field is affected by source effects and in particular that directivity-induced amplification effects can be recognized. We show that the quasi-unilateral rupture propagation along the fault can explain the high-intensity patterns observed along specific direction at relatively large distance from the instrumental epicentre, in accordance with the seismological source models derived from the analysis of instrumental observations.