ALBERT

Description: Macroseismic intensities are the only available data for most historical earthquakes and often represent the unique source of information for crucial events in the definition of seismic hazard. In this paper, we attempt at getting insight into source characteristics by reproducing the observed intensity field. As a test case, we study the source of 1908 Messina Straits earthquake ( M W  = 7.1), by testing three distinct fault models deduced from the analysis of geodetic data. Starting from the static slip distribution, we develop kinematic source models for the investigated fault and compute full waveform synthetic seismograms in a 1-D structural model, also accounting for anelastic attenuation. Then, we convert both computed peak-ground acceleration (PGA) and peak-ground velocity (PGV) to macroseismic intensity at 100 selected sites, by means of specific empirical relations for the Italian region. By comparing the original data separately with PGA- and PGV-based intensity fields, we discriminate among the tested faults and determine the best values for the investigated kinematic parameters of the source. We also perform a misfit analysis for the best source model, in order to investigate the dependence of the results on the selected parametrization. The results of the analysis indicate that among the tested models, the one characterized by an east-dipping fault, with strike-oriented NS slightly rotated clockwise, better explains the observed macroseismic field of the 1908 Messina Straits earthquake. Besides, the fracture nucleated at the southern end of the fault and ruptured northward, producing considerable directivity effects. This is in agreement with the published results obtained from the investigation of the historical seismograms. We also determine realistic values for the rupture velocity and the rise-time. Our study confirms the great potential of the macroseismic data, demonstrating that they contain enough information to constrain important characteristics of the fault, which can be retrieved by using complex source models and computing complete wavefield. Moreover, we also show that the simultaneous comparison of both PGA- and PGV-based synthetic macroseismic fields with the original intensities provides tighter constraints for discriminating among different source models, with respect to what attainable from each of them.

Description: Preferential direction in rupture propagation of earthquakes is known to have strong consequences on the azimuthal distribution of the ground motion. While source directivity effects are well established for large seismic events, their observation for moderate and small earthquakes are still restricted to a few cases. This is mainly due to intrinsic difficulties in recognizing source directivity unambiguously for less energetic/shorter ruptures. Therefore, we propose the use of multiapproach analysis for revealing the possible directivity for small-to-moderate earthquakes, taking advantage of the different sensitivity of each approach to various source and propagation characteristics. Here, we demonstrate that the application of six diverse and independent methods converges in giving consistent information on the rupture kinematics of the 2013 December 29, M w  = 5.0 earthquake. The results indicate a distinct rupture propagation direction toward S-SW, which correlates with observed asymmetry of damage and felt area. Overall, we conclude that the use of a single technique cannot provide a univocal solution, whereas the application of distinct analyses helps to strongly constrain source kinematics and should be preferred, in particular when dealing with small-to-moderate earthquakes.