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http://hdl.handle.net/2122/10031
Authors: | Tinti, E.* Scognamiglio, L.* Cirella, A.* Cocco, M.* |
Title: | Up-dip directivity in near-source during the 2009 L'Aquila main shock | Journal: | Geophysical Journal International | Series/Report no.: | 3/198(2014) | Publisher: | Wiley-Blackwell | Issue Date: | 16-Jun-2014 | DOI: | 10.1093/gji/ggu227 | Keywords: | earthquake ground motion, earthquake source observations, computational seismology | Subject Classification: | 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous | Abstract: | In this study we have investigated the forward directivity associated with the initial up-dip rupture propagation during the April 6th 2009 (MW 6.1) L’Aquila normal-faulting earthquake. The objective is the understanding of how the peculiar initial behavior of rupture history during the main shock has affected the near-source recorded ground motions in the L’Aquila town and surrounding areas. We have modeled the observed ground velocities at the closest near-source recording sites by computing synthetic seismograms using a discrete wavenumbers and finite difference approach in the low frequency bandwidth (0.02-0.4 Hz) to avoid site effects contaminations. We use both the rupture model retrieved by inverting ground motion waveforms and continuous high sampling-rate GPS time series as well as uniform-slip constant-rupture speed models. Our results demonstrate that the initial up-dip rupture propagation, characterizing the first three seconds of the rupture history during the L’Aquila main shock and releasing only ∼25% of total seismic moment, controls the observed ground motions in the near-source. This initial stage of the rupture is characterized by the generation of clear ground velocity pulses, which we interpret as a forward directivity effect. Our modeling results confirm a heterogeneous distribution of rupture velocity during the initial up-dip rupture propagation, since uniform rupture speed models overestimate up-dip directivity effects in the footwall of the causative fault. The up-dip directivity observed in the near field during the 2009 L’Aquila main shock is that predicted for a normal faulting earthquake by Somerville’s directivity model, but it differs from that inferred from far-field observations that conversely provide evidence of along-strike directivity. This calls for a careful analysis as well as for the realistic inclusion of rupture directivity to predict ground motions in the near source. |
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