Complexity of the rupture process during the 2009 L’Aquila, Italy, earthquake

In this study, we investigate the rupture history of the April 6th 2009 (Mw 6.1) L’Aquila normal faulting earthquake by using a nonlinear inversion of strong motion, GPS and DInSAR data. Both the separate and joint inversion solutions reveal a complex rupture process and a heterogeneous slip distribution. Slip is concentrated in two main asperities: a smaller shallow patch of slip located up-dip from the hypocenter and a second deeper and larger asperity located southeastward along strike direction.
The key feature of the source process emerging from our inverted models concerns the rupture history, which is characterized by two distinct stages. The first stage begins with rupture nucleation and with up-dip propagation at relatively high (∼ 4.0 km/s), but still sub-shear, rupture velocity. The second stage starts nearly 2.0÷2.5 seconds after nucleation and it is characterized by the along strike rupture propagation. The largest and deeper asperity fails during this stage of the rupture process. The rupture velocity is larger in the up-dip than in the along-strike direction. The up-dip and along-strike rupture propagation are separated in time and associated with a Mode II and a Mode III crack, respectively.
The comparison between the source models inferred in this study with the Poisson ratio anomalies in the crustal volume containing the fault plane (Di Stefano et al., 2011) allows the interpretation of the delay in along-strike rupture propagation in terms of a structural control of the rupture history. Our results show that the L’Aquila earthquake featured a very complex rupture, with strong spatial and temporal heterogeneities suggesting a strong frictional and/or structural control of the rupture process.