Radiography of a normal fault system by 64,000 high‐precision earthquake locations: The 2009 L'Aquila (central Italy) case study

Abstract

.[1] We studied the anatomy of the fault system where the 2009 L'Aquila earthquake (M W 6.1) nucleated by means of~64 k high-precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross-correlation and double-difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high-angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co-seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

Structure of the Suasselkä postglacial fault in northern Finland obtained by analysis of local events and ambient seismic noise

“…The rough estimate of the width of this zone corresponds to a smaller wavelength in the considered frequency band of 0.1-1 Hz and is about 1.5 km. This is in a good agreement with the recent results of Valoroso et al (2013), who estimated the width of the seismically active L'Aquila fault zone using analysis of high-precision earthquake locations spanning 1 year. They showed that the width of the fault zone varies along strike from 0.3 km, where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips, with an increase in the geometrical complexity.…”

On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (<i>M</i><sub>w</sub> 6.3)

“…4c), although in nearby areas the observed Vp and Vs values correspond to the characteristic velocities of the Meso-Cenozoic limestones and terrigenous rocks Di Stefano et al, 2011). The sequence is well described by a detailed doubledifference catalog of relocated events (3000 events with M ≥ 1.9 from Chiaraluce et al, 2011; ∼ 64 000 events of all magnitudes from Valoroso et al, 2013). When coupled with moment tensor solutions for the largest shocks (M w ≥2.7; Herrmann et al, 2011), these data allow imaging of individual faults activated during this complex sequence in great detail (Fig.…”

Between-event and between-station variability observed in the Fourier and response spectra domains: comparison with seismological models

“…It includes both velocimetric and accelerometric signals recorded by temporary and permanent stations of the National Seismic Network (RSN, Rete Sismica Nazionale) operated by INGV and by the national Accelerometric Network (RAN, Rete Nazionale Accelerometrica) operated by the Italian Civil Protection Department (DPC). For each event, the local magnitude is computed accordingly to Di Bona (2016) and the P-and S-wave onsets obtained by an automatic phase-picker (Spallarossa et al 2016) are used to relocate the hypocentres using the regional 1-D velocity model reported in Valoroso et al (2013). A final visual inspection is applied to verify the automatic arrival times and to discard clipped or distorted signals.…”