A Clustered Preparatory Phase of the 2009 Mw 6.3 L’Aquila Earthquake

Abstract:

One of the biggest challenges in seismology is to understand whether precursory seismicity sequences can alert us about an upcoming earthquake. On April 6^th 2009 the Mw 6.3 L’Aquila earthquake struck central Italy causing damage and fatalities, preceded by a productive foreshocks sequence. In order to gain insight about the precursory phase of this earthquake, we study a specific region surrounding the main fault, where most of the foreshock activity was observed. We apply template matching using a high-quality catalog of 271 templates and 10 broad-band stations in a wide frequency range between 5-30 Hz to scan 6 months before and 5 months after the mainshock. With this, we create a dense catalogue containing more than 5,000 foreshocks starting ~3 months before the mainshock. We observe clustered seismicity across the two main faults, active during different periods of time. Furthermore, we observe that there are some aftershocks detected using foreshocks waveforms, which likely represent faults re-activated during the postseismic sequence. On the other hand, in addition to the foreshock-mainshock-aftershock behavior of the main sequence, the precursory clusters or bursts of seismicity exhibit their own mainshock-aftershock and swarms-type sequences. To characterize these, we compute several parameters such as the largest event time delay, skew moment release and diffusivity among others. The results suggest that the preparatory phase could be linked to a diffusive fluid migration process. However, some other physical processes (i.e., slow slip event and/or stress transfer triggering) could also play a role.