Brune Stress Drop and Rupture Directivity During the 2016-2018 Amatrice-Visso-Norcia Seismic Sequence: Key Parameters for Earthquake Source Physics

Abstract:

The 2016-2018 central Italy seismic sequence is a multiple mainshock sequence with nine earthquakes with magnitude M_w > 5 whose largest events of the 24 and 26 August and 30 October (M_w 6.0, 5.9, and 6.3, respectively) ruptured a 60-km-long Apenninic-trending normal-fault system. The seismicity pattern of the 2016-2018 Amatrice seismic sequence showed the strong interaction between the inherited compressional thrusts and the younger and active normal faults. The study of the stress drop and rupture directivity of the strongest earthquakes of this seismic sequence can provide important constraints on hazard scenarios.

This seismic sequence represents an excellent opportunity for investigating the source properties of the strongest events given the high number of seismic events that occurred, the large number of seismic stations, the extended range of magnitude and the extensive area affected by the sequence. We perform a detailed investigation of uncertainties and resolution of stress drop and directivity estimates to check the real significance of inter-event variability. We apply and compare two different approaches to calculate the source spectra and then the source parameters. One involves correcting the observed amplitudes for along‐path propagation and site terms. The other is based on an empirical Green’s function (EGF) deconvolution, where two co-located earthquakes of different magnitude with similar source mechanisms are assumed to be affected by the same propagation and site effects. In this approach, the spectral ratio between target and EGF events yields the source spectrum of the target event.

We start by analyzing possible sources of uncertainty in the stress drop estimates, including the choice of the moment catalog, the frequency range in which the source parameters are estimated, and directivity effects. Then we compare the stress drop estimates obtained with the two methods. We accept stress drop estimates as reliable when the results of the two methods are within 1 standard deviation.

Preliminary results suggest that stress drop values and rupture directivity spatially characterize the different sectors of the Apennines. Investigation is ongoing to discover the causes of these different behavior.