Breakdown of Earthquake Self-Similar Scaling and Source Rupture Directivity in the 2016-2017 Central Italy Seismic Sequence

摘要

To analyze the source characteristics of the 2016-2017 central Italy seismic sequence, source spectra of 78 earthquakes of M-L=3.5-6.1 were separated from the S wave Fourier spectra using the two-step nonparametric generalized inversion technique. Source parameters (e.g., stress drop) were estimated from the source spectra following the -square model. Stress drops were found mainly in the range 0.113-12.190MPa. The significant dependence of stress drop on magnitude indicates the breakdown of earthquake self-similar scaling in this sequence. The low stress drops for small events following the release of high stress by the mainshock might have led to stress accumulation on the unruptured fault, which could explain the subsequent occurrence of multiple major events. We investigated the source rupture directivity for 36 events through the azimuthal variation of apparent source spectra. Significant variation was observed at specific frequency bands (generally, over corner frequencies) in 10 events caused by rupture directivity, which was verified by the stable estimation of rupture plane. The rupture parameters confirmed unilateral ruptures predominantly on the NNW-SSE striking fault with fast rupture velocity (2.52-2.84km/s) for the 10 events. Rupture parameters for an additional four events with stable estimated rupture plane were also analyzed. These were characterized by slow rupture velocity causing weak directivity effects. According to the rupture parameters for the 14 events, prevailing SSE or SEE (NNW or NNE) rupture directivity was a common feature for events to the north (south) of the mainshock in this earthquake sequence.