Empirical Evidence of Frequency-Dependent Directivity Effects From Small-To-Moderate Normal Fault Earthquakes in Central Italy

Abstract

Rupture directivity and its potential frequency dependence is an open issue within the seismological community, especially for small-to-moderate events. Here, we provide a statistical overview based on empirical evidence of seismological observations, thanks to the large amount of high-quality seismic recordings (more than 30,000 waveforms) from Central Italy, which represents an excellent and almost unique natural laboratory of normal faulting earthquakes in the magnitude range between 3.4 and 6.5 within the time frame 2008-2018. In order to detect an anisotropic distribution of ground motion amplitudes due to the rupture directivity, we fit the smoothed Fourier Amplitude Spectra (FAS) cleared of source-, site- and path-effects. According to our criteria, about 36% of the analyzed events (162 out of 456) are directive and the distribution of rupture direction is aligned with the strikes of the major faults of the Central Apennines. We find that the directivity is a band-limited phenomenon whose width may extend up to five times the corner frequency. The results of this research provide useful insights to parameterize directivity, to be explicitly implemented in future ground motion modeling and scenario predictions.