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
of 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
strongest, up to approximately 5 times the event’s corner frequencies.
The results of this research provide useful insights to parameterize
directivity as a frequency-dependent band-limited phenomenon, to be
explicitly implemented in future ground motion modeling and scenario
predictions.