Evidence of frequency-dependent directivity effects from non-ergodic ground motion modelling of Spectral Acceleration in Central Italy

Rupture directivity and its potential frequency dependence is an open issue in the seismological community, especially for small-to-moderate earthquake. Directivity itself is the focusing of the radiated seismic wave energy due to the rupture propagation along the direction of the fault.

In this research, we calibrate a non-ergodic ground motion model for the ordinates of the 5% acceleration response spectra (computation interval 0.04-2 sec) and we analyse, earthquake by earthquake, the azimuthal dependence of the aleatory component, i.e. the residual terms corrected for systematic source, site and path contributions. The final aim is the calibration of a prediction model including directivity effects that can be used for engineering purposes such as seismic hazard assessment and shaking scenarios generation.

The study area is the Central Italy, which was affected by several seismic sequences in the last 20 years, occurred on normal fault systems. The dataset we used is composed by almost 300,000 seismic recordings of 456 earthquakes in the magnitude range from 3.4 to 6.5 within the time frame 2008-2018. We find that about one-third of the analysed events are directive, characterized by unilateral ruptures along the Apennine faults direction.

Directivity effects occur over a wide frequency band and can be described by spectral curves peaked in different frequency ranges according to the event magnitude: the stronger the earthquake, the lower the frequency at which these effects are visible. Vice versa, we find no correlation between the amplitude of such peaks and the events magnitude. When normalized to the peak, the directivity curves can be grouped into families characterized by similar amplification trends variable with frequency, with the exception of 16 events, which we classify as "super-directive", that differ markedly from the others generating broadband amplifications.

Preliminary results suggest that is possible to obtain similar shapes of directivity curves for defined frequency families and that they can consequently be modeled for non-ergodic ground motion model and predictive shaking scenarios.