Along-strike rupture directivity of earthquakes of the 2009 L'Aquila, central Italy, seismic sequence

We perform a systematic investigation of along-strike rupture directivity of 70 earthquakes (3.0 a parts per thousand currency sign M-w a parts per thousand currency sign 6.1) of the 2009 L'Aquila seismic sequence by analysing azimuthal variations of broad-band seismograms recorded in the distance range 60 km < R < 230 km. We use reference spectra of events with little directivity (similar to the empirical Green's function method) to deconvolve propagation-site effects and focus on source properties. A directivity index (0 a parts per thousand currency sign I-DIR a parts per thousand currency sign 1) calculated for each earthquake quantifies the spectral separation above the corner frequency of the target event at opposite along-strike directions. A large number (73 per cent) of events including the M-w 6.1 main shock show high (> 0.7) I-DIR values indicating predominantly unilateral rupture propagation. The preferred rupture propagation direction is generally to the southeast with no dependence on the earthquake magnitude or occurrence time. Events on two main faults (L'Aquila and Campotosto) show somewhat different behaviour. Almost all earthquakes on the L'Aquila fault have strong unilateral directivity to the southeast, whereas earthquakes on the Campotosto fault show more diverse behaviour. However, there is a predominance of unilateral ruptures (14 out of 22) also on the Campotosto fault, and the few (five) earthquakes with ruptures to the northwest are limited to the most northwestern segment of the fault. The spectral results are consistent with time-domain analysis when the latter samples adequately the frequency band above corner frequency. The preferred rupture direction may be produced at least in part by a velocity contrast across the fault. The results provide important input for estimates of seismic motion and physics of earthquake ruptures.