Transient anomaly in fault zone‐trapped waves during the preparatory phase of the 6 April 2009, M_w 6.3 L'Aquila earthquake

Abstract

Fault zone-trapped waves generated by repeating earthquakes of the 2009 L'Aquila seismic sequence show a sudden, up to 100% increase of spectral amplitudes 7 days before the main shock. The jump occurs 10 to 20 h after the M L 4.1, 30 March 2009 largest foreshock. The amplitude increase is accompanied by a loss of waveform coherence in the fault-trapped wave train. Other geophysical and seismological parameters are known to have shown a sudden change after the 30 March foreshock. The concomitance of a consistent change in the fault zone-trapped waves leads us to interpret our observation as due to a sudden temporal variation of the velocity contrast between the fault damage zone and hosting rocks in the focal volume. Fault zone-trapped waves thus provide a refined time resolution for changes occurring near the rupture nucleation, with the indication of a strong variation in 1 day.

Foreshocks and short-term hazard assessment of large earthquakes using complex networks: the case of the 2009 L'Aquila earthquake

“…The M w = 6.3, 6 April 2009 L'Aquila earthquake was preceded by a long suite of foreshocks; the largest one of magnitude M L = 4.1 (30 March at 13:38 UTC) marked the beginning of an abrupt temporal change in different seismic parameters, such as the b value (Papadopoulos et al, 2010;Sugan et al, 2014), the spatio-temporal distribution of the events (Telesca, 2010) and the P -to-S wave velocity ratio (Di Luccio et al, 2010;Lucente et al, 2010). Calderoni et al (2014) state that before that event, seismicity was concentrated to the north of the volume where the main shock nucleated. After the M L = 4.1 event, rate and magnitude of foreshocks increased and seismicity migrated toward the main shock nucleation zone.…”

Stress Drop, Apparent Stress, and Radiation Efficiency of Clustered Earthquakes in the Nucleation Volume of the 6 April 2009, Mw 6.1 L'Aquila Earthquake

“…The data set of this study comprises nine of the repeating earthquakes used by Calderoni, Rovelli, and Di Giovambattista (). These earthquakes are part of a cluster located a few kilometers to the southwest of the main shock nucleation point that were selected as having precise hypocentral determinations and moment magnitude available from independent moment tensor determinations.…”

“…These earthquakes are part of a cluster located a few kilometers to the southwest of the main shock nucleation point that were selected as having precise hypocentral determinations and moment magnitude available from independent moment tensor determinations. The data set in analysis also includes 17 earthquakes located in a 2.5 × 2.5 × 2.5‐km 3 crust volume (see Figure ) adjacent to those used by Calderoni, Rovelli, and Di Giovambattista (). The selection criterion for these earthquakes was based on a cross‐correlation analysis of waveform similarity as already done by Calderoni, Rovelli, and Di Giovambattista ().…”

“…The earthquakes used by Calderoni, Rovelli, and Di Giovambattista () have a spatial distribution that is optimal for an investigation of the state of stress in the nucleation volume. A study of repeating earthquakes yielded important results on the San Andreas Fault at Parkfield where Abercrombie () found a sharp decrease of stress drop with the 2004, M w 6 earthquake followed by a fast return to previous values.…”

“…In this study, we have enlarged the data set of Calderoni, Rovelli, and Di Giovambattista () including as much as possible nearby earthquakes with the goal of seeking for temporal and/or spatial variation of stress parameters in the nucleation volume of the destructive M w 6.1 L'Aquila earthquake. The attention is focused on earthquakes between the M w 4.1 foreshock and the aftershocks that occurred immediately after the M w 6.1 main shock.…”