Transient anomaly in fault-zone trapped waves during the preparatory phase of the 6 April ͳ 2009, Mw 6.3 L’Aquila earthquake ʹ ͵ Giovanna Calderoni, Antonio Rovelli, and Rita Di Giovambattista Ͷ Istituto Nazionale di Geofisica e Vulcanologia ͷ Via di Vigna Murata Ͳͷ ͲͲͳͶ͵ Rome, Italy ͺ ͻ Abstract ͳͲ Fault-zone trapped waves generated by repeating earthquakes of the 2009 L’Aquila seismic ͳͳ sequence show a sudden, up to 100% increase of spectral amplitudes seven days before the ͳʹ mainshock. The jump occurs ten to twenty hours after the M L 4.1, 30 March 2009 largest foreshock. ͳ͵ The amplitude increase is accompanied by a loss of waveform coherence in the fault-trapped ͳͶ wavetrain. Other geophysical and seismological parameters are known to have shown a sudden ͳͷ change after the 30 March foreshock. The concomitance of a consistent change in the fault-zone ͳ trapped waves leads us to interpret our observation as due to a sudden temporal variation of the ͳ velocity contrast between the fault damage zone and hosting rocks in the focal volume. Fault-zone ͳͺ trapped waves thus provide a refined time resolution for changes occurring near the rupture ͳͻ nucleation, with the indication of a strong variation in one day. ʹͲ