GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Figure 6. Average atenuaton parameter calculated for the North American side (black) and for the Pacifc side of the SAF (red), in the indicated tme windows (2001-2004 and 2005-2019). Below 8-10 Hz, all curves show a fatening that is stronger for the atenuaton parameter calculated on both sides of the SAF in the 2001-2004 tme window. Such behavior is consistent with the fact that the ground moton in the 2-8 Hz band is rich with surface waves, if not dominated by them. Our study shows that, along the fault segment under scrutny, the signature of the precritcal state of the 2004 mainshock is represented by the variatons in opposite directons of the low- and the high-frequency atenuaton parameters as a functon of tme (low-frequency atenuaton increases sharply, high-frequency atenuaton decreases). The bifurcatons are observed on both sides of the fault, and last 6-8 weeks. The precritcal state of the Parkfeld asperity emerges afer a period of smooth variability of the atenuaton parameter that lasts for about one year.  The bifurcaton of the stacked seismic atenuaton is the result of the diferent trends followed by the populatons of cracks in the two ranges of frequency at both ends of the available spectrum: 2-5 Hz and 30-50 Hz. In order to interact with the seismic energy, cracks’ lengths must form permeability structures with dimensions comparable to the wavelengths carried by the seismic radiaton: 400-1500 m for the 2-5 Hz band, and 60-100 m for the 30-50 Hz band. The cracks in the ( Q −1 S ( f ) )