GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 permeability structures with dimensions comparable to the short-wavelength radiaton tend to close because crack coalescence causes strain localizaton into the macroscopic fracture, and compacton and stress relaxaton in the surrounding volume. The long-wavelength radiaton, on the contrary, gets more atenuated because the longer cracks that dominate the coalescence process increase the bulk permeability in the volume immediately around the fault plane by enhancing their aperture and interconnecton. Finally, we observe that the 2-5 Hz band is dominated by surface waves that sample a shallower crustal structure than the direct S-waves of the 30-50 Hz band (see Figure 6). About the intertwined behaviour of and the NVT actvity, it is possible that the increased permeability along the very fault plane under subcritcal conditons allows pore pressure to diminish at NVT depth (in our case, this would correspond to the observed decrease in NVT actvity that starts in 2020), and to an upward pore fuid migraton along the fault plane. In turn, the later may weaken the fault and facilitate the occurrence of the mainshock. Within the limits of this hypothesis, monitoring the fuctuatons of NVT actvity along the SAF would be useful in recognizing the precritcal state of the fault.  In the next few months (or years) the Parkfeld segment of the SAF will provide most of the answers to our questons. In the meantme, it may be worthwhile applying our technique on diferent tectonic setngs characterized by more complex fault geometries and ongoing NVT actvity. Aside from more advanced theoretcal developments and laboratory experiments on seismic atenuaton under increasing diferental stress, the most important ingredient for a successful use of non-geometric atenuaton and NVT informaton would be the use of data from dense(r) seismic networks equipped with borehole sensors recording at high sampling rate.  References Guilhen, A. and Nadeau, R.M. (2012), Episodic tremors and deep slow-slip events in Central California, EPSL, 357-358, 1-10. Malagnini, L., Dreger, D. S., Bürgmann,R., Munafò, I., & Sebastani, G. (2019). Modulaton of seismic atenuaton at Parkfeld, before and afer the 2004 M6 earthquake. Journal of Geophysical Research: Solid Earth,124, 5836–5853. htps://doi.org/10.1029/2019JB017372. Malagnini, L., & D.S. Dreger (2016). Generalized Free-Surface Efect and Random Vibraton Theory: a new tool for computng moment magnitudes of small earthquakes using borehole data, Geophys J Int (2016) 206 (1): 103-113. htps://doi.org/10.1093/gji/ggw113.  Nadeau, R.M., Guilhem, A. (2009). Nonvolcanic Tremor Evoluton and the San Simeon and Parkfeld, California, Earthquakes,Science 325, 191,  DOI: 10.1126/science.1174155. Sebastani, G., & L. Malagnini (2020). Forecastng the Next Parkfeld Mainshock on the San Andreas Fault (California), Journal of Ecology and Natural Resources, vol. 4, issue 3, htps://doi.org/10.23880/jenr-16000218 .   Corresponding author: luca.malagnini@ingv.it δ ( Q −1 S ( t , f ) )