GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Seismic atenuaton and stress on the San Andreas Fault at Parkfeld: are we critcal yet? L. Malagnini *1,2 , R. M. Nadeau 2 , and T. Parsons 3 1 Isttuto Nazionale di Geofsica e Vulcanologia, Rome, Italy 2 Berkeley Seismological Laboratory, University of California Berkeley, USA 3 U.S. Geological Survey, Mofet Field, CA, 94035, USA The Parkfeld transitonal segment of the San Andreas Fault (SAF) is characterized by the producton of frequent quasi-periodical M6 events that  break the very same asperity. The last Parkfeld mainshock occurred on September 28 th 2004, 38 years afer the 1966 earthquake, and afer the segment showed a ~22 years average recurrence tme. The main reason for the much longer interevent period between the last two earthquakes is thought to be the reducton of the Coulomb stress from the M6.5 Coalinga earthquake of May 2 nd 1983, and the M6 Nuñez events of June 11 th  and July 22 nd  1983. Plausibly, the transitonal segment of the SAF at Parkfeld is now in the late part of its seismic cycle and current observatons may all be relatve to a state of stress close to critcality. This study aims at the recogniton of the precritcal state of the transitonal segment of the SAF at Parkfeld, in the hypothesis that the fuctuatons of the atenuaton parameter represent a proxy for the stress conditons in the crustal volume crossed by an actve fault, even though the behaviour of the atenuaton parameter in the last few years seems substantally diferent from the one that characterized the years prior to the 2004 mainshock. A few questons arise from this study: (i) Does a detectable preparaton phase for the Parkfeld mainshocks exist, and is it the same for all events?; (ii) How dynamically/kinematcally similar are the quasi-periodic occurrences of the Parkfeld mainshocks? (iii) Are some dynamic/kinematc characteristcs of the next mainshock predictable from the analysis of current data? (e.g., do we expect the epicenter of the impending failure to be co-located to that of 2004?) (iv) Should we expect the duraton of the current interseismic period to be close to the 22-year “undisturbed” average value? We respond to the questons above by analyzing the non-geometric atenuaton of direct S-waves along the transitonal segment of the SAF at Parkfeld, in the close vicinity of the fault plane, between January 2001 and November 2023. Of partcular interest is the preparatory behaviour of the atenuaton parameter as the 2004 mainshock approached, on both sides of the SAF. We also show that the non-volcanic tremor actvity modulates the seismic atenuaton in the area, and possibly the seismicity along the Parkfeld fault segment, including the occurrence of the mainshocks. The hypothesis is that the non-geometric atenuaton is directly related to the bulk permeability of crustal rocks, which in turn is driven to crack density and interconnecton. Figure 1 shows a map of the investgated region, with the locatons of the earthquakes in our dataset and more informaton of interest. An interestng observaton is the 10-day smoothed rate of non-volcanic tremor (NVT) collected along the transitonal segment of the SAF (Guilhem and