GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Nadeau, 2012). The long-term NVT rate (~8500 days, updated through November 15 2023) is ploted in red in Figure 2A, the cumulatve NVT is ploted in black, and its detrended version is shown in Figure 2B. About 30-days prior to the 2004 Parkfeld event a signifcant NVT fore tremor was observed (Nadeau and Guilhem, 2009; Guilhem and Nadeau, 2012). Prior to this fore tremor, a relatvely quiescent period of ~120-days of NVT actvity was also observed (Figure 2C).  Figure 1. A: map of the region, showing the background seismicity and four mainshocks of specifc interest in this study (white stars), whose focal mechanisms are indicated in B. C: i) epicentral locatons of the SAF earthquakes used in this study; white triangles indicate the seismic statons of the HRSN. A similar quiescent period also precedes the proposed foretremor of the next possible Parkfeld M6 (Figure 2D). This 120-day quiescence is unusual for the majority of NVT episodes throughout the long-term NVT catalog (Figure 2A). The two pairs of dashed red vertcal lines in Figures 2A,B correspond to the tme windows of Figures 2C,D. Figure 2A suggests the presence of an anomaly in the tremor rate at Parkfeld, which we call foretremor, between 15 and 30 days before the 2004 mainshock. A similar anomaly (another foretremor?) seems to characterize the tremor rate in