GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 Identification of shallow events by residual analysis of seismic ground motion Mascandola C. 1 , Pacor F. 1 , Lanzano G. 1 , Ramadan F. 1 , and Traversa P. 2 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Milano (Italy) 2 EDF-DIPNN-DI-TEGG, Aix-en-Provence, France Despite the importance of the earthquake depth, just this parameter is less accurately determined than the epicenter position, except when stations are located near or above the epicenter. The hypocentral depth is usually determined with P or S waves arrival times and needs the knowledge of the crustal structure, employed in most of the location procedures, which is often fairly limited (Janský et al., 2009). A wrong focal depth can occur when a superficial earthquake is located in an available crustal model, not appropriate enough in terms of the shallow low-velocity structure, and if the nearest station is relatively distant from the epicenter. Therefore, the very shallow earthquakes might be systematically mislocated, i.e. moved to larger depths, while deeper events are much less vulnerable to these problems. Inadequacy of the models in the shallow crust, combined with often sparse location networks (thus missing very near stations) might systematically bias the seismicity pattern in many regions of the world. Indeed many seismic catalogs present prevalent occurrences at fixed depths, which are defined a-priori due to the inability to reduce their uncertainty (Yang and Yao, 2021). Therefore, delineation of the top of the seismogenic layer might be less certain than believed so far. An example regarding tectonic studies is the 2001 Meran earthquake (Mw 4.8; Pondrelli et al., 2004). Being at the border of the existing national seismic networks of the Italian, Austrian and Swiss territories, the area was not well monitored seismically and large differences in the epicentral locations computed by the different agencies were observed (Caporali et al., 2005). In this case, the addition of geodetic and hydrologic data to the seismological data enabled both the hypocenter and the fault plane solution to be constrained more uniquely than with seismological data alone, allowing the earthquake association with the Giudicarie fault system, which crosses the town of Meran (Caporali et al., 2005). The space and time distribution of earthquakes represents a key input for seismotectonic models, but also for the seismic hazard assessment. In particular, the systematic uncertainty on the hypocentral depth of shallow events can significantly affect the near-source ground-motion of both weak and strong events, with significant implications on hazard estimates. The correct identification of very shallow events (< 5km) becomes an important task that we try to address by using seismic records and analyzing the residual distribution with respect to a reference ground motion model (GMM). In particular, we explore the between-event component of the