Applying the ETAS Model to Simulate Earthquake Sequences for Hazard Assessment in Crustal Seismicity Regions with Scarce Data

Considering earthquake sequences in seismic hazard and risk analyses has become increas-ingly important. Many sequences around the world demonstrated that limiting the consideration to only mainshocks leads to underestimation of hazard and, more importantly, risk. Among the methods to simulate earthquake sequences, the epidemic-type aftershock sequence (ETAS) model has shown notable accuracy in representing these complex sequences making it a valuable tool for seismic hazard and risk assessment. However, the ETAS model is rather complex and for regional applications it requires many parameters to be calibrated. The calibration is most effective when a long instrumental catalog complete to low-magnitude events is available. However, this is rarely the case. Our study investigates the possibility of using the ETAS model with parameter values calibrated for a region with many recent sequences (namely central Italy) to simulate realistic sequences in two other crustal seismicity regions, namely Türkiye and Croatia. We compare three options (1) using a set of ETAS parameters calibrated based on local data, (2) fixing most ETAS parameters to values derived from a well-studied region with many recent sequences (central Italy) while locally calibrating the total background rate and productivity term (SimplETAS [SE] parameter set, akin to the simplETAS methodology of Mancini and Marzocchi (2023), and (3) fixing all ETAS parameters to values from central Italy except for the total background rate (Set of Central Italy [SCI] parameter set). The results show that simplified approaches based on central Italy data (SE and SCI) can produce simulated earthquake sequences that align well with the observed data even for Croatia and Türkiye. This reflects the applicability of modeling earthquake sequences for hazard and risk applications in many crustal regions where good quality observed instrumental catalogs complete to low-magnitude events are not available or they are available but do not contain significant sequences.