Inverse analysis of seismic activity rate changes for severely incomplete sequences: comparison of aftershock activity patterns immediately following the 2023 M6.5 and 2024 M7.6 Noto Peninsula earthquakes

Abstract We estimate the true aftershock activity rate from the detected initial aftershocks of the M7.6 Noto Peninsula earthquake on January 1, 2024, in addition to the preceding M6.5 Noto Peninsula earthquake on May 5, 2023. Estimating the initial aftershock activity rates has the serious problem of missing data, but in itself contains important clues to the physical structure associated with aftershocks and other external factors. We first estimate the detection rate models of the two aftershock sequences assuming the Gutenberg–Richter frequency laws, and we have found the contrasting time evolution of b -values immediately after the mainshocks, in addition to the contrasting detection rate evolutions. Furthermore, by applying the compensated nonstationary ETAS (epidemic-type aftershock sequence) model, we have revealed the contrasting time evolution of the background intensity rates, which characterizes the external factor of the original ETAS model besides the static triggering seismic activity. We discuss the implications of these differences of the analyzed outputs. Graphical Abstract