Statistical Characteristics of Seismicity Correlated With Crustal Fluids in the Noto Region

Abstract

Earthquake swarms and strong shocks have frequently occurred in Noto Peninsula, Japan, for decades, resulting in a seismicity pattern characterized by a mixture of background earthquakes, swarms, and foreshock/aftershock sequences. Using an improved version of the space-time Epidemic-Type Aftershock Sequence model, we extract statistical features of the aftershocks hidden within earthquake sequences from 2000 to 2024. The aftershock productivity density patterns in the source regions of the 2007 Mw6.7, 2023 Mw6.2, and 2024 Mw7.5 earthquakes exhibit strong spatial heterogeneity, with high productivity density values extending from surface to depths of 15 km, 15 km, and 27 km, respectively. More importantly, the model parameters which characterize productivity and decay rates of aftershocks vary significantly in both space and time, as demonstrated by the results obtained from stochastic reconstruction. Two regions, located in the southwest and northeast of the Noto Peninsula, where the 2024 Mw7.5 mainshock has ruptured and earthquake swarms constantly occur, are characterized by higher $A$, lower $\alpha$, relatively low $p$ and high $b$ values. Temporal variations in the parameters indicate that these anomalies are primarily observed between 2021 and 2023 in the swarm activated region. We attribute the statistical characteristics of seismicity in these two local regions—such as a larger proportion of indirectly triggered aftershocks, a slower decay rate of aftershock numbers, and a relative higher proportion of small events—to the presence and migration of fluids in the lower crust or upper mantle beneath the Noto Peninsula.