Spatio-temporal evolution in stress drop during earthquake sequences from the swarm to aftershocks in the Noto Peninsula, central Japan

We have investigated the spatial and temporal variations in stress drop during a series of earthquakes, from the swarm to the early aftershocks in the Noto Peninsula, central Japan. The swarm began in 2018 and lasted more than five years until the devastating 2024 Mw 7.5 Noto earthquake. We estimated stress drop from a comparison between the observed and theoretical Frequency Index (FI). Employing a commonly used source model and the relations between corner frequency and stress drop, the theoretical FI is a function of S-wave velocity, attenuation factor, and stress drop. By assuming a S-wave velocity and using the separately measured attenuation factor, the theoretical FI depends solely on stress drop. We estimate the stress drop that minimizes the difference between the observed and theoretical FI. We validated the method by comparing the obtained stress drop with that from the ordinary method to measure the corner frequency. The result is consistent, though our method gives slightly higher stress drop than a one-to-one relationship. As long as we discuss the relative values of stress drop, this difference has little effect on subsequent observations and interpretations.

We estimated stress drop for 3,490 earthquakes with magnitudes from 1.5 to 4.0. The obtained stress drop shows an apparent spatial variation: Low-Stress-Drop Events (LSDEs) are dominant in the southern part of the swarm area, whereas both LSDEs and High-Stress-Drop Events (HSDEs) coexist in the northern part. Previous studies classified the swarm area into four subareas of S (southeast), W (southwest), N (northwest), and E (northeast). The swarm areas temporarily expanded, including subareas in this order. Our results show that earthquakes in the S subarea, where the swarm started, mostly have low stress drop. Some events with extremely low stress drop exhibit a unique waveform with a low-frequency band and a decaying amplitude over time, resembling volcanic low-frequency earthquakes. Previous studies have suggested that crustal fluids contribute to seismogenesis in the area. Our results give further and strong support for this suggestion. HSDEs mainly occurred in subarea N, with the highest seismicity, and in subarea E, which hosted many large swarm earthquakes. They are located sandwiched between and around the band of LSDEs. During the early aftershock stage, HSDEs are absent in these locations. The rupture of the mainshock originated in these subareas and was "quiet," with only minor moment release. Previous studies suggested that these subareas experienced strain release preceding the mainshock due to the swarm. The spatio-temporal variation of HSDEs is consistent with the interpretation. On the other hand, LSDEs in subarea S occurred during both the swarm and the aftershock sequences, implying continuous fluid supply from the anticipated fluid source beneath the swarm area. Our method of stress drop estimation does not aim for high accuracy, but rather to obtain estimates for many earthquakes with acceptable accuracy. The results of this study indicate that the approach works well to investigate the seismogenesis of complex earthquake sequences.