Many unknowns exist regarding the energy radiation processes of the inland low-frequency earthquakes (LFEs) often observed beneath volcanoes. To evaluate their energy radiation characteristics, we estimated the scaled energy for LFEs and regular earthquakes in and around the focal area of the 2008 Mw 6.9 Iwate-Miyagi earthquake. We computed the source spectra for regular earthquakes, deep LFEs, and shallow LFEs by correcting for the site and path effects from direct S-waves. We computed the radiated energy and seismic moments, and obtained the scaled energy (eR) for 1464 regular earthquakes, 169 deep LFEs, and 52 shallow LFEs. The eR for regular earthquakes is in the order of 10-5 to 10-4, typical for crustal earthquakes, and tends to become smaller near volcanoes and shallow LFEs. In contrast, eR is in the order of 10-7 and 10-6 for deep and shallow LFEs, respectively, one to three orders of magnitude smaller than that for regular earthquakes. This result suggests that LFEs are associated with a much lower stress drop and/or slower rupture and deformation rates than regular earthquakes. Although the energy magnitudes derived from radiated energy generally show good agreement with the local magnitudes for the three types of earthquakes, the moment and local magnitudes show a large discrepancy for the LFEs. This suggests that the local magnitude based only on the maximum amplitude of the observed seismic records may not provide good information on the static sizes of LFEs whose eR values are substantially different from those of regular earthquakes.

Determining fluid migration and pore pressure changes within the Earth is key to understanding earthquake occurrences. We investigated the spatiotemporal characteristics of intense fore- and aftershocks of the 2017 ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. The results show that the foreshock hypocenters moved upward on a sharply defined plane with steep dip. The mainshock hypocenter was located at the edge of a seismic gap formed by foreshocks along the plane. This spatial relationship suggests that the mainshock ruptured this seismic gap. The corner frequency of the mainshock supports this hypothesis. The aftershock hypocenters migrated upward along several steeply dipped planes. The aftershock activity slightly differs from the simple mainshock–aftershock type, suggesting that aseismic processes controlled this earthquake sequence. We established the following hypothesis: First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing a foreshock sequence and potentially aseismic slip. The continuous decrease in the fault strength associated with an increase in the pore pressure and the increase in shear stress associated with aseismic slip and foreshocks caused the mainshock in an area with relatively high fault strength. The change in the pore pressure associated with post-failure fluid discharge contributed to aftershocks, causing the upward migration of the earthquake. These observations demonstrate the importance of considering fluid movement at depth not only earthquake swarms but also foreshock—mainshock–aftershock sequences.

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting the information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region by using GNSS data. The earthquake detection and hypocenter relocation succeeded in delineating three planar structures of earthquakes. The results suggest that individual aftershocks were caused by a slip on the macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault) similar to the recent earthquake swarm sequences triggered by the 2011 Tohoku-Oki earthquake in the stress shadow in the upper plate. East–west contraction strain rate in the source region of the Yamagata-Oki earthquake with E–W compressional reverse fault mechanism changed to the E–W extension as a result of Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that the reduction in the fault strength due to the uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.

We conducted a detailed investigation of an earthquake cluster distributed from the lower crust to the upper crust beneath Hakodate, Hokkaido, which included both low-frequency earthquakes (LFEs) and regular earthquakes. Relocated hypocentres clearly show that both the LFEs and regular earthquakes occurred close to each other in the brittle upper crust of this non-volcanic area, while only LFEs occurred in the lower crust. This indicates that LFEs can occur not only in the ductile lower crust, but also in the brittle upper crust, which suggests that LFEs can occur in an environment similar to that of regular earthquakes. Regular earthquakes that occur in close vicinity of LFEs have very similar waveforms and nearly overlapping source regions, which indicate that they reflect the repeated rupture of the same asperity patch on a fault. Temporally, the intervals between events in the repeating earthquake sequence were very short, thus suggesting that they were caused by a sudden increase in pore pressure. The cluster of LFEs and repeating earthquakes, which has a rod-like distribution extending from the bottom of the crust to the surface and tilted slightly eastward, might represent a pathway of aqueous fluid movement sourced from the subducting slab.

Low-frequency earthquakes (LFEs) are categorized as slow earthquakes whose spectral power is concentrated at 1–4 Hz. While the tectonic LFEs along megathrust boundaries occur as shear failure, LFE generation in the continental plate, which is widespread in the lower crust and rarely occurs in the brittle upper crust, is poorly understood due to the diversity of focal mechanism solutions. Here we conduct a systematic survey of LFEs using two metrics (frequency index and peak frequency) that characterize the frequency content of the waveforms, and show that LFEs are prevalent in the upper crust beneath the Japanese Islands, even in non-volcanic regions. Shallow LFEs are most common near tectonic boundaries, and are temporarily activated in the aftershock sequences of large (M³6.5) crustal earthquakes. The widespread distribution of shallow LFEs suggests that a lower crustal rheology is not necessary for their genesis. We infer that failure along frictionally weakened faults due to high pore-fluid pressures is a primary control for the enrichment of low-frequency energy. The observed differences in the frequency content are probably due to differences in the pore-fluid pressure along each fault, which influences the rupture velocity and magnitude of the tensile component during shear failure. Our observations may lead to a more unified model of earthquake generation, thereby providing a better understanding of how earthquakes release the stress accumulated in the Earth.