This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The hypocenter relocation of 10,940 earthquakes (M>1) reveals that they are all crustal and migrated upward, activating a complex network of faults at depths shallower than 20 km. The initiation of this earthquake swarm occurred at a locally deep depth (z = 17 km). We find a distinctive S-wave reflector in the immediate vicinity. The local hypocenter distribution shows a characteristic circular pattern. A low-velocity anomaly exists below the reflector, and a high helium isotope ratio and a low-gravity anomaly were observed at the surface. These observations suggest that the current seismicity is associated with fluids released by ancient or possibly unrecognized modern magmatic activity, although no volcanic activity has been documented in this area for 15 million years. The strongest M5.4 earthquake (as of October 2022) initiated near the migration front on the largest planar structure and propagated in the same direction. Significant crustal deformation was also observed during this earthquake swarm and is probably related to the fluid movement and aseismic deformation that contributed to this earthquake swarm. 

Upward earthquake migration in the northeastern Noto Peninsula, Japan, initiated from a deep ring‐shaped cluster: Possibility of fluid leakage from a hidden magma system

Stress accumulation and release in the crust remains poorly understood compared to that at the plate boundaries. Spatiotemporal variations in foreshock and aftershock activities can provide key con...

Stress Release Process Along an Intraplate Fault Analogous to the Plate Boundary: A Case Study of the 2017 M5.2 Akita-Daisen Earthquake, NE Japan

Stress drops of small earthquakes have been estimated under the assumption that the rupture propagates symmetrically within a circular fault. However, recent studies have observed directivity effects on seismic waves even for small earthquakes. In this study, rupture directivity was investigated systematically for small-to-moderate-sized earthquakes (M 3.5–5.5) that occurred beneath inland Japan from 2004 to April 2019. Apparent moment rate functions were determined for 1463 earthquakes, and their corner frequencies were inverted for their rupture parameters. The results indicate that most of the analyzed earthquakes (1217 of 1463) are characterized by significantly asymmetrical rupture propagation. The stress drops of the earthquakes estimated by considering asymmetrical rupture propagation were 16.8 MPa, which are almost twice the estimates based on the commonly used symmetrical rupture model. This shows the importance of recognizing the diversity of ruptures, even for small earthquakes, for extracting information about earthquake sources and the Earth’s structure. The prevailing rupture directivity can provide useful information on source parameters including the fault size, fault geometry, and rupture velocity of small earthquakes.

/pdf/prevalence-of-asymmetrical-rupture-in-small-earthquakes-and-3v7njov522.pdf

Prevalence of asymmetrical rupture in small earthquakes and its effect on the estimation of stress drop: a systematic investigation in inland Japan

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 afters...

Fault-Valve Behavior Estimated From Intensive Foreshocks and Aftershocks of the 2017 M 5.3 Kagoshima Bay Earthquake Sequence, Kyushu, Southern Japan


 Time-domain analyses of seismic waveforms have revealed diverse source complexity in large earthquakes (Mw > 7). However, source characteristics of small earthquakes have been studied by assuming a simple rupture pattern in the frequency domain. This study utilises high-quality seismic network data from Japan to systematically address the source complexities and radiated energies of Mw 3–7 earthquakes in the time domain. We first determine the apparent moment-rate functions (AMRFs) of the earthquakes using the empirical Green's functions. Some of the AMRFs show multiple peaks, suggesting complex ruptures at multiple patches. We then estimate the radiated energies (𝐸𝑅) of 1736 events having more than ten reliable AMRFs. The scaled energy (𝑒𝑅 = 𝐸𝑅/𝑀0) does not strongly depend on the seismic moment (𝑀0), focal mechanisms, or depth. The median value of 𝑒𝑅 is 3.7 × 10−5, which is comparable to those of previous studies; however, 𝑒𝑅 varies by approximately one order of magnitude among earthquakes. We measure the source complexity based on the radiated energy enhancement factor (𝑅𝐸𝐸𝐹). The values of 𝑅𝐸𝐸𝐹 differ among earthquakes, implying diverse source complexity. The values of 𝑅𝐸𝐸𝐹 do not show strong scale dependence for Mw 3–7 earthquakes, suggesting that the source diversity of smaller earthquakes is similar to that of larger earthquakes at their representative spatial scales. Applying a simple spectral model (e.g. the ω2-source model) to complex ruptures may produce substantial estimation errors in source parameters.

Time-domain source parameter estimation of Mw 3–7 earthquakes in Japan from a large database of moment-rate functions

Rupture directivity, stress drop, and hypocenter migration of small earthquakes in the Yamagata-Fukushima border swarm triggered by upward pore-pressure migration after the 2011 Tohoku-Oki earthquake

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Rupture directivity, stress drop, and hypocenter migration of small earthquakes in the Yamagata-Fukushima border swarm triggered by upward pore-pressure migration after the 2011 Tohoku-Oki earthquake