A good understanding of the rupture patterns of small earthquakes is essential to understand the differences between earthquakes of different sizes. However, resolving the source complexity of small events (Mw<5) is challenging, because their seismic waveforms are distorted during propagation. In this study, we used high-quality seismic waveforms recorded by an excellent downhole sensor in Japan to directly examine the source complexities of 64 Mw3.3-5.0 short-range earthquakes (< 8 km). We found that even the waveforms of microearthquakes (Mw < 2) were simple at the sensor, indicating that the waveforms were scarcely disturbed by structural inhomogeneities. We inferred the moment rate functions from the shapes of the direct P-waves, which showed diversity in their complexity. Even conservatively estimated, 30% of the events had multiple subevents. The results suggest that methods that account for complexity, rather than those that assume a simple source pattern, are required to characterize even small earthquakes.

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.

Estimating the radiated energy of small-to-moderate (Mw < 5) events remains challenging because their waveforms are strongly distorted during wave propagation. Even when near-source records are available, seismic waves pass through the shallow crust with strong attenuation; consequently, high-frequency energy may be significantly dissipated. Here, we evaluated the degree of energy dissipation in the shallow crust by estimating the depth-dependent attenuation (Q-1) by modeling near-source (< 12 km) waveform data in northern Ibaraki Prefecture, Japan. High-quality waveforms recorded by a downhole sensor confined by granite with high seismic velocity helped to investigate this issue. We first estimated the moment tensors for M1–4 events and computed their synthetic waveforms, assuming a tentative one-dimensional -model. We then modified the -model in the 5–20 Hz range such that the frequency components of the synthetic and observed waveforms of small events (Mw < 1.7) matched. The results show that the Q-value is 55 at depths of < 4 km and shows no obvious frequency dependence. Using the derived -model, we estimated the moment-scaled energy (eR) of 3,884 events with Mw 2.0–4.5. The median eR is 3.6×10-5 , similar to the values reported for Mw >6 events, with no obvious Mw dependence. If we use an empirically derived Q-model (~350), the median eR becomes a one-order underestimation (3.1×10-6). These results indicate the importance of accurately assuming the Q-value in the shallow crust for energy estimation of small events, even when near-source high-quality waveforms are available.

An intense earthquake swarm is occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but the last volcanic activity in this area occurred in the middle Miocene (15.6 Ma), and no volcanic activity has occurred since then. In this study, we investigated the cause of this earthquake swarm using spatiotemporal variation of earthquake hypocenters and seismic reflectors. Hypocenter relocation revealed that earthquakes moved from deep to shallow areas via many planes, similar to earthquake swarms in volcanic regions. The strongest M5.4 earthquake initiated near the migration front of the hypocenters. Moreover, it ruptured the seismic gap between the two different clusters. The initiation of this earthquake swarm occurred at a locally deep depth (z = ~17 km), and we found a distinctive S-wave reflector, suggesting a fluid source in the immediate vicinity. The local hypocenter distribution revealed a characteristic ring-like structure similar to the ring dike that forms just above the magma reservoir and is associated with caldera collapse and/or magma intrusion. These observations suggest that the current seismic activity was impacted by fluids related to ancient or present hidden magmatic activity, although no volcanic activity was reported. Significant crustal deformation was observed during this earthquake swarm, which may also be related to fluid movement and contribute to earthquake occurrences. A seismic gap zone in the center of the swarm region may represent an area with aseismic deformation.

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 constraints on time-dependent stress and deformation processes in the crust. The 2017 M5.2 Akita-Daisen intraplate earthquake in NE Japan was preceded by intense foreshock activity and triggered a strong sequence of aftershocks. We examine the spatiotemporal distributions of foreshocks and aftershocks and determine the coseismic slip distribution of the mainshock. Our results indicate that seismicity both before and after the mainshock was concentrated on a planar structure with N-S strike that dips steeply eastward. We observe a migration of foreshocks towards the mainshock rupture area, suggesting that foreshocks were triggered by aseismic phenomena preceding the mainshock. The mainshock rupture propagated toward the north, showing less slip beneath foreshock regions. The stress drop of the mainshock was 1.4 MPa and the radiation efficiency was 0.72. Aftershocks were intensely triggered near the edge of large coseismic slip regions where shear stress increased. The aftershock region expanded along the fault strike, which is attributed to the post-seismic aseismic slip of the mainshock. The postseismic slip possibly triggered repeating earthquakes with M ~3. We find that the foreshocks, mainshock, aftershocks, and post-seismic slip released stress at different segments along the fault, which may reflect differences in frictional properties. Obtained results were similar to those observed for interplate earthquakes, which supports the hypothesis that the deformation processes along plate boundaries and crustal faults are fundamentally the same.

The behaviour of fluids in the crust is key to understanding earthquake occurrence due to the effect of fluid behaviour on fault strength. The attenuation of seismic waves may be locally high in fault zones as fluids are intensely distributed in these zones. This study uses a novel, simple approach to examine near-source attenuation in the focal region of intense swarm activity in the Yamagata-Fukushima border region, Japan. Near-source attenuation was estimated by determining the decay of amplitude ratios of nearby earthquake pairs with travel time differences precisely quantified using a waveform correlation. In the initial ~50 d, Q^(-1) was high, then it significantly decreased to become almost constant for the subsequent period. This pattern is similar to those independently observed for background seismicity rate, b-value, stress drop, and fault strength. These patterns can be attributed to the hypothesis that the swarm was triggered by fluid movement following the 2011 Tohoku-Oki earthquake, and the source and seismicity characteristics and the seismic attenuation were altogether affected by the temporal change in pore pressure. The method demonstrated in this study may be a useful tool to detect high pore pressure anomaly at depth and understand its relationship with earthquake occurrence.

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.

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 utilized 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 determined the apparent moment-rate functions (AMRFs) of the earthquakes using the empirical Green’s functions. Some of the AMRFs showed multiple peaks, suggesting complex ruptures at multiple patches. We then estimated the radiated energies (𝐸𝑅) of 1736 events having more than ten reliable AMRFs. The scaled energy (𝑒𝑅=𝐸𝑅/𝑀0) did not strongly depend on the seismic moment (𝑀0), focal mechanisms, or depth. The median value of 𝑒𝑅 was 3.7×10-5, which is comparable to those of previous studies; however, 𝑒𝑅 varied by approximately one order of magnitude among earthquakes. Additionally, we measured the source complexity based on the radiated energy enhancement factor (𝑅𝐸𝐸𝐹). The values of 𝑅𝐸𝐸𝐹 differed among earthquakes, implying diverse source complexity. The values of 𝑅𝐸𝐸𝐹 did 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 of source parameters.

Tsunamis with maximum amplitudes of up to 40 cm, related to the Mw 7.1 normal-faulting earthquake off Fukushima, Japan, on November 21, 2016 (UTC), were clearly recorded by a new offshore wide and dense ocean bottom pressure gauge network, S-net, with high azimuthal coverage located closer to the focal area. We processed the S-net data and found that some stations included the tsunami-irrelevant drift and step signals. We then analyzed the S-net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (~40 km) and a large peak (~200 cm) was obtained. The other near-coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests that the stress drop of this earthquake is ~10 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ~2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to the stress change.The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around this region even before the 2011 Tohoku earthquake. The present study demonstrates that the S-net high-azimuthal-coverage pressure data provides a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate around the offshore region. Our analysis provides an implication for the crustal stress state, which is important for understanding the generation mechanisms of the intraplate earthquake.

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.

The development of seafloor seismic observations facilitates reliable estimation of the rupture directivities of offshore earthquakes. We used seismic waveforms obtained by a new seafloor seismic network (S-net) and onland stations to systematically examine the rupture directivities of interplate earthquakes along the Japan trench. We estimated the rupture directions of 206 (M w 3.5-5) events, most of which occurred near the base of the seismogenic zone. We found that most earthquake ruptures (>~80 %) were directional, primarily propagating in the updip direction. This tendency cannot be explained by the effect of the bimaterial interface. The prevalence of updip rupture in the data suggests that deep, steady creep and upward fluid migration along the plate interface affected earthquake ruptures in the subduction zone. The updip ruptures redistributed the accumulated shear stress from the base of the seismogenic zone to the shallow large seismic patches. Furthermore, the updip ruptures may open up ways for deeper fluids to migrate further upward along the plate interface. Both the stress redistribution and the upward fluid migration facilitate the occurrence of shallow megathrust earthquakes.