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We assessed the seismic potential around the northeastern edge of the Longmenshan fault zone (LFZ) by analyzing the aftershocks of the 2008 Mw 7.9 Wenchuan earthquake, focusing on the right-lateral strike-slip Qingchuan fault (QCF). The data used in the present study were collected from a dense network of broadband seismographs, installed after the Mw 7.9 mainshock by the China Earthquake Administration. We analyzed 440 aftershocks that occurred around the northeastern part of the aftershock area between December 2008 and June 2009, the local magnitudes (ML) of which ranged from 2.0 to 5.2. We determined 127 well-constrained focal mechanism solutions for these aftershocks down to ML 2.5 by using P-wave polarity data in conjunction with body-wave amplitudes. An analysis of the stress tensor inversion revealed that, upon approaching the QCF from the LFZ, the stress field changes from a reverse faulting regime to a mixed regime containing strike-slip components and the orientation of the maximum compressional stress changes from a NW–SE to a WNW–ESE direction. This stress field favors a right-lateral strike-slip movement along the fault. The slip tendency analysis revealed that the QCF is regarded as being favorably oriented to the present-day stress field. The coseismic Coulomb stress changes induced by the mainshock increased along the preferred fault plane in the northeastern half of the QCF, which is further loaded by viscoelastic relaxation. This evidence suggests that the potential for a future earthquake in the QCF is high. [ABSTRACT FROM AUTHOR]

The 1944 Tonankai and 1946 Nankai megathrust earthquakes are likely to have been immediately preceded by a huge slow slip event (SSE) in the deeper brittle‐to‐ductile transition zone, which accumulates slip of a meter or more within several days. The present study investigates the interaction between such huge SSEs and the characteristic earthquakes through earthquake cycle simulations obeying the rate‐ and state‐dependent friction. To simulate huge SSEs, we employ a cutoff velocity (Vcx) above which the friction changes from velocity weakening to velocity strengthening. We assume that Vcx decreases from 10 to 10−9 m/s with depth over the transition zone located between 20‐ and 35‐km depths. Depending on the characteristic slip distance L of rate and state friction, simulation results showed three patterns of earthquake nucleation. If L is small, an SSE in the transition zone directly grows into an earthquake without experiencing a separate nucleation at the brittle zone (brittle nucleation). If L is moderate, an SSE triggers brittle nucleation, which develops into an earthquake. If L is large, an SSE occurs but ceases without triggering an earthquake nor brittle nucleation, then slip deficit accumulates further to finally start brittle nucleation without the direct help of SSE, and an earthquake occurs without a large precursor. In addition, isolated huge SSEs, resembling precursory ones, occur in all cases, at most a few times per cycle. Key Points: We investigate the nucleation of characteristic earthquakes in simulated cycles involving deep huge SSEsThree distinct nucleation patterns were found; two involve a huge SSE; a huge SSE does not necessarily trigger an earthquakeSSEs and earthquake nucleation grow huge because of a unique slip profile characterized by a flatter maximum than an ellipse [ABSTRACT FROM AUTHOR]

A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone. [ABSTRACT FROM AUTHOR]

Earthquake ruptures cause mass redistribution, which is expected to induce transient gravity perturbations simultaneously at all distances from the source before the arrival of P-waves. A recent research paper reported the detection of such prompt gravity signals from the 2011 Tohoku-Oki earthquake by comparing observed acceleration waveforms and model simulations. The 11 observed waveforms presented in that paper recorded in East Asia shared a similar trend above the background seismic noise and were in good agreement with the simulations. However, the signal detection was less quantitative because the significance of the observed signals was not discussed and the waveforms at other stations in the region were not shown. In this study, similar trends were not observed in most of the data recorded near the stations used in the aforementioned study, suggesting that the reported signals were only local noises. We thus took a different approach to identify the prompt signals. We optimized the multi-channel data recorded by superconducting gravimeters, broadband seismometers, and tiltmeters. Though no signal was identified in the single-trace records, the stacked trace of the broadband seismometer array data in Japan showed a clear signal above the reduced noise level. The signal amplitude was 0.25 nm/s2 for an average distance of 987 km from the event hypocenter. This detection was confirmed with a statistical significance of 7σ, where σ is the standard deviation of the amplitude of the background noise. This result provided the first constraint on the amplitude of the observed prompt signals and may serve as a reference in the detection of prompt signals in future earthquakes. [ABSTRACT FROM AUTHOR]