Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic tremor, low-frequency earthquakes, very-low-frequency earthquakes, slow slip events and silent earthquakes. Each of these has been demonstrated to arise from shear slip, just as do regular earthquakes, but with longer characteristic durations and radiating much less seismic energy. Here we show that these slow events follow a simple, unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes. We find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant, with a spectral high-frequency decay of f(-1). This scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category. The observed scale dependence of rupture velocity for these events can be explained by either a constant low-stress drop model or a diffusional constant-slip model. This new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation.
Studies were made of the 1-70 keV persistent spectra of fifteen magnetars as a complete sample observed with Suzaku from 2006 to 2013. Combined with early NuSTAR observations of four hard X-ray emitters, nine objects showed a hard power-law emission dominating at 10 keV with the 15-60 keV flux of ∼1-11 × 10 −11 ergs s −1 cm −2 . The hard X-ray luminosity L h , relative to that of a soft-thermal surface radiation L s , tends to become higher toward younger and strongly magnetized objects. Updated from the previous study, their hardness ratio, defined as ξ = L h /L s , is correlated with the measured spin-down rateṖ as ξ = 0.62 × (Ṗ /10 −11 s s −1 ) 0.72 , corresponding with positive and negative correlations of the dipole field strength B d (ξ ∝ B
d) and the characteristic age τ c (ξ ∝ τ −0.68 c ), respectively. Among our sample, five transients were observed during X-ray outbursts, and the results are compared with their long-term 1-10 keV flux decays monitored with Swift/XRT and RXTE/PCA. Fading curves of three bright outbursts are approximated by an empirical formula used in the seismology, showing a ∼10-40 d plateau phase. Transients show the maximum luminosities of L s ∼1035 erg s −1 , which is comparable to those of the persistently bright ones, and fade back to 10 32 erg s −1 . Spectral properties are discussed in a framework of the magnetar hypothesis.
The 2016 Kumamoto-Oita earthquake sequence involving three large events (M w ≥ 6) in the central Kyushu Island, southwest Japan, activated seismicities in two volcanic areas with unusual and puzzling spatial gaps after the largest earthquake (M w 7.0) of April 16, 2016. We attempt to reveal the seismic process during the sequence by following seismological data analyses. Our hypocenter relocation result implies that the large events ruptured different faults of a complex fault system. A slip inversion analysis of the largest event indicates a large slip in the seismicity gap (Aso gap) in the caldera of Mt. Aso, which probably released accumulated stress and resulted in little aftershock production. We identified that the largest event dynamically triggered a mid-M6 event at Yufuin (80 km northeast of the epicenter), which is consistent with existence of the 20-km long zone where seismicity was activated and surface offset was observed. These findings will help us study the contribution of the identified complexity in fault geometries and the geotherm in the volcanic areas to the revealed seismic process and consequently improve our understanding of the seismo-volcano tectonics.
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