Takahiko Uchide

A scaling law for slow earthquakes.

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.

Stress drop variations among small earthquakes before the 2011 Tohoku‐oki, Japan, earthquake and implications for the main shock

It is important to assess the likely rupture characteristics of future megathrust earthquakes. One approach is to study the spatiotemporal variation of geophysical properties in active subduction zones. We explore this idea by examining stress drops of 1563 small earthquakes (Mw 3.0–4.5) shallower than 80 km in the Tohoku-oki region before the 2011 Tohoku-oki earthquake. Although individual stress drop estimates exhibit considerable scatter, we find a strong increase in stress drop with depth between 30 km and 60 km, whereas stress drops for shallower and deeper events, respectively, are nearly constant. We also identify lateral variations in stress drop along strike. Higher-than-average stress drops are found in East Aomori-oki and Miyagi-oki, whereas Sanriku-oki is a moderate stress drop area. The high stress drop zone in Miyagi-oki is located just south of the large slip area of the 2011 Tohoku-oki earthquake, and possibly acted as a barrier to further rupture propagation during the event. In addition, the frequency dependence of the seismic radiation observed during the main shock, with proportionally higher frequencies coming from the deeper parts of the fault, mimics the depth dependence we see in small earthquakes in the same region. These results imply that smaller pre-main shock earthquakes can provide insight into the fault properties and consequent rupture processes of future megathrust earthquakes.

Determination of focal mechanisms of nonvolcanic tremor using S wave polarization data corrected for the effects of anisotropy

We propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy-corrected S polarizations measured directly from noisy tremor signals. The successful retrieval of polarization angles in noisy signals owes much to the observation that, in general, tremors propagate slowly and therefore do not change their location immediately. This feature of tremors enables us to use longer time windows to compute polarization angles (e.g., 1 min or longer), resulting in a stack of particle motions. We applied the method to a tremor episode that occurred beneath the Kii Peninsula in April 2013. Although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface, some events with strike-slip components were also found to have occurred. The proposed method provides us with a tool for investigating smaller-scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones.

Magnetar Broadband X-ray Spectra Correlated with Magnetic Fields: Suzaku Archive of SGRs and AXPs Combined with NuSTAR, Swift, and RXTE

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

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

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Non-self-similar source property for micro-foreshocks of the 2014 Mw 6.2 Northern Nagano, central Japan, earthquake

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Supershear rupture on multiple faults for the Mw 8.6 Off Northern Sumatra, Indonesia earthquake of April 11, 2012

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The 2016 Kumamoto–Oita earthquake sequence: aftershock seismicity gap and dynamic triggering in volcanic areas

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Development of multiscale slip inversion method and its application to the 2004 mid-Niigata prefecture earthquake

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Scaling of earthquake rupture growth in the Parkfield area: Self-similar growth and suppression by the finite seismogenic layer

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