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Togo T.,Japan National Research Institute for Earth Science and Disaster Prevention | Shimamoto T.,China Earthquake Administration
Journal of Structural Geology | Year: 2012

Brunauer-Emmett-Teller (BET) surface area of quartz gouge was measured before and after shearing deformation at subseismic to seismic slip rates to determine how much of frictional work and fracture energy was consumed during grain crushing. Measurement of BET surface area provides more direct data on surface-area change than grain-size measurement does. Twenty-four experiments on quartz gouge composed of Ottawa sands or crushed quartz were conducted dry (with room humidity) or wet (with added water) at slip rates of 0.006∼1.3 m/s and normal stresses of 0.76∼3.37 MPa, using a high-velocity friction apparatus and a low to high-velocity friction apparatus. About 1 mm-thick gouge was placed between cylindrical specimens of Belfast dolerite of 25 mm in diameter. Results show that grain crushing absorbed only 0.02∼0.22% of frictional work and only 0.05∼1.06% of fracture energy under dry and wet conditions. Thus, grain comminution is unlikely to be an important energy sink at least for mature faults with well-developed slip zone. Surface area of gouge decreases with deformation at seismic slip rates. This and SEM observations suggest partial sintering due to frictional heating. Thus a simple scenario of grain crushing as an energy sink does not hold for estimating the energy budget during earthquakes. © 2012 Elsevier Ltd.

Iizuka S.,Japan National Research Institute for Earth Science and Disaster Prevention
Journal of Geophysical Research: Atmospheres | Year: 2010

In the present study, the winter precipitation in the vicinity of Japan is simulated by the Weather Research Forecasting model by using two sets of sea surface temperature (SST) data with different spatial resolutions. On comparing the simulated mean precipitations, we found that SST resolution has a significant influence on the simulated precipitation along the northwestern coast of Japan; in this region, the coarse-resolution SST data have a systematic cold bias. In the simulation using high-resolution SST data, the moisture supply to the atmosphere increases over the relatively warm coastal SST. The increase in the moisture supply leads to an increase in the moisture convergence near the mountain ranges in Japan on the Japan Sea side, leading to an increase in precipitation amount. The result suggests that coastal SST must be carefully used for dynamic downscaling of the climate simulation, in particular, in Japan, which is surrounded by boundary currents. We also found that a small-scale SST anomaly in the Kuroshio-Oyashio Extension (KOE) region near Japan enhances the interannual variance of local precipitation in the regions downwind of the SST anomaly. The associated anomalous ascent extends to the midtroposphere and is accompanied by an increase in cloud ice, suggesting that the interannual SST variation over the KOE region may affect the free atmosphere. Moisture budget analysis indicates the influence of moisture advection by mean wind on the spatial phase difference between the SST and precipitation anomalies. Copyright 2010 by the American Geophysical Union.

Obara K.,Japan National Research Institute for Earth Science and Disaster Prevention | Obara K.,University of Tokyo
Journal of Geophysical Research: Solid Earth | Year: 2010

The phenomenology of deep slow earthquakes, including low-frequency tremors, very low frequency earthquakes, and short-term slow slip events, on the subducting plate interface in southwest Japan is investigated on the basis of their spatiotemporal characteristics. The belt-like distribution of tremors is divided into segments bounded by gaps. The repetition of tremor episode depends on the magnitude of the episode, which is defined by the number of detected tremors within each episode. Major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months, accompanying the short-term slow slip events in large segments. In small segments, tremor episodes with small numbers of tremors have recurrence intervals of a few months. Some of them occasionally accompany the slight ground tilting caused by a small-size slow slip event. Even in the same segment, there exist different recurrence intervals according to the magnitude of the tremor episode. The migration pattern varies greatly in initiation, termination, direction, and speed; however, the migration direction has a general tendency for each segment. On both sides of the gap, the tremor starts next to the gap and propagates to the other side. Assuming all tremor episodes are caused by slow slip events, the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode. The equivalent slip history estimated from all tremor episodes is almost constant in each segment. This indicates that tremor activity is a good proxy for slow slips. Copyright 2010 by the American Geophysical Union.

Tanaka S.,Japan National Research Institute for Earth Science and Disaster Prevention
Geophysical Research Letters | Year: 2010

I observed tidal triggering of earthquakes precursory to the three giant earthquakes occurring off Sumatra on 26 December 2004 (Mw 9.0), 28 March 2005 (Mw 8.6), and 12 September 2007 (Mw 8.5). I measured the correlation between the Earth tide and earthquake occurrence in and around the focal regions of these megathrust earthquakes. The result of statistical analysis indicates that a high correlation appeared for several to ten years preceding the occurrence of the large earthquakes. The correlation vanished after the main events. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak near the angle where the tidal shear stress is at its maximum to accelerate the fault slip. This implies that the high correlation observed in the pre-seismic stage is not a stochastic chance but is likely a physical consequence of the tidal stress change. © Copyright 2010 by the American Geophysical Union.

Saito T.,Japan National Research Institute for Earth Science and Disaster Prevention
Geophysical Journal International | Year: 2010

This study formulates Love-wave excitation in terms of the interaction between a propagating ocean wave and the sea-bottom topography. By assuming a Fraunhofer diffraction range, or far-field approximation, I theoretically derive an equivalent point force for the Love-wave excitation. The equivalent point force acts in the same direction as the propagation direction of the ocean wave. The excited Love wave has a radiation pattern characterized by sin θ, where θ is the angle between the propagation directions of the Love and ocean waves. The efficiency of the excitation is then investigated by employing both deterministic and stochastic models for sea-bottom topography. When a seamount given by a Gaussian function is used as a deterministic model, the equivalent point force has a narrow peak against the wavenumber of the ocean wave; a strong interaction occurs at λ= 2.2. d, where λ is the ocean-wave wavelength and d is the characteristic scale of the seamount. On the other hand, when randomly fluctuating sea-bottom topography characterized by a power-law spectrum is used, the interaction can occur over a wide range of the ocean wave wavelength. © 2010 The Author Journal compilation © 2010 RAS.

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