Japan National Research Institute for Earth Science and Disaster Prevention

Tsukuba, Japan

Japan National Research Institute for Earth Science and Disaster Prevention

Tsukuba, Japan
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Hasegawa A.,Tohoku University | Yoshida K.,Japan National Research Institute for Earth Science and Disaster Prevention
Geoscience Letters | Year: 2015

The 2011 Tohoku-Oki earthquake ruptured a large area of the megathrust east of NE Japan. The earthquake’s magnitude was 9.0, substantially larger than predicted. It is important to know what occurred in the source area prior to this great megathrust earthquake to improve understanding of the nucleation processes of large earthquakes and risk assessments in subduction zones. Seafloor observation data revealed the existence of two extremely large slip patches: one just updip of the mainshock hypocenter and the other 80–100 km to the north near the trench axis. For 70–90 years before 2003, M > 6 events and slips of M > c. 7 events on the megathrust occurred in the areas surrounding these two large slip patches. Seismic activity had increased since at least 2003 in the downdip portion of the source area of the Tohoku-Oki earthquake. In addition, long-term accelerated slow slip occurred in this downdip portion of the source area in the decades before the Tohoku-Oki earthquake. About 1 month before the earthquake, a slow slip event (SSE) took place at relatively shallow depths between the two large slip patches, accompanied by foreshock activity. Both the slow slip and foreshocks propagated from north to south toward the southern large slip patch. Two days before the earthquake, an M 7.3 foreshock and an associated postseismic slip began at relatively deep depths in the megathrust between the two large slip patches. In addition, a slow slip type event seems to have occurred approximately half a day after the M 7.3 foreshock near the mainshock hypocenter. This slow slip event and the foreshock activity again propagated from north to south toward the mainshock hypocenter. These long- and short-term preceding seismic and aseismic slip gradually reduced the interplate coupling, increased shear stresses at the two large slip patches (i.e., two strong asperity patches), and finally led to the rupture of the great Tohoku-Oki earthquake. © 2015, Hasegawa and Yoshida; licensee Springer.

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.

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.

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.

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.

Tanaka S.,Japan National Research Institute for Earth Science and Disaster Prevention
Geophysical Research Letters | Year: 2012

I observe highly significant tidal triggering of earthquakes prior to the 2011 M w 9.1 Tohoku-Oki earthquake. Strong statistical correlations between tidally-induced stresses and earthquake occurrence times are identified in the northern part of the Tohoku-Oki source region, where the mainshock rupture initiated, in the several to ten years before the Tohoku-Oki earthquake. The tidal phase distribution of earthquakes in this period exhibits a peak where the shear stress is at its maximum to promote failure. On the other hand, no significant tidal correlation is found after the Tohoku-Oki mainshock. These observations suggest that tidal triggering occurs over a decade-long period preceding the Tohoku-Oki earthquake, and the initial rupture site of this event is already critically stressed in this precursory stage. Copyright 2012 by the American Geophysical Union.

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.

Saito T.,Japan National Research Institute for Earth Science and Disaster Prevention
Earth, Planets and Space | Year: 2013

Recording ocean-bottom pressures in offshore regions has increased our understanding of tsunami sources and promoted the development of rapid source estimation for early tsunami warning. Analytical solutions for the water height at the surface have often played an important role as a reference model in these efforts. To understand the system better and to develop the techniques further, not only the water height but the velocity in the sea and the pressure at the sea bottom are important. The present study obtains a solution for the velocity potential for sea-bottom deformation with an arbitrary source time function, and derives analytical solutions for the velocity distributions in the sea and the pressure at the bottom. These enable us to visualize the tsunami generation process, including the velocity field. The solutions can also give a theoretical background for setting the initial conditions of height and velocity in 2-D tsunami propagation simulations. Furthermore, the solution of the ocean-bottom pressure indicates that when the sea-bottom uplifts at an increasing rate, the sea-bottom pressure increases through a dynamic effect. This dynamic contribution will not be negligible as we develop more rapid and precise source estimation techniques using ocean-bottom pressure gauges within the source region. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

Fukuyama E.,Japan National Research Institute for Earth Science and Disaster Prevention
Earth, Planets and Space | Year: 2015

There have been reports of conjugate faults that have ruptured during earthquakes. However, it is still unclear whether or not these conjugate faults ruptured coseismically during earthquakes. In this paper, we investigated near-fault ground tilt motions observed at the IWTH25 station during the 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9). Since near-fault tilt motion is very sensitive to the fault geometry on which the slip occurs during an earthquake, these data make it possible to distinguish between the main fault rupture and a rupture on the conjugate fault. We examined several fault models that have already been proposed and confirmed that only the models with a conjugated fault could explain the tilt data observed at IWTH25. The results support the existence of simultaneous conjugate faulting during the main rupture. This will contribute to the understanding of earthquake rupture dynamics because the conjugate rupture releases the same shear strain as that released on the main fault, and thus it has been considered quite difficult for both ruptures to accelerate simultaneously. © 2015 Fukuyama; licensee Springer.

Okada Y.,Japan National Research Institute for Earth Science and Disaster Prevention
Journal of Physics: Conference Series | Year: 2013

Before the occurrence of disastrous Kobe earthquake in 1995, the number of high sensitivity seismograph stations operated in Japan was nearly 550 and was concentrated in the Kanto and Tokai districts, central Japan. In the wake of the Kobe earthquake, Japanese government has newly established the Headquarters for Earthquake Research Promotion and started the reconstruction of seismic networks to evenly cover the whole Japan. The basic network is composed of three seismographs, i.e. high sensitivity seismograph (Hi-net), broadband seismograph (F-net), and strong motion seismograph (K-NET). A large majority of Hi-net stations are also equipped with a pair of strong motion sensors at the bottom of borehole and the ground surface (KiK-net). A plenty of high quality data obtained from these networks are circulated at once and is producing several new seismological findings as well as providing the basis for the Earthquake Early Warning system. In March 11, 2011, "Off the Pacific coast of Tohoku Earthquake" was generated with magnitude 9.0, which records the largest in the history of seismic observation in Japan. The greatest disaster on record was brought by huge tsunami with nearly 20 thousand killed or missing people. We are again noticed that seismic observation system is quite poor in the oceanic region compared to the richness of it in the inland region. In 2012, NIED has started the construction of ocean bottom seismic and tsunami observation network along the Japan Trench. It is planned to layout 154 stations with an average spacing of 30km, each of which is equipped with an accelerometer for seismic observation and a water pressure gauge for tsunami observation. We are expecting that more rapid and accurate warning of earthquake and tsunami becomes possible by this observing network.

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