4-1. Researches for Earthquake Prediction

 

Our recent researches have revealed the characteristics of the places that have potential for generating earthquakes.  In particular, progress in the studies on the interplate earthquakes is remarkable: earthquake data analyses, laboratory experiments and numerical simulations show that large earthquakes are caused by ruptures of asperities where two plates are strongly coupled in the interseismic periods.  Moreover, large episodic creep events (very slow events) have been found also.  It is expected that such a slow event will cause a stress concentration and accumulation on a nearby asperity and the asperity will eventually rupture to generate a large earthquake.  Thus, we believe that the ruptures of asperities (i.e., earthquakes) can be predicted to some extent by monitoring the slow events.  Before making prediction, however, we have to know the nature of the asperities and slow events in detail.  Intensive studies for this purpose are now going on.

 

4-1-1. Study of the relation between seismic process and physical properties of the subuction plate boundary in the forearc slope of the Japan trench

 

By a seismic experiment in 1996, we found intense PP reflections from the subduciton plate boundary at 10km below seafloor in the aseismic region existing in the forearc slope of the Japan trench(green ellipsoid in Fig. 1). To confirm this observation in whole aseismic region, we carried out another seismic experiment in 2001. As the result, we confirmed the previous findings. Composite record sections for Lines 3 and 4 (Fig. 2) show intense PP reflections from the plate boundary at 0-sec. Records from Line 3 to Line 7, we can conclude almost the same result as before. Seismic characteristics and variety of interpolate earthquake generations may be controlled by physical properties at the plate boundary.

Fig. 1. Epicenters with M>3 and depth<100km during 1985 and 1989. Green ellipsoid : aseismic region.

 

Fig.2. Composite move-out record section along Line 3 (N-S blue line in Fig. 1) and Line 4 of 20km west of Line 3.  Vertical axis: observed travel time –PP reflected travel time at plat boundary. 0-sec : plate boundary. Horizontal axis: location of refection.

 

4-1-2. Plate boundary at the Tokai region

 

The Philippine Sea plate is descending into the mantle beneath Japan with a velocity of several cm/year at the Tokai and Nankai trough region.  Large earthquakes with magnitudes of about 8 have repeatedly occurred along the Nankai trough where the Philippine Sea plate is descending beneath central Japan.  The Tokai region is one of the very important fields for understanding the mechanism of large interplate earthquakes. The geometry of the subducting plate is one of the very important parameters in the numerical simulation, but has not yet been determined well.

A joint seismic experiment was conducted in the Tokai and central Japan area in August 2001 (Fig. 3) with explosive sources by the Research Group for Seismic Expedition in Central Japan, which is organized by universities, JAMSTEC(Japan Marine Science and Technology Center), and other government organizations. A 261.6 km profile was extended in N-S direction to traverse island-arc Japan from south coast (Iwata, Shizuoka prefecture) to north coast (Hakui, Toyama prefecture).  We put 391 seismic stations along the survey line.  Six explosive sources were shot on the seismic survey line.  The objectives of the experiment are to know the large-scale structural variation of island-arc crust across central Japan and to know the configuration of the subducting Philippine Sea plate.

The most remarkable feature of the record sections is two clear later arrivals observed in the long distance range for shot J5.  J5 is the southernmost shot point. The later arrivals are explained by the reflected waves at the upper boundary of the subducting Philippine Sea plate by an analysis by the use of a ray tracing method. The configuration of the subducting Philippine Sea slab was revealed.  The reflection coefficient at the upper boundary of the Philippine Sea slab is expected to be large because the observed amplitude of the reflected waves are much larger than those of direct waves.  A detailed analysis with amplitude data is necessary to know the acoustic-impedance contrast at the boundary. The analysis of the amplitude and waveform data will reveal the physical properties at the upper boundary of the subducting Philippine Sea plate.

Fig.3.　Location of the seismic profile line and expected fault plane of Tokai earthquake.

 

4-1-3. Plate convergence and long-term deformation in central Japan

 

Surveys by continuous Global Positioning System in and around Japan revealed that the Amurian Plate collides with the North American Plate in central Japan by ~2 cm/yr.  Long-term crustal deformation seems to be influenced mainly by this collision although subduction of oceanic plates governs short-term elastic deformation over the arc.  Here we study the long-term deformation field by carefully removing the short-term signals inferred from a-priori plate convergence vectors and coupling strengths predicted by a thermal model.  The obtained field shows that the change in velocities occurs along the longitude 135°~137°E, and there exist a relatively rigid block and zones accommodating strains (Fig. 4).  Characteristic compressional deformation is found northwest of Izu due possibly to the collision of the Izu-Bonin arc with Honshu.  Plate convergence rate along the Nankai-Suruga trough is considerably smaller in eastern parts, due partly to the transition from the Amurian to the North American Plate of the landward side, and partly to the motion of the Izu microplate relative to the Philippine Sea Plate (Fig. 5).  This accounts for longer recurrence intervals of interplate earthquakes in the Suruga trough where the Tokai earthquake is anticipated to occur.

Fig.4.Departure from rigid plate motions at GPS stations. One of red, green and blue　colors was given to each GPS point to show which plate best explains its long-term velocity vector.  Vivid colors show that the points move little with　respect to that plate (numbers in the scale indicate velocities relative to that plate). Niigata-Kobe Tectonic Zone [Sagiya et al., 2000] is shown by a yellow broad line.

 

Fig. 5. Plate convergence rates at the centers of the fault segments along the Nankai-Suruga trough.

 

4-1-4. Distribution of asperities and seismic coupling

 

Waveform inversion has been carried out to derive the asperities of recurrent earthquakes off Sanriku, Japan, region. It is obtained that the 1968 Tokachi-oki event mainly consists of two asperities (large co-seismic slip areas), one of which is coincident with the asperity of the 1994 Sanriku-oki event (Fig. 6). It is also shown that the seismic coupling in this region is nearly 100%. In the southern Sanriku-oki region, there is no large earthquake, indicating a very small seismic coupling. In off-Miyagi region, on the other hand, a moderate seismic coupling is observed. It is also shown that episodic slip often occurs in surrounding area of the asperities.

Fig. 6. Distribution of asperities in Sanriku-oki region.

 

4-1-5. Numerical simulation of complicated slip behaviors on a plate boundary

 

  Recent studies of earthquake source processes and geodetic observations indicate that sliding behavior on a plate boundary is nonuniform. Seismic slip repeatedly occurs at asperities, significant aseismic slip follows some large earthquakes in the adjacent area, and episodic aseismic slip events occur at some regions. This suggests that frictional property on a plate boundary is nonuniform. In order to understand nonuniform and unsteady sliding behavior on a plate boundary, we conduct numerical simulation studies of seismic cycles using laboratory-derived rate- and state-dependent frictional laws.

  We consider a simple two-block model, in which Block 1 and Block 2 are connected by a liner spring and driven by a slowly moving driver. We assume the friction parameters so that Block 1 becomes unstable while Block 2 is stable. It is found that episodic slow slip occurs when the friction parameters of Block 2 are near the stability transition (Fig. 7). After the stress is reduced due to dynamic event, both blocks stick during a period. When the stress is accumulated to a curtain level, Block 2 starts slow slip. When the slip of Block 2 is approaching a steady state slip, decaying oscillation in the stress and the slip velocity occurs around the steady state values. The decaying oscillation approaching a steady state could be a plausible generation mechanism of the episodic slip which started in 2001 in the Tokai area.

  Figure 8 shows an example of simulation result assuming a more realistic continuum model, where two velocity-weakening friction patches are embedded on the plate boundary which is loaded by a constant plate velocity. In the figure, four snapshots of sliding velocity normalized by the plate velocity are shown with colors, where yellow shows slow slip with slip rate of about 1 cm/day and red shows seismic slip of about 1 m/s. An episodic aseismic slip event with a slip duration of about 10 days takes place at one of the patches. When the slow slip reaches the other patch, unstable (seismic) slip starts. These simulations may explain various complicated observed phenomena such as preslip, afterslip, episodic aseismic slip events (silent earthquakes), and delayed rupture. It will be possible to estimate the spatial distribution of frictional constitutive parameters by comparing the simulated slip histories with observed data. Our goal is to forecast slip events through numerical simulations with the estimated friction parameters.

Fig. 7. Simulation with a two-degree-of-freedom block-spring model. Episodic slow slip occurs when the friction parameters of Block 2 are near the stability transition.

 

Fig. 8. Snapshots of simulated sliding velocity normalized by the plate velocity.