Microseismicity Induced by Heavy Rainfall Around Flooded Vertical Ore Veins

—?Microseismicity (M?M?M?M??6 were also monitored at a crustal movement monitoring station located several hundred meters from the veins. It was found that the opening of the vertical ore veins primary led to significant strain and tilt, but not to seismicity, because the delay and the longer duration of the seismicity were significant. Most seismic events involve thrusting mechanisms that are consistent with the present stress state of E-W-oriented tectonic compression, but are not consistent with the opening of the deepest ore vein. Interstingly, all the events within a few months of the heavy rainfall occurred near the faults that offset the deepest ore veins, wheareas all those events located away from the deepest ore veins occurred many months after the heavy rainfall. Consequently, the delayed diffusion of water appears to have played a dominant role in reducing rock strength, which led to seismicity in the Ikuno mine.     

Frequency-dependent Attenuation of High-frequency P and S Waves in the Upper Crust in Western Nagano, Japan

—Borehole seismograms from local earthquakes in the aftershock region of the 1984 western Nagano Prefecture, Japan earthquake were analyzed to measure the frequency-dependent characteristics of P- and S-wave attenuation in the upper crust. The records from a three-component velocity seismometer at the depth of 145m exhibit high S/N-ratio in a wide frequency range up to 100 Hz. Extended coda normalization methods were applied to bandpass-filtered seismograms of frequencies from 25 to 102 Hz. For the attenuation of high-frequency P and S waves, our measurements show Q _P ^-1? 0.052?^-0.66 and Q _S ^-1? 0.0034?^-0.12 respectively. The frequency dependence of the quality factor of S waves is very weak as compared with that of P waves. The ratio of Q _P ^-1/Q _S ^-1 is larger than unity in the entire analyzed frequency range.     

Water-weakened lower crust and its role in the concentrated deformation in the Japanese Islands

The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata-Kobe tectonic zone) was investigated by carefully analyzing the GPS data and qualitatively modeling the lower crust in NKTZ. It was concluded that this deformation zone is not a plate boundary between the Amurian plate (AMU) and the North America plate but is rather an internal deformation zone near the eastern margin of AMU. The data previously obtained on the conductivity anomalies in the lower crust and the ³He/⁴He ratios suggest that the concentrated deformation in NKTZ results from the lower crust in NKTZ being weakened by a high water content. The high water content is thought to result from the dehydration of subducting slabs. NKTZ has a higher water content in the lower crust than other regions do because there is no Philippine Sea plate (PHS) seismic slab beneath NKTZ. In other regions, it is estimated that the mantle wedge above the seismic Philippine Sea slab prevents the water dehydrated from the slab from rising to the lower crust, and that the lithosphere within PHS itself prevents the water dehydrated from the Pacific plate from rising up through it.     

由精确震源机制推断的日本海岸地震带的局部应力集中:板内地震断层应力积累过程的意义

在日本西南的板内地区发现了最大主应力轴（σ1）方位角的空间变化。根据大量精确震源机制进行应力反演推测,沿日本海岸地震带的口。轴方位角取向为N110°E～N130°E,而在周边地区几乎为东西方向,即N90°E～N100°E。沿南海海沟内陆板块的地壳浅部广泛观测到东西方向的最大水平压应力。然而,在该地震带上仅观测到WNW-ESE方向的最大水平应力。用该地震带之下下地壳中的无震断层或延性断层带的形变解释了该地震带内及其周围应力场的这种空间变化。     

Terrestrial heat flow at Hirabayashi on Awaji Island, south-west Japan

Abstract Terrestrial heat flow at Hirabayashi in Awaji Island, south-west Japan, was investigated using the deep borehole penetrating through the Nojima Fault, which was activated during the 1995 Hyogo-ken Nanbu earthquake, by measuring the thermal conductivity of basement rocks. Using the temperature logging data, the value of terrestrial heat flow in Hirabayashi was found to be 56.6 ± 5.2 mW/m². The relationship between cut-off depth of aftershocks of the Hyogo-ken Nanbu earthquake in Hirabayashi and terrestrial heat flow are discussed. The cut-off depth roughly corresponds to isotherms of 300°C.     

Large earthquakes initiate by the acceleration of slips on the downward extensions of seismogenic faults

In order to examine the relationship between anomalous crustal deformations and subsequent large earthquakes, we compiled and examined data on preseismic anomalous crustal deformations for two earthquakes that occurred in and around the Japanese Islands in recent years. We found that the slip on a downward extension of a seismogenic fault accelerates before the main shock and that the average slip velocity is inversely proportional to the time remaining until the main shock occurs.     

In situ stress measurements in NIED boreholes in and around the fault zone near the 1995 Hyogo-ken Nanbu earthquake, Japan

Abstract The 1995 Hyogo-ken Nanbu (Kobe) earthquake, M 7.2, occurred along the north-east–south-west trending Rokko–Awaji Fault system. Three boreholes of 1001 m, 1313 m and 1838 m deep were drilled in the vicinity of the epicenter of the earthquake. Each borehole is located at characteristic sites in relation to active faults and the aftershock distribution. In particular, the Nojima–Hirabayashi borehole [Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drilling] in Awaji Island was drilled to a depth of 1838 m, approximately 320 m southeast from the surface rupture of the Nojima Fault, and it crosses fracture zones below a depth of 1140 m. In situ stress measurements by the hydraulic fracturing method were conducted in these boreholes within 1.5 years after the earthquake. Measurement results suggest the following: (i) Differential stress values are very small, approximately 10 MPa at a depth of 1000 m at each site; (ii) the orientation of maximum horizontal compression is almost the same in the boreholes, perpendicular to the surface trace of the faults, north-west–south-east; (iii) fault types estimated from the state of stress differ among these sites; and (iv) the differential stress value just beneath the fault fracture zone decreases abruptly to one-half of that above the fault zone in the Hirabayashi NIED drilling. These features support the idea that the shear stress along the Rokko–Awaji Fault system decreased to a low level just after the earthquake.     

Is the plastic flow uniformly distributed below the seismogenic region?

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10⁻¹⁵ s⁻¹. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10⁻¹³ s⁻¹, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region.