Hiroaki Tsukahara

EARTHQUAKE SWARMS AND DIKE INTRUSIONS OFF THE EAST COAST OF IZU PENINSULA,CENTRAL JAPAN

Abstract Relationships between earthquake swarms and dike intrusions are investigated in the monogenetic volcano region off the east coast of the Izu Peninsula, central Japan. Hypocentral distribution of an earthquake swarm associated with the 1989 Ito-oki submarine eruption and those of eight major earthquake swarms for 7 years before the eruption in the same region are precisely relocated. Comparison of the hypocentral distribution with a dike model obtained from crustal movement measurements by Okada and Yamamoto (1991) ensures that hypocentral areas of the earthquake swarms can be used as reliable indicators of intruded dikes. The epicentral area of each earthquake swarm shows an elliptic shape with a 3–5-km-long axis and a 2–4-km-short axis, the long axis trending from a NW-SE to WNW-ESE direction. The total hypocentral distribution of the nine swarms forms a seismic zone of about 20 km long and about 5 km wide, with each of the swarm areas overlapping. Focal migrations from deep (∼ 10 km) to shallow (∼ 3 km) depths are found in the two largest events in 1988 and 1989, indicating that dikes propagated from deep to shallow areas in the upper crust. These features of the hypocentral distributions suggest that the seismic swarms were induced by dike intrusions which ascended from a deep-seated magma reservoir with dike-like geometry located beneath the seismic zone. Solidification time of a dike with thickness comparable to that of the dike model of Okada and Yamamoto (1.45 m at the bottom of the dike) is several days, indicating that the dikes inducing the earthquake swarms solidified almost during the swarm period. The physical mechanisms of the dike-induced earthquake swarms are investigated for the Izu region. The focal depth migration and fault type of the earthquake swarm suggest that stress change in the region surrounding the dike due to dike emplacement is the most important factor for the cause of the earthquakes.

Hydraulic fracturing stress measurements and in-situ stress field in the Kanto — Tokai area, Japan

Abstract Hydraulic fracturing stress measurements have been made in fifteen boreholes of various depths from 100 to 800 m in the Kanto-Tokai area since 1978. About 90 sets of in-situ stresses have been obtained successfully. The maximum and minimum horizontal compressive stresses increase linearly with depth in each borehole. The difference between the maximum and minimum principal stresses also increases with depth. The increase of stress difference with depth is interpreted in terms of large stress relaxation in shallow parts of the boreholes where low confining pressure and many pre-existing microcracks are dominant. The maximum shear stress at a depth of 400 m ranges from 1 to 8 MPa depending on the site. It is not always the case that the regions of small shear stresses are inactive in microseismicity and crustal movement. This phenomenon is attributed to the relaxation of shear stress at the measured depths and to the regional variation in increasing rate of shear stress. The maximum compressive stress direction is obtained from detection of the fracture azimuth after hydraulic fracturing. Stress direction measured at each borehole agrees well with that estimated from geologic and seismic methods near the measurement site. Several stress provinces where the stress directions appear almost uniform are defined in the Kanto-Tokai area. The regional distribution of the stress directions is understood in terms of the relative movement of three plates, the Philippine Sea, Pacific, and Eurasian plates, which are in contact with each other in this area.

In-situ stress measurement in an earthquake focal area

Abstract A 2-km-deep borehole was drilled into granitic rock where many shallow earthquakes, with focal depths from 2 to 15 km, have occurred. The drill site, Ashio, is 100 km north of Tokyo. Downhole testing and measurements were conducted five times: four times after each 500 m drilling and the fifth time after completing the 2000 m borehole. Measurements of in-situ stress orientation and magnitude were conducted by the hydraulic fracturing method, stress-induced well bore breakout analysis, and drilling-mud pressure induced hydraulic fracturing analysis. Breakouts and mud pressure induced hydraulic fractures were observed below 650 m and 1250 m, respectively. The circular well bore is maintained only in limited spots below 650 m because of breakouts indicating a large differential stress condition between the maximum and the minimum principal stresses. The differential stress is calculated at 90 ± 20 MPa at the depth of 2000 m based on the condition under which the breakout with some degree of width appears. It is interpreted that this large differential stress is representative of the regional crustal stress condition in the earthquake swarm area. Each spot of the circular well bore is always adjacent to a fracture zone. This suggests that the fracture zone has small differential stress. The stress values were measured where the well bore is circular by the hydraulic fracturing method. For example, the maximum and the minimum horizontal compressive stresses are about 35 MPa and about 25 MPa, respectively, at the depth of 1650 m; giving the differential stress of 10 MPa. The water pressure in pre-existing fractures was also measured, and found that they were nearly equal to the hydrostatic water pressure at the corresponding depths. The stress direction estimated from the azimuth of the breakouts and the hydraulic fracture is consistent with that estimated from the earthquake focal mechanisms. These results support the following conclusions. The differential stress is large in the earthquake swarm region. But, it is extremely small at narrow zones adjoining fracture zones. This is due to the relaxation of the differential stress at fractures of low frictional strength. However, water pressure is not so large that the low differential stress causes sliding of the fractures. Clay minerals seems to play an important role in decreasing of the frictional strength.

Moho and Philippine Sea plate structure beneath central Honshu Island, Japan, from teleseismic receiver functions

We used teleseismic P waves recorded by the J-array, the Hi-net and a temporal local seismic network to investigate the three-dimensional topography of the Moho and the Philippine Sea plate beneath central Honshu Island, Japan. An image of the subsurface discontinuities beneath the region, derived from receiver function analysis, depicts the Philippine Sea plate dipping toward the north with complex local curvatures. The Moho is clearly detected in the northern part of the area studied, and its depth increases to the center of the island. Receiver functions from the stations adjacent to the Itoigawa-Shizuoka tectonic line indicate the step-like topography of the Moho directly beneath this tectonic line.

Some issues to be considered in establishing age models for the long Lake Baikal sediment records

Abstract The middle part of core BDP98 from Lake Baikal, which contains the oldest lake sediments ever obtained in the world from an existing lake, is structurally different from other parts of the core. Age models that disregard this difference are of little meaning. This part of the core must be examined and discussed in detail if we are to propose physically reasonable models for dating this long terrestrial record from Lake Baikal.

Hydraulic fracturing technique: Pore pressure effect and stress heterogeneity

Abstract Hydraulic fracturing stress measurements have been conducted in 19 boreholes at depths from 100 m to a maximum of 900 m. From detailed observation of the pressure-time record for each test, it is found that the increase of pore pressure during pressurization for hydraulic fracturing affects the pressure-time curve considerably. A new “curve fitting method” is proposed for identification of the shut-in point on the pressure-time record which is affected by the pore pressure. Concerning the stress variation with depth, stress concentrations and/or relaxation zones are found at some sites. Test procedures and field equipment are described. A dual impression packer system, which has been developed for its efficiency and overall low cost, is also described.

Deep-Borehole Techniques for Continuous Monitoring of Crustal Activity and In-Situ Stress Measurement in Japan

Deep borehole measurements have been carried out in two fields in Japan; the geothermal energy extraction project in the Tohoku (Sengan) geothermal area, and the earthquake prediction research in the Kanto-Tokai area (as “an area of specific observation” (e.g., Mogi 1981) for prediction of the next hazardous earthquake). I introduce deep borehole observatories for continuous observation of microearthquakes and ground tilt, and in-situ stress measurements in the Kanto-Tokai area.

Long-Term Late Cenozoic Global Environmental Changes Inferred from Lake Baikal Sediments

Long, high-resolution records preserved in Lake Baikal sediments (BDP-98) suggest that the climate of the Asian continental interior cooled gradually, although not steadily. Major climatolimnological fluctuations had long periods of about 2000 kyr, 1000 kyr, or 600 kyr, in addition to shorter 400-kyr and 100-kyr periods. These possibly correspond to Milankovitch parameters of eccentricity, implying that changes in solar insolation were printed as long-term environmental variations in the eastern Eurasian continental interior. The amplitude of these fluctuations began to increase at about 4.0 Ma, and most orbit-related periods have fluctuated more distinctly since that time. In particular, the 400-kyr filtered fluctuations in the Lake Baikal datasets correspond well with 400-kyr filtered insolation. The beginning of larger fluctuations seems to have been related to obliquity-related insolation minima.