MANABU HASHIMOTO
Disaster Prevention Research Institute, Kyoto University Uji, Kyoto, 611, Japan

Tectonophysics, Vol.84, P.247-266, 1982.

It has been inferred from the focal mechanism of earthquakes and their hypocenter distribution (Shiono, 1977) that the stress field in southwestern Japan indicates complicated features; a NW-SE compression at shallow depths along the Nankai trough, an E-W or ESE-WNW compression in the inland crust, an extension parallel to the leading edge of the Philippine Sea plate at subcrustal depths in the region from the southern Chubu to northwestern Shikoku, and a down-dip tension beneath the Kyushu island. In order to investigate possible sources of these complex features of the stress state, a three-dimensional finite element method is employed to model the configuration of the subducting Philippine Sea plate. taking into consideration the following three possible types of forces:
(1) A negative buoyancy due to the density contrast between the subducting plate and the surrounding mantle.
(2) A northwestward compressive force generated by the movement of the Philippine Sea plate.
(3) A westward compressive force due to the movement of the Pacific plate.
For various combinations of different magnitudes of these forces, and of different elastic moduli, the stresses at a number of selected sites are calculated, and their directions are compared with those inferred from the focal mechanism and other geophysical information. It is found that the observed extensional stresses parallel to the leading edge of the subducting Philippine Sea plate may be caused mainly by the negative buoyancy. The northwestward compressive force may not play an important role in generating the complex stress field in southwestern Japan. The observed E-W compressional stress field prevailing in the inland region appears to result mainly from the westward-moving Pacific plate. The present results suggest that if a thin low-velocity transitional layer exists just above the subducting Philippine Sea plate, it could give appreciable effects on the mechanism of low-angled thrust faulting off the Kii peninsula and the Shikoku island. The magnitude of the shear stress in the continental crust and in the subducting plate is estimated to be of the order of several hundred bar, although the calculated shear stresses are considerably affected by the configuration of the subducting plate and also by the applied forces. It is interesting that the stress distribution appears to have some relations to seismicity of subcrustal earthquakes, and to the rupture process of large thrust earthquakes along the plate boundary.

MANABU HASHIMOTO and TAKASHI TADA
Geographical Survey Institute, Kitazato 1, Tsukuba, Ibaraki 305, Japan

Physics of Earth Planetery Interiors, Vol.60, P.324-338,1990.

We have investigated crustal deformations associated with the 1986 eruption of lzu-Oshima volcano, Japan, which was accompanied by an intensive fissure eruption. Two fissure crater chains, with NW-SE trend were created in the northem part of the caldera and on its northwestern flank. Their trend is consistent with the direction of compressive stress in this region. Depression of > 30 cm in the central zone including the caldera, and in the northwestern and southeastem parts in the island. was detected by precise leveling. On the other hand, uplifts up to 20 cm in the northeastern and southwestern parts were observed. Tide observations revealed that the Okada tide station, the leveling datum in lzu-Oshima, may have subsided by 5 cm after the eruption. An ~1 m opening of fissure craters was detected by distance measurements of the baselines which cross fissure craters. Horizontal displacements obtained by reoccupation of control points showed a symmetrical pattern which was consistent with the opening of fissure craters. Anomalous strain changes were also observed in the surrounding regions-contractions were observed in the Boso and the Miura peninsula, northeast of lzu-Oshima, and extensions in the lzu peninsula.
To interpret these crustal deformations, a model which consists of a nearly vertical tensile fault and a deflation source is presented. The tensile fault lies parallel to the fissures and is divided into two parts according to depth. The deeper part of the tensile fault is 12 km long, 10 km wide, and has 2 km burial depth and 2.7 m opening displacement. The shallower part, which may represent the fissure craters, is 4 km long, 2 km wide, and the amount of opening is estimated to be 1 m. However, the deflation source may be located at a depth of 10 km beneath the northwestem flank of the caldera and depression just above the source is estimated to be 30 cm. A deflation source is required to explain the subsidence at the Okada tide station and the extension in the lzu peninsula. This model suggests that the eruption might have released tensile stresses in and around the lzu region which result from bending of the subducting Philippine Sea plate.

Co-seismic Displacements of the 1995 Hyogo-ken Nanbu Earthquake

MANABU HASHIMOTO, TAKESHI SAGIYA, HIROMICHI TSUJI, YUKI HATANAKA, and TAKASHI TADA
Geographical Survey Institute, Tsukuba 305, Japan

Journal of Physics of the Earth, Vol.44, P.255-279, 1996

We present co-seismic displacements of the Hyogo-ken Nanbu earthquake of January 17. 1995, detected by continuous GPS (Global Positioning System) observation, campaign type GPS survey and leveling. Continuous GPS observation gives a consistent pattern of displacements with those expected from a right lateral slip on a NE-SW trending vertical fault in far field: stations about 50 km east and west of the epicenter moved toward the epicenter by about 4 cm, while stations north and south moved away from the epicenter. By comparing with line lengths obtained by geodolite about 10 years ago, the campaign type GPS revealed most control points on Awaji Island moved to the southwest or south, which may be attributed to the movement of the Nojima Fault which cut the surface. On the other hand, control points northwest of the Rokko fault system moved toward the northeast and those on the other side moved slightly to the west, in and around Kobe. Leveling data revealed uplift of 19 cm on the northwestern side of the Suma Fault, a member of the Rokko fault system, and subsidence of 7 cm just east of this fault. Furthermore, up]ift of about 5 cm was observed in the central part of Kobe City, and subsidence of 5 cm was detected east of Kobe. There is no significant gap in horizontal and vertical displacements around the northern extension of the Nojima Fault, which implies a complicated rupture process of this event. Leveling on the east coast of Awaji Island revealed a significant uplift of about 20 cm with slight subsidence at both edges of this uplift region during the past 20 years. By fitting the above geodetic data, we searched for an optimal set of parameters of a dislocation model. We assumed six nearly vertical faults trending NE-SW from Kobe to Awaji Island on the basis of aftershock distribution and focal mechanism. About 250cm of the right lateral slip for the fault on Awaji Island is derived from large horizontal displacements near the Nojima Fault. The fault in Kobe may be divided into two segments with 100-200 cm slip by a slip-free zone which roughly corresponds to the cluster of aftershocks. The southern part of the Nojima Fault, segments near the Akashi Strait. and north of central Kobe may have significantly large thrust components of 100 cm. We also examined the possibility of buried faults bcneath the zones of severe damage. Since this model cannot explain the observed geodetic data and the estimated slips are inconsistent with focal mechanism, these possible buried faults may not play a significant role, if any.

Plate Tectonics and Crustal Deformation Around the Japanese Islands

MANABU HASHIMOTO
Geographical Survey Institute, Tsukuba, Ibaraki, Japan
DAVID D. JACKSON
Department ofEarth and Space Sciences, University of California, Los Angeles

Journal of Geophysical Research, Vol.98, No.B9, P.16149-16166, September, 1993

We analyze over a century of geodetic data to study crustal deformation and plate motion around the Japanese Islands, using the block-fault model for crustal deformation developed by Matsu'ura et al. (1986). In this model, crustal deformation corresponds to the sum of relative movement among designated blocks and displacement due to dislocations along the locked parts of designated faults. We model the area including the Japanese Islands with 19 crustal blocks and 104 faults based on the distribution of active faults and seismicity. Three kinds of data are used; rates of change of angles and lengths, and average velocities of very long baseline interferometry stations. With the inversion of these geodetic data we obtain block motions and average slip rates of faults. This geodetic model predicts that the Pacific plate moves N69 +- 2W at about 80 +- 3 mm/yr relative to the Eurasian plate which is much lower than that predicted in geologic models. Substantial aseismic slip occurs on the subduction boundaries. The block containing the lzu Peninsula may be separated from the rigid part of the Philippine Sea plate. The faults on the coast of Japan Sea and the western part of the Median Tectonic Line have slip rates exceeding 4 mm/yr, while the Fossa Magna does not play an important role in the tectonics of the central Japan. The geodetic model requires the division of northeastern Japan, contrary to the hypothesis that northeastern Japan is a part of the North American plate. Owing to rapid convergence, the seismic risk in the Nankai trough may be larger than that of the Tokai gap.

Changes in Coulomb Failure Function due to the Occurrence of the M7.2 Kobe Earthquake of January 17, 1995, as a Possible Measure of the Change in Seismicity

MANABU HASHIMOTO
Crustal Dynamics Departent, Geographical survey Institute, Kitasato 1, Tsukuba, Ibaraki 305, Japan

"Earthquake Proof Design and Active Faults", ed. Y. Kanaori, Developments in Geotechnical Engineering, Vol.81, p.181-197, Elsevier Science B.V.,1997

We calculated changes in Coulomb failure function (CFF) for a geodetic fault model of the M7.2 Kobe Earthquake of January 1 7, 1995, to investigate the possible relationship between the occurrence of the main shock and the following increase in seismicity in the surrounding area. The modeled fault, which was derived from geodetic data, consists of six nearly vertical planes trending in the NE-SW direction with dominant right-lateral strike-slip and some thrust components. We calculate CFF changes for several focal mechanisms which are usually seen in the Kinki district or are consistent with the type of active faults there, and compare its distribution with the seismicity observed after the main shock. Calculated CFF for right-lateral slip on NE-SW trending vertical planes or left-lateral slip on NW-SE trending vertical planes, which are dominant focal mechanisms in this area, increases along the northeastern extension of the source region and around the Yamasaki fault, a conjugate fault located northwest of the source region. In the former region a drastic increase in number of earthquakes were observed after the main shock. The activation of seismicity was also observed near the Yamasaki fault. CFF decreases in the north of the source region, where the size of earthquakes became smaller than before the main shock. CFF may decrease a little in the Wakayama region, where no notable changes in seismicity were observed. The Kobe Earthquake may have loaded stresses which might lead to right-lateral slip on the eastern half of the Arirna-Takatsuki Tectonic Line and segments of the Median Tectonic Line in the Wakayama and Tokushima regions. Since these faults have no record of M7 or greater events during at least 400 years, we should be aware of the seismicity around these tectonic lines.

Simulation of Temporal Variation in Coulomb Failure Functions in the Source Region of the Hyogo-ken Nanbu Earthquake

Manabu HASHIMOTO
Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

Journal of Seismological Society of Japan, Vol.50, Supplement, P.229-249, 1998

We synthesize temporal variation in Coulomb Failure Functions (CFF) for the mechanism similar to the 1995 Hyogo-ken Nanbu earthquake taking both interseismic stress loading and coseismic changes due to large events into account. A block-fault model derived from the inversion of triangulation and trilateration data during 100 years by HASHIMOTO and JACKSON (1993) is adopted as the model for interseismic loading. Temporal changes in CFF is calculated as a sum of secular changes for back slip of block boundary faults in their model during arbitrary period since 1855 and coseismic changes due to large events which occurred during the corresponding period with simple pure elastic dislocation model. Among the several factors which affect CFF change in the source region of the 1995 Hyogo-ken Nanbu earthquake, increasing back slip of faults along the Nankai trough, which may represent strengthening of coupling, decreases CFF for right-lateral strike-slip mechanism on NE-SW trending vertical faults by 0.2 MPa/100yr at (34.6N, 135.04E and 10 km deep). Back slip along the Arima-Takatsuki Tectonic Line and Rokko fault system increase CFF most (1.6 MPa/100yr), though the calculated point is closely located to the edge of modeled block-boundary fault. These calculations seem contradictory to the hypothesis that strengthening of coupling between the subducting Philippine Sea plate and the Japanese islands induces the activity of large inland events. Loading of locked part due to stable sliding of deeper part of block boundary faults may play a main role for the accumulation of stress. Among large events, the 1946 Nankaido earthquake may have caused eminently large increase in CFF (0.1 MPa) near the epicenter of the Hyogo-ken Nanbu earthquake. The 1944 Tonankai earthquake may have increased CFF by about 0.01 MPa, but the 1927 Tango earthquake decreased it by 0.02 MPa. Other events may not have larger effect than 0.01 MPa. Summing up these contributions, our results suggest that the CFF for the Hyogo-ken Nanbu earthquake increased larger than 2 MPa since 1855.

Key words: Stress changes. Coulomb failure function, Hyogo-ken Nanbu earthquake, Back-Slip model, Dislocation model.

NORIHIKO ISHIKAWA
Geographical Survey Institute, Kitasato 1, Tsukuba, Ibaraki 305-0811, Japan
MANABU HASHIMOTO
Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho. Uji, Kyoto 611-0011, Japan

Journal of Seismological Society of Japan, Vol.52, P.299-315, 1999

This is a revision of Hashimoto's (1990) study on average horizontal crustal strain rates in Japan derived from geodetic data collected during the past 100 years. This new study includes Hokkaido since the completion of the latest nationwide trilateration enables us to apply the methods of the previous study and obtain strain rates there. In this method, side lengths of triangulation networks are calculated using adjusted coordinates for different epochs and then the rate of change of each side length is estimated by regression. Finally the principal strain rate in each triangular region is obtained. In order to avoid the effects of large earthquakes or volcanic eruptions, data before or after these events are simply discarded. Unfortunately, due to frequent occurrence of large events in northern Japan, we can use data from only two epochs for most of Hokkaido, which causes large uncertainties in the estimated strain rates. Hokkaido appears to be divided into four provinces according to the characteristics of the horizontal strain rates, although the uncertainties are too large to be definitive. Eastern Hokkaido is characterized by large WNW-ESE contraction. In northern Hokkaido E-W contraction is prevailing, while extension is dominant in southwestern Hokkaido. In other regions the directions of principal axes of strain rates are not noticeably different from the of the previous study. In the Tohoku region, tensile strain rates in a N-S direction are prevailing. A NNW-SSE or NW-SE trending contraction is dominant along the coast from southern Kanto to Shikoku, though it is smaller in the Tokai area and Kii peninsula than in southern Kanto and Shikoku In the same Shikoku region, there is a remarkable region of NE-SW extension that is as large as contraction. In central Japan, there is a NE-SW trending region of contraction. An E-W oriented contraction is dominant west or northwest of this region, such as in the Kinki and eastern Chugoku districts, while a NW-SE trending contraction is prevailing in the southeast. Kyushu is under an extensional regime with tensional directions of N-S and NW-SE in central and southern Kyushu, respectively. Magnitude of strain rates obtained in this study ranges from 1 to 3 x 10^-7/yr in most regions except in the Shikoku, southern Kanto and Fukui areas where it reaches 6 x 10^-7/yr. Over all, magnitude of strain rates are as large as or slightly larger than those derived from continuous GPS observations, but are larger than geological or seismological strain rates by a factor of 10.

Key words: Geodetic survey, Triangulation, Trilateration, Horizontal strain rates, Interseismic deformation.