Masataka Ando

Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, Japan

Abstract Two recent and three historical earthquakes which occurred along the Nankai trough, marking the northern plate boundary between the Philippine Sea and the Asian Plate, are studied mainly on the basis of the data of crustal deformations and tsunami waves. These earthquakes are the 1946 Nankaido, the 1944 Tonankai, the 1854 Ansei I, II and the 1707 Hoei earthquakes. They are all interpreted as low-angle thrust faults at the plate boundary, with the oceanic side underthrusting northwestward against southwestern Japan. The fault parameters of the historical earthquakes are assumed here to be common to those of the recent two earthquakes, except for the magnitude of dislocation. The entire fault region, which extends for 530 km from western Shikoku Island in the west to the Tokai district in the east, is divided into four fault planes, which are denoted the planes A, B, C and D, from west to east, respectively. Then, the five earthquakes may be attributed to the planes A, B, C and D, in the following manner: the Nankaido earthquake, A + B; the Tonankai earthquake, C; the Ansei II earthquake, A + B; the Ansei I earthquake, C + D; and the Hoei earthquake, A + B + C + D. The latest cycle of earthquake migration seems incomplete as proved by the recent inactivity in D. Consequently, the future major earthquake next to occur is expected there, off the Tokai district. Eight further ancient earthquakes from A.D. 684 to 1605 are also discussed. Taking the results of the foregoing studies into consideration, their sequence is well interpreted by the four migration cycles. Topographical data, tilt of coastal terraces and location of hinge lines, prove that the thrusting has continued all along the extension of the Nankai trough for at least 300,000 years.

A fault model of the 1946 Nankaido earthquake derived from tsunami data

Abstract A fault model of the 1946 Nankaido earthquake (M = 8.2) is determined by the use of tsunami records of Uwajima, Shimotsu and Hososhima which were located within or near the area of major coseismic crustal deformation. Synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters. A low-angle thrust model slightly revised from a previous model by Ando is consistent with the observed tsunamis. The duration of faulting is constrained as less than 10 min based upon the tsunami. The fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks, respectively. The fault area and dislocation for the western segment are 150 × 70 km2 and 6 m, and those for the eastern segment are 150 × 70 km2 and 3 m, respectively. The total seismic moment is 4.7 × 1028 dyn·cm, significantly smaller than that obtained from a geodetic model by Fitch and Scholz, but still larger than that of the seismic model by Kanamori. The discrepancy in seismic moment between the seismic and the present models (RAN2) could be interpreted in terms of a slow dislocation on the fault, but this interpretation does not match the seismic intensity distribution and damage pattern, and the slow-slip model for the Nankaido earthquake is rejected. The discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions. If the revised geodetic model (RAN2) is modified, the seismic moment required to explain the observed tsunamis would be reduced further by ∼30%. If we consider the uncertainties involved in the fault model of Kanamori and the fault-finiteness effect affecting the amplitude of seismic waves, the seismic moment required to interpret the seismic-wave data could be increased, possibly being more than twice that of Kanamori. Thus, the two seismic moments from the different data sets could be close to each other within allowable tolerance. This implies that the rise time of the Nankaido earthquake was short enough to generate short-period seismic waves from both the western and the eastern fault segments.

Possibility of a major earthquake in the tokai district, Japan and its pre-estimated seismotectonic effects

Abstract Crustal deformations, tsunamis and seismic intensity are pre-estimated for a large hypothetical earthquake, which it is feared may occur in the Tokai district along the Nankai trough. The long-term seismic quiescence since 1854, as well as the high rate of the present crustal movements in the district, form the principal evidence for the risk of the approaching catastrophe. The location and the mode of faulting in this earthquake are hypothesized in reference to the source mechanisms of the recent and historical earthquakes there. The fault parameters thus assumed are as follows: dip direction: N30°W; dip angle: 25°; fault dimension : 100 km × 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The following are the principal conclusions: (a) the eastern part of the epicentral region including the Point Omaezaki will rise up about 100 cm, whereas the western part covering Ise and Mikawa bays will subside about 10–30 cm; (b) the coast extending from Omaezaki to the Shima peninsula will receive tsunami waves as high as 3 m in maximum, which may be locally amplified by the factor 2 or more on the rias coast along the Shima peninsula; (c) the Tokai coastal region with thick alluvium layers may suffer seismic damages as severe as those experienced in the 1854 Ansei I earthquake.

Incidence of and risk factors for incisional hernia after abdominal surgery

Few larger studies have estimated the incidence of incisional hernia (IH) after abdominal surgery.

Induced earthquakes accompanying the water injection experiment at the Nojima fault zone, Japan: Seismicity and its migration

The 1995 Hyogo-ken Nanbu (Kobe) earthquake of M7.2 occurred on January 17, 1995. After the earthquake, a scientific drilling program called the Nojima Fault Zone Probe was carried out at the Nojima fault which ruptured during the mainshock. Water was injected during two periods, February 9–13 and March 16–25, 1997. The pumping pressure at the surface was about 4 MPa. Pressurized water was injected into a 1800-m-deep borehole and supplied to the surrounding rock at depths between 1480 and 1670 m. The total amount of injected water was 258 m3. An increase in earthquake activity was observed 4 or 5 days after the beginning of each water injection. The seismicity increased in the region around 3 or 4 km from the injection point. This suggests that these earthquakes are likely to have been induced by the water injections. The induced earthquakes were located between 2 and 4 km in depth and had magnitudes ranging from −2 to +1. The hypocenters of the induced earthquakes migrated with speeds of ∼2–40 m/h. The speeds decreased with time, suggesting a relationship with the diffusion of water. Values of intrinsic permeability of 10−14–10−15 m2 were obtained from the time dependency of induced seismicity change. The coefficient of friction in the area where the induced earthquakes occurred was estimated to be less than 0.3. Twenty earthquake clusters were found, and cross-spectrum analysis was applied to them. We could distinguish between the induced and noninduced earthquakes from the analysis. The induced earthquakes forming each cluster migrated with speeds of 20–80 m/h, which means microscale water migration or permeation. The present water injection experiment revealed that the Nojima fault zone was highly permeable and could slip with small (∼10% or less) increases in pore fluid pressure or shear stress.

Deficiency of Large Equivalent Width Lyα Emission in Luminous Lyman Break Galaxies at z ~ 5-6?*

We report a deficiency of luminous Lyman break galaxies (LBGs) with a large rest-frame equivalent width (EWrest) of Lyα emission at z ~ 5-6. Combining our spectroscopic sample of LBGs at z ~ 5 and those from the literature, we found that luminous LBGs at z ~ 5-6 generally show weak Lyα emissions, while faint LBGs show a wide range of Lyα EWrest and tend to have strong (EWrest 20 A) Lyα emissions; i.e., there is a deficiency of strong Lyα emission in luminous LBGs. There seems to be a threshold UV luminosity for the deficiency; it is M1400 = -21.5 to -21.0 mag, which is close to or somewhat brighter than the M* of the UV luminosity function at z ~ 5 and 6. Since the large EWrest of Lyα emission can be seen among the faint LBGs, the ratio of Lyα emitters to LBGs may change rather abruptly with the UV luminosity. If the weakness of Lyα emission is due to dust absorption, the deficiency suggests that luminous LBGs at z = 5-6 tend to be in dusty and more chemically evolved environments and that they begin star formation earlier than faint LBGs, although other causes cannot be ruled out.

S wave splitting in the aftershock region of the 1995 Hyogo-ken Nanbu earthquake

The 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake; M7.2) occurred on January 17, 1995. A temporary aftershock observation was carried out by the Geophysical Research Group Organized by Universities for Prediction Seismology in 1995 from October 1995 to January 1996. A dense seismic network was deployed in and around the aftershock region, and some seismic stations were set directly on the active fault traces. We analyze the S wave splitting in and around the aftershock region of the Hyogo-ken Nanbu earthquake. We detect a spatial variation of S wave splitting which can be related to crack orientations in an area where a large earthquake has occurred. At stations more than 1 km away from the Hyogo-ken Nanbu earthquake fault zone, the leading shear wave polarization directions (LSPD) are found to be parallel to the axis of the regional maximum horizontal compressional stress (E-W), suggesting that cracks caused by regional tectonic stress exist in the area. However, at stations within 500 m of the earthquake fault zones, the LSPDs are parallel to the fault strikes (N45°–50°E). This suggests that new fractures were produced parallel to the faults by shear faulting of the Hyogo-ken Nanbu earthquake and resulted in S wave splitting along the earthquake fault zone. Nevertheless, shear fault origin anisotropy was not evident at stations on the active faults which ruptured in 1596. This implies that healing processes have already closed the fractures produced by shear faulting in 1596. Therefore we can expect that S wave splitting can be a useful tool for monitoring healing processes of active faults.

Shear-wave splitting above small earthquakes in the Kinki district of Japan

Abstract We have identified shear-wave splitting, diagnostic of the effective anisotropy induced by aligned microcracks, in the wavetrains of micro-earthquakes at four stations of the Abuyama network in the Kinki District of Japan. We find that the directions of polarization of the faster split shear-waves are nearly parallel for all azimuths of arrival, and for all angles of incidence less than the critical angle at three of the four stations. These directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault-plane mechanisms, and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination. These results suggest that crack-induced anisotropy is present in the brittle upper crust beneath Japan, as has been found elsewhere, but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field, or results from the general trends of surface geology. Although the delay times between faster and slower shear waves are difficult to estimate reliably, because of their high sensitivity to internal interfaces, the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0.04. The consistency or lack of consistency of the directions of the shear-wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear-wave polarizations.

Lyman Break Galaxies at z ~ 5: Rest-Frame Ultraviolet Spectra*

We report initial results for spectroscopic observations of candidates of Lyman break galaxies (LBGs) at z ~ 5 in a region centered on the Hubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with IC ≤ 25.0 mag, including one active galactic nucleus, are confirmed to be at 4.5 < z < 5.2. The rest-frame UV spectra of seven LBGs commonly show no or weak Lyα emission lines (rest-frame equivalent width of 0-10 A) and relatively strong low-ionization interstellar metal absorption lines of Si II λ1260, O I+Si II λ1303, and C II λ1334 (mean rest-frame equivalent widths of them are -1.2 to -5.1 A). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at z ~ 3 but are quite similar to those of subgroups of LBGs at z ~ 3 with no or weak Lyα emission. The weakness of Lyα emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at z ~ 5 are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at z ~ 5 in our sample is larger than that at z ~ 3, we may witness some sign of evolution of LBGs from z ~ 5 to z ~ 3, although the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyα emission, because our spectroscopic sample is bright (brighter than L*) among LBGs at z ~ 5. More observations are required to establish spectroscopic nature of LBGs at z ~ 5.

Timing and displacement of holocene faulting on the median tectonic line in central shikoku, southwest japan

Abstract The Median Tectonic Line (MTL) extends about 900 km parallel to the general trend of the southwest Japan island arc. The central segment of the MTL has been active during the Quaternary. Although the average slip-rate along the MTL is up to several mm a-1, there is no historical record of destructive earthquakes along it. In order to collect data on the behavior of the MTL during late Holocene times, especially on the timing of seismic events and the amount of displacement during each event, trench studies were conducted on the active segment in central Shikoku in 1984 and 1988. The results are summarized as follows: (1) the latest two seismic events produced by slip on the MTL in central Shikoku occurred during the early 8th century A.D., and between 3250 and 2820 years B.P.; (2) the amount of displacement during the latest event is inferred to have been about 5.7 m; (3) the earthquake magnitude for the latest event is estimated at 7.9 (Richter scale), and a fault-segment trace about 70 km long ruptured during that event; (4) an average slip-rate along the active trace of the MTL in central Shikoku during the late Quaternary is estimated to be 5–8 mm a-1.