Shin-ichi Kawakami

Systematic variations in C, O, and Sr isotopes and elemental concentrations in Neoproterozoic carbonates in Namibia: implications for a glacial to interglacial transition

Abstract We analyzed C, O, and Sr isotopic compositions of Neoproterozoic cap carbonates overlying a glacial deposit in Namibia to trace environmental changes following the glaciation. The three isotopic records showed coherent and phased changes that corresponded to the stratigraphic variation. A lower rhythmite unit had three intervals, which were distinguished by isotopic variations and mineral type. In the basal interval, δ 13 C increased exponentially from a negative value (−4.3‰, Pee Dee Belemnite [PDB]). δ 13 C values were relatively constant in the middle interval, and increased again to positive values in the top interval, indicating a phased removal of 13 C -depleted carbon from the surface ocean. The gradual increase to larger values (>6‰) in the upper stromatolite unit was interpreted to reflect a change in seawater composition resulting from biological activity after environmental recovery from the glacial episode. δ 18 O values and Sr isotopic ratios decreased in the basal interval and approached stable values in the middle interval. They increased in the upper interval and fluctuated in the stromatolite unit. On the basis of geochemical screening criteria for detecting diagenetic alteration of Sr isotopes, the middle interval of the rhythmite unit, with a minimum 87 Sr / 86 Sr ratio of 0.70685, was interpreted to have retained the primary value, or to have the least-altered Sr isotopic composition. The basal interval, which had ratios (∼0.718) at its base as high as those of old, sialic rocks, was interpreted as having altered Sr isotopic ratios. The high Sr isotopic ratios at the base of this interval gradually decreased to near the minimum value at the top of the interval, and other isotopic compositions and trace element concentrations showed systematic variations. Such systematic variations, except for the variation in Na concentration, can be explained by a progressive diagenetic fluid–rock interaction. Alternatively, they might reflect a compositional transition from a local, transport-limited water following the glaciation to global, well-mixed seawater.

The block structure and Quaternary strike-slip block rotation of central Japan

Central Japan is situated on the inflection point of the bow-shaped Japanese islands. Numerous NW-SE trending active faults, arranged in parallel at intervals of 20 to 80 km are found in this area. These active faults are more than 30 km long with shattered zones from 30 to 300 m wide. Several active faults constitute a given block boundary, which serves as the dividing line for one of the four blocks that make up central Japan. The block boundaries require careful study since numerous historical earth-quakes have occurred along these lines. Offset measurements of basement rocks, created during the Quaternary period due to left-lateral faulting, amount to 1 to 7 km. Gravity lineaments, which link points of sudden change and saddles of Bouguer anomalies, are clearly found along the block boundaries. The NW-SE trending active faults appearing on the ground surface are associated with motions of the block boundaries. Block rotational movement, caused by left-lateral faulting, plays an important role in the crustal deformation of central Japan. Rotational angles of the blocks calculated from the amount of displacement of basement rocks, initiated during the Quaternary period, are estimated to be 3° to 7° in a clockwise manner.

A nested fault system with block rotation caused by left-lateral faulting: the Neodani and Atera faults, central Japan

Abstract A detailed study of outcrops existing around the Neodani and Atera faults, which are large-scale, left-lateral faults oriented NW-SE in central Japan, revealed they were accompanied by two distinct fault systems of different scale and slip sense; intermediate-scale, parallel, right-lateral faults and small-scale, parallel, left-lateral, kink bands or faults. A nested fault model with different geometric scales is proposed to explain the hierarchical occurrence of the fault systems. In this model, the secondary fault system is contained within the primary fault system as a mirror image of it. Left-lateral movement of the Atera or Neodani fault induces right-lateral movement of intermediate-scale faults and results in counterclockwise rotation of the blocks between the faults. This right-lateral movement in turn induces left-lateral movement of small-scale faults or kink bands, which causes clockwise rotation of the small blocks between them. Since the angle of the counterclockwise rotation of the primary fault systems is the same as that of the clockwise rotation of the secondary fault system, these rotations cancel each other. While the large-scale fault movements cause block rotation, the small blocks eventually show no rotation.

Space—time correlations between inland earthquakes in central Japan and great offshore earthquakes along the Nankai trough: Implication for destructive earthquake prediction

Based on the tectonic framework of central Japan, including the surrounding submarine areas, the space-time relationship between destructive inland earthquakes of magnitudesM 6.4 or greater and great offshore earthquakes along the Nankai trough was examined. From east to west, four tectonic lines are defined as lines linking active faults: the Itoigawa-Shizuoka tectonic line (ISTL), the Tsurugawan-Isewan tectonic line (TITL), the Hanaore-Kongo fault line (HKFL), and the Arima-Takatsuki tectonic line (ATTL). The TITL divides central Japan into the Chubu and Kinki districts, and probably extends southward to the Nankai trough. The Chubu district is subdivided into four blocks by boundary lines linking NW-SE trending active faults having left-lateral strike slip. In the Kinki district, N-S trending, active reverse, steep-dip faults are dominant in the triangular region north of the Median Tectonic line, between the TITL and HKFL, forming a basin-and-range province. Starting from 1586 A.D., a seismic space-time sequence of high seismic activity in the Chubu district in which earthquake occurrence migrates from the eastern to western tectonic lines of central Japan was identified. The sequence also revealed that inland earthquakes preceded great offshore earthquakes which occurred along the Nankai trough. It was also found that a destructive earthquake tends to occur on the HKFL within 30 years after the occurrence on the TITL, and that the western Nankai trough generated great earthquakes ofM≥7.0 at intervals ranging from 8 to 49 years after the HKFL earthquakes. If the eastern Nankai trough is coupled with the western Nankai trough, a forthcoming greater earthquake measuringM 8.5 may be expected. Since such great earthquakes are always accompanied by large tsunamis, much attention should be focussed on possible tsunami disasters along the Pacific coast of central Japan. Based on its tectonic structure, a tectonic model of central Japan is proposed. The seismic space-time sequence, which attempts to explain the cause of the sequential earthquake generation, is also discussed.

Space-time distribution patterns of destructive earthquakes in the inner belt of central Japan: Activity intervals and locations of earthquakes

Abstract Based on a block structure model of the inner belt of central Japan, an examination was conducted of the space-time distribution patterns of destructiv magnitudes M 6.4 or greater (M =Japan Meteorological Agency Scale). The distribution patterns revealed a periodicity in earthquake activit seismic gaps. Major NW—SE trending left-lateral active faults divide the inner belt of central Japan into four blocks, 20–80 km wide. The occurrenc A.D. with M ≥ 6.4, which have caused significant damage, were documented in the inner belt of central Japan. The epicenters of these earthquakes close to the block boundaries. Using the relationship between the magnitude of earthquakes which occurred in the Japanese Islands and the active length of faults that generated them, movement is calculated for each historical earthquake. Space—time distributions of earthquakes were obtained from the calculated lengths, the latitud of generation. When an active period begins, a portion or segment of the block boundary creates an earthquake, which in turn appears on the ground surf active period ends when the block boundary generates earthquakes over the entire length of the block boundary without overlapping. Five seismic gaps with fault lengths of 20 km or longer can be found in the inner belt of central Japan. It is predicted that the gaps will generate ea magnitudes of 7.0. These data are of significance for estimating a regional earthquake risk over central Japan in the design of large earthquake resist The time sequences of earthquakes on the block boundaries reveal a similar tendency, with alternating active periods with seismic activity and quiet pe activity. The inner belt of central Japan is now in the last stage of an active period. The next active period is predicted to occur around 2500 A.D.

Space-time distribution patterns of destructive earthquakes in the inner belt of central Japan (part 2): moment-release rates and earthquake prediction

The inner belt of central Japan between the Itoigawa-Shizuoka tectonic line (ISTL) and Tsurugawan-Isewan tectonic line (TITL) can be divided into four blocks by boundaries linking major NW-SE trending left-lateral active faults. Historical earthquakes of magnitude 6.4 or greater, which caused significant damage, mainly occurred along the four block boundaries and have been recorded or documented since 715 A.D. The average rates of seismic moments released along the block boundaries in the inner belt of central Japan have been estimated. Since no destructive earthquakes were recorded or documented between 888 A.D. and 1585 A.D., this period is regarded as a time of seismic quiescence. The average rates (re*) of the seismic moments released by destructive earthquakes since 1586 A.D. (the present active period) were estimated from the amount of the released moment inferred from the magnitude of the earthquakes. Average rates (rs) of seismic moments are also calculated from the average rates of Quaternary displacements along the major active faults constituting the block boundaries. The average rates,re*, during the active period were found to be0.031 × 1026dyne·cm/yr for the ISTL. Those for the other three boundaries ranged from 0.087 to0.095 × 1026dyne·cm/yr. The average release rate (re) through the quiet and active periods since 888 A.D. ranged from 0.023 to0.031 × 1026dyne·/yr, with exception of0.008 × 1026dyne·cm/yr for the ISTL. These values were found to be in good agreement with the estimation ofs. This suggests that destructive earthquakes are a manifestation of the movement of the block boundary. Thus, it is important to assess the block boundary movements when designing large earthquake-resistant structures. Regions having no incidences of earthquakes (seismic gaps) during the present active period (since 1586 A.D.) have already been detected by us in previous work. Destructive earthquakes are predicted to occur in these seismic gaps. The dates of the future earthquakes are then estimated, assuming that the moment accumulated since the last earthquake exceeds the moment inferred from the possible magnitude in the region of the seismic gap. Future earthquakes are predicted to occur between 2007 and 2076 A.D.

Liquefaction and flowage at archaeological sites in the inner belt of central Japan: Tectonic and hazard implications

Abstract All archaeological data gathered at 55 archaeological sites excavated in the alluvial plains and basins of the inner belt of central Japan revealed the occurrence and age of events of liquefaction and flowage. The alluvial plains and basins are bounded on one or both sides by active faults, representing tectonic or block boundary lines. The excavated sites with liquefaction and flowage events identified, are found distributed within a 15-km-wide zone along the tectonic or block boundary lines of the alluvial plains and basins. This suggests that seismic hazards caused by liquefaction and flowage mainly occur within this 15-km-wide zones along the tectonic lines. Sand-filled dikes of various shapes were found and are thought to represent evidence of liquefaction and flowage. The sand-filled dikes can be classified into two main types; a lenticular type arrangeden echelon; and a cylindrical type opening upward in a fan shape. Archaeological age estimates indicate that the liquefaction and flowage were induced by earthquakes which occurred during three periods; around 1000 B.C., between 0 and 500 A.D., and after 1500 A.D., suggesting that active seismic periods occurred at an interval of approximately 1000 years.

Seismotectonics of the Median Tectonic Line in Southwest Japan: Implications for Coupling among Major Fault Systems

The relationship between the slip activity and occurrence of historical earthquakes along the Median Tectonic Line (MTL), together with that of the fault systems extending eastward has been examined. The MTL is divided into three segments, each containing diagnostic active faults. No historical earthquakes have been recorded along the central segment, although the segment has faster Quaternary slip rates compared with the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explained by a microplate model of southwest Japan. The microplate model also provides spatial and temporal coupling of slip on adjacent fault systems. In the context of this model, slip on adjacent faults reduces the normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong ground motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where historical earthquakes were recorded. Alternatively, the earthquakes at the eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that the MTL characteristically does not produce great earthquakes. The microplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough.

Space—time distribution patterns of destructive earthquakes in the inner belt of Central Japan (Part 3): Seismic hazard assessment

Abstract On the basis of the strike and sense of active faults, the inner belt of central Japan can be divided into the Chubu and Kinki districts. The Chubu district can be subdivided into four blocks by lines linking major NW—SE trending active faults. A number of inner belt earthquakes of magnitudes M 6.4 or greater ( M : Japan Meteorological Agency scale) have been documented or recorded since 599 A.D. Whereas most epicenters of these earthquakes were located along lines linking active faults, a few occurred in intra-block areas. In the first paper of this series, the analysis of space—time distribution patterns of destructive earthquakes revealed seismic cycles which consist of an active period of earthquakes and a quiet period with no activity. Seismic gaps were also detected in which no earthquakes have occurred during the present active period. The second paper demonstrated that the average rates of seismic-moment release calculated from the average slip rates of active faults, which constitute the major tectonic or block boundary lines, are nearly equal to those released by the historic earthquakes. On the basis of the results of the two former papers, two seismic maps, a hazard map and an earthquake potential map of the inner belt of central Japan are constructed. Since the seismic activity along tectonic and block boundary lines is much greater than that of the intra-block areas, the lines and intra-block areas are separately evaluated in these seismic maps. The hazard map shows the seismic activity by making use of the average rate of seismic-moment release. While the average rates of the lines range from 0.010to0.030 × 10 26 dyne·cm/y , those of the intra-blocks have values of less than 0.005 × 10 26 dyne·cm/y , with one exception. Since regions of higher-moment release rate experienced historic earthquakes with larger maximum magnitude, the moment-release rate is a good indicator for estimating the maximum magnitude of future earthquakes. During the present active period, future earthquakes will probably occur along seismic gaps. These seismic gaps are illustrated in the earthquake potential map. The two seismic maps are useful in reducing earthquake disasters by designing earthquake-resistant buildings and structures in the earthquake-prone regions of central Japan.

A new opportunity to detect paleo-earthquake events dating back to the past 10 millennia : a record from lacustrine sediment

Bulk density and grain density profiles of the uppermost 90 cm portion of lacustrine sediment from Lake Suigetsu, one of the five major lakes of Lake Mikatagoko, located in central Japan, indicate the occurrence of at least 10 depositional events characterized by anomalously high density spikes. Two events are accompanied by turbidites. The dates of the sedimentary events estimated here are correlated with the occurrence of destructive earthquakes since 1100 A.D. Close examination of the sedimentological signatures recorded in the lake sediment and historical literature describing destructive earthquake phenomena provides a new opportunity to detect paleo-earthquake events in a high seismic area having a long history.