地理科学

If vertical deformation has not been accumulated, seismic reflection profiles are not indicative of active structure. Deformed landforms are indispensable for detecting recent crustal movement. We should understand tectonic landforms to make sure we have not missed any active fault out. Even though reflection seismic records show clear geological structures, we have to check them for contradiction to the geomorphologic characteristics. Wide discrepancies can lead the discussion into unreal issue. Both geophysical prospecting and geomorphic investigation are useful for studying active tectonics. However, prospected records do not always coincide with surface data. The latter takes precedence over the former, in clarifying the active tectonics. This is an important stance not only on studying geomorphic development but also on examining how to reduce seismic hazard. Tectonic landforms are simple but highly effective subjects of structural geology and paleo-seismology. Geomorphologist had better expand their research in these close fields of geo-sciences, which allows tectonic geomorphology to progress.

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We propose hypothetical methods to predict the rupture initiation point and the directivity of rupture propagation of future earthquakes associated with active fault systems based on the morphometric unit model. The morphometric unit model including the fault branching model is a geometric criteria such as branching features of active fault traces and dip-slip distribution along the fault traces related to the dynamic rupture processes. Following high demand on detailed information for active faults, we compiled completely new active faults map of Japan with a scale of 1:25,000 and digitized to GIS database. This digital fault trace map with the distribution of measured surface displacements may provide useful information in order to predict the strong ground motions.

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The timing of past faulting events from individual faults is the most critical parameter to understand the mechanism of recurring large earthquakes and the potential of seismic hazards. High-resolution and reliable radiocarbon dating is the most fundamental method to know the time-series. Among the various physical conditions, the common lack of the Latest Pleistocene (10 to 20 ka) sediments affects the completeness of the time series most deeply in Japan. There are a number of missing events in this period and reasonable estimation of recurrence interval is usually impossible. On the other hand, recent advance of computational refinement of radiocarbon dates enables objective determination of the timing of events. The Bayesian methods are applied to dendrocorrection and recalculation of probability density distribution of a series of faulting events as well as radiocarbon dates. The recalculated chronology, given the dates are not too much affected by rework and contamination, gives high-precision objective estimates of the timing of events. The utilization of new quantitative techniques together with better survey strategy against unfavorable physical conditions will lead us to better understanding of paleoseismic activities.

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The change of diatom flora showed that the Pacific coast of eastern Hokkaido rose at least 0.5m through the decades after a abnormally large tsunami in 17th century. The uplift produced an upward transition from tidal-flat mud to forest peat at an estuary previously after the invasion of the tsunami. Volcanic-ash layers show that the uplift began before 1667 and probably ended by 1694. Beneath the uplifted area, which extends at least 70km along the Kuril trench, oceanic crust subductys beneath the Kuril arc at depths of 60-80km. The simulation of crustal deformation suggests that shallow rupture seaward of the coarst along hundreds of kilometers of the Kuril trench probably generated the tsunami and the rupture triggered deep afterslip that raised the coast.

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