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| index | Vol. 51 | 1 | 2/3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | Japanese Index | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Analyses of active faults in shallow marine and lake areas -case studies and arguments about what to do-
Kohsaku ARAI(Marine Geology Department, GSJ)
2000
vol. 51 (2/3) P.49-58
5 figs
Keywords: active fault, shallow marine, bay, lake, seismic survey
Abstract : Studies of active faults are reviewed regarding the geological setting of shallow marine and lake environments in which they occur. In order to be able to reveal each event on the faults, surveys with a time resolution at least one 100 years and a vertical resolution in the order of several tens of cm are required. High resolution seismic survey that uses several kHz sound source is useful for studies of active faults. Sedimentation rate and grain size of sediments have a strong bearing on studies of active faults. If the sedimentation rate is higher than the slip rate of an active fault, the datum plane of each event was formed within the sediment and they can provide a record of each activity. Finer grained sediments are advantages on for seismic survey and for sampling of sediments by coring. This implies that closed environments, such as bay and lake areas, are the best suited geological settings for analyses of active faults. On the other hand, open marine environments, such as shelf and shelf edge, that are characterized by coarse grain sediments and slow sedimentation, are less well suited and should be further studied in detail by new techniques.
Identification of offshore active faults on seismic profiles - application of fault related fold, growth strata and growth triangle -
Yukinobu OKAMURA(Marine Geology Department, GSJ)
2000
vol. 51 (2/3) P. 59-77
18 figs
Keywords: fault related fold, growth strata, growth triangle, offshore active fault, seismic profile
Abstract: Seismic profiles generally show fault-related folds and flexures rather than faults themselves. In order to infer fault geometry and active fault movements, studies of fault-related folding, growth strata and growth triangles were reviewed. Folding styles can be divided into hinge migration folding and fixed hinge folding. The former is characterized by an increase of the width of a limb while its dip and thickness remain constant. In contrast, the latter type of folding occurs by limb rotation. A hinge migration folding is widely recognized in low-angle thrust belts developed in sedimentary sequences. Based on the difference of relationship between folds and faults, fold styles are divided into fault-bend folding, fault-propagation folding and detachment folding. The strata deposited during growth of folds form a "growth triangle" that preserves the history of fold growth. In contrast, faults involving basement rocks accompany folds of which hinges are almost fixed, that are characterized by a downward increase of limb rotation and change in thickness. The trishear model is one of the models, which can explain characteristics of fixed hinge folding. Growth strata may show growth triangle like structure in the trishear model. In and around the Transverse Ranges in southern California, balanced cross sections including detachments at mid-crustal level have been constructed. The amount and rate of contraction have been estimated based on restoration of the sections. It is expected that collecting deep seismic profiles and drilling data will clarify the amount and rate of contraction in Japan.
Initiation processes of turbidity currents: implications for assessment of recurrence intervals of offshore earthquakes using turbidites
Takeshi NAKAJIMA(Marine Geology Department, GSJ)
2000
vol. 51 (2/3) P. 79-87
1 fig., 1 table
Keywords: turbidity current, initiation, trigger, turbidite, earthquake, recurrence interval
Abstract: nitiation processes of turbidity currents are reviewed with the aim to clarify validity of offshore seismic hazard assessment using turbidite occurrence. Turbidity currents are generated from either injection of concentrated-sediment suspension or mass failures on submarine slopes. The former processes include direct discharges from rivers, ignitive flows resulting from storm or tidal surges, density undercurrents derived from suspension layers on shelves and discharges of volcaniclastic sediments. Possible triggers of turbidity currents are not only earthquakes, but also floods, storms, tides, tsunamis, volcanic eruptions, oversteepening of slopes, overloading of sediments, generation of gasses, sea-level falls and so on. While most turbidity currents are generated on shelves, mass failures of submarine slopes triggered by earthquakes are the most possible process for generating turbidity currents in deeper settings. Therefore, it is concluded that recurrence intervals of earthquakes can not be estimated confidently on the basis of frequency of turbidites when this method is applied to coastal basins with narrow shelves. Turbidity currents originated from shelves may intrude into such basins. However, this method can be applied with much more confidence for deeper basins which are separated from shelves by structural highs (e.g. ridges). Further studies are required to clarify initiation mechanisms of turbidity currents and their resulting deposits.
Paleoseismicity analysis using earthquake-induced sediments
Ken IKEHARA(Marine Geology Department, GSJ)
2000
vol. 51 (2/3) P. 89-102
4 figs., 1 table
Keywords: paleoseismicity, radiocarbon dating, recurrence interval, earthquake-induced sediment
Abstract: Some of the large earthquakes are recorded in sedimentary sequences as event deposits and rapid change of depositional environments. Abrupt subsidence of coastal plains by earthquakes allowed the intrusion of sea water into coastal lowlands, and the deposition of tidal mud on the soil. Strong shaking of aerial and subaqueous slopes triggered their failure, that produced avalanched or gravity flow deposits just below the slope. In marine environments, turbidites and subaqueous debris flow deposits may be formed. The strong shaking of the ground also induces liquefaction/fluidation of sediments, as a result of which deformed beds and sand dykes formed. Tsunamis, associated with earthquakes, may deposit unusual coarse-grained strata. From the depositional ages of the event deposits, age and recurrence intervals of large earthquakes are inferred. For marine sediments, however, the difficulty to calbrate radiocarbon and calendar ages poses a problem to correlate evidence in marine sediments with the records in historical literature and archaeological sites with high accuracy.
Interaction between subducting seamounts and overriding forearc wedges, and its relation to large earthquakes: a review
Toshitsugu YAMAZAKI(Marine Geology Department, GSJ)
2000
vol. 51 (2/3) P. 103-111
5 figs
Keywords: Seamount subduction, asperity, earthquake, trench
Abstract: This paper presents a review of recent studies on the interaction between the overriding plates and topographic highs on subducting plates like seamounts, and its relation to large earthquake generation. Seamount subduction produces characteristic structures on forearc wedges. A topographic swell and new backthrusts occur landward of a subducting seamount. Behind the seamount, a topographic depression with a scarp is formed at first, and landward dipping normal faults are produced when the seamount subduction proceeds further. Recently, the possibility that topographic highs on subducting plates act as seismogenic asperities is calling attention of many researchers. It was proposed that large thrust-type earthquakes at Chilean-type margins are generated by rupture of jammed seamounts at the base of the overriding plates. Another model postulates that subduction of oceanic plateaus and seamount chains increases seismic coupling. Based on precise bathymetry and seismicity data in the Middle America convergent margin off Costa Rica, however, it was proposed that subduction of rough seafloor could cause low interplate coupling along relatively small asperities compared with the regions of flat seafloor covered with sediments which could have large and strong contact zones. A similar interpretation was presented for the relation between seismicity along the Northeast Japan margin and relief of the subducting Pacific plate. Swath mapping of margins around Japan are in progress, which will enable us a closer examination of the relationship.
