3:30 PM - 3:45 PM
[SCG50-07] Source fault models of the onshore Northern Honshu, Japan
Keywords:Seismogenic source fault, Northern Honshu, Overriding plate
Geometry of seismogenic source faults provides the essential information for calculation of strong ground motions and tsunamis. To evaluate the seismic risk in the overriding plate, stress accumulation rates on the source faults are recognized as an important factor. To construct source fault models in the overriding plate, we carried out deep seismic reflection profiling in the onshore northern Honshu since 1996. Together with geological, tectonic geomorphological, geophysical information, we constructed the rectangular models of source fault models in the onshore Northern Honshu.
Rectangular source fault models were developed using mainly active fault traces. For the earthquakes occurred in 20thcentury in Japan, only 40% of the events shows surface ruptures. Thus, we tried to pick up more source faults using seismic reflection sections, geological and geomorphological data. Along the onshore offshore boundary, possible source faults were estimated based on gravity anomaly and distribution of marine terraces. The bottom of the source faults was estimated based on D90 deduced by Hi-net seismic network.
Sesimogenic source faults in Northern Honshu are largely divided into two types; half graben type and aborted rift type.
Half graben type is most common. It occurs in the backarc stretched continental crust. As Northern Honshu received arc perpendicular extension during the Sea of Japan opening in Miocene and followed by arc perpendicular compression since Pliocene, reactivation of normal faults as reverse faults is common feature. Such fault reactivation is recognized by thicker sediment cover and negative Bougue anomaly in hanging wall. Deep seismic profiles and distribution of aftershocks suggest listric geometry of source faults. with rotation of domino blocks make the dip angle lower than that of original normal fault. Commonly the dip angles of this type of source faults show 40 to 50 degrees.
Miocene aborted rift zones are distributed long the Sea of Japan coast. It characterized by thick accumulation of post rift sediments, floored basaltic rocks and faster P-wave velocity in the lower crust. Fold-and-thrust belt well developed in the thick sediments. The boundary between mafic rich crust in failed rift and outside continental crust forms low angle geometry dipping to outside of rift axis. The boundary forms thrust by the subsequent compression. The rift boundary thrust reached into the rift basin fill and formed a large wedge thrust. The western boundary of Nagano basin fault is one of such examples. As suggested by the source fault of the 2007 Chuetsu-Oki earthquake, the dip angle of the rift boundary is 30 degrees.
Stress state in overriding plate is strongly controlled by the coupling of subduction megathrust. To evaluate the middle term seismic risk in overriding plate, we need the approach using physical model on subduction system. The physical model which explains the geodetic data can predict the stress state in the overriding plate, and it makes us possible to calculate the loading stress on source faults. Together with the data on physical property of faults, such as slip rate, which reflects the strength of faults, water contents by seismic tomography, and fault material, we can construct the physical model for the evaluation of middle term seismic risk.
Rectangular source fault models were developed using mainly active fault traces. For the earthquakes occurred in 20thcentury in Japan, only 40% of the events shows surface ruptures. Thus, we tried to pick up more source faults using seismic reflection sections, geological and geomorphological data. Along the onshore offshore boundary, possible source faults were estimated based on gravity anomaly and distribution of marine terraces. The bottom of the source faults was estimated based on D90 deduced by Hi-net seismic network.
Sesimogenic source faults in Northern Honshu are largely divided into two types; half graben type and aborted rift type.
Half graben type is most common. It occurs in the backarc stretched continental crust. As Northern Honshu received arc perpendicular extension during the Sea of Japan opening in Miocene and followed by arc perpendicular compression since Pliocene, reactivation of normal faults as reverse faults is common feature. Such fault reactivation is recognized by thicker sediment cover and negative Bougue anomaly in hanging wall. Deep seismic profiles and distribution of aftershocks suggest listric geometry of source faults. with rotation of domino blocks make the dip angle lower than that of original normal fault. Commonly the dip angles of this type of source faults show 40 to 50 degrees.
Miocene aborted rift zones are distributed long the Sea of Japan coast. It characterized by thick accumulation of post rift sediments, floored basaltic rocks and faster P-wave velocity in the lower crust. Fold-and-thrust belt well developed in the thick sediments. The boundary between mafic rich crust in failed rift and outside continental crust forms low angle geometry dipping to outside of rift axis. The boundary forms thrust by the subsequent compression. The rift boundary thrust reached into the rift basin fill and formed a large wedge thrust. The western boundary of Nagano basin fault is one of such examples. As suggested by the source fault of the 2007 Chuetsu-Oki earthquake, the dip angle of the rift boundary is 30 degrees.
Stress state in overriding plate is strongly controlled by the coupling of subduction megathrust. To evaluate the middle term seismic risk in overriding plate, we need the approach using physical model on subduction system. The physical model which explains the geodetic data can predict the stress state in the overriding plate, and it makes us possible to calculate the loading stress on source faults. Together with the data on physical property of faults, such as slip rate, which reflects the strength of faults, water contents by seismic tomography, and fault material, we can construct the physical model for the evaluation of middle term seismic risk.