A possible factor controlling fault behavior of reverse fault zone along the island arc - Heterogeneity of crustal structure in the Ou backbone range, NE Japan -

Abstract:

We report crustal structure in the Ou backbone range, NE Japan by a seismic tomography analysis using the campaigned micro-earthquake observation data and discuss a role of the heterogeneity to control active fault behavior. In the present, a lot of N-S striking active faults dominated by dip-slip are found along the backbone range in the E-W compressive stress perpendicular to the arc. Some large (~M7) crustal earthquakes of reverse faulting has occurred under the backbone range in the last century. On the other hand, it is known that the NE Japan arc used to be pulled away from the Asian continent by back arc rifting in the middle Miocene. Consequently many normal faults along the arc and some strike slip faults oblique to the arc had been initially formed in this stage. Some of the ancient normal faults are seemed to reactivate as reverse faults in the present E-W compression.

We carried out a dense arrayed micro-earthquake observation with 30 temporary stations in the Ou backbone range during 2011-2012. A seismic tomography analysis using the acquired data with the surrounding seismic network clearly shows heterogeneity of the velocity structure. The results indicate 1) a remarkable heterogeneity (discontinuity) in NW-SE direction along the strike slip faults oblique to the arc, 2) an apparent change of the distribution and the activity of the present reverse faults across the discontinuity, 3) a good correspondence between the end of the source area of the large earthquakes in this century and the discontinuity. We discuss a possibility that the present fault behavior is controlled by the heterogeneous structure formed by the ancient faults.