High-angle normal faults in the oceanic plate entering the Japan-Kuril Trench subduction zones

Abstract:

Activation of seismicity in the outer trench region following large shallow megathrust earthquakes has been reported (e.g., the 1960 Chile, 1965 Aleutians). In some of those cases, a large normal fault earthquake (M > 8) occurred in the outer trench. The 2007 Kuril Trench earthquake (Mw=8.1) is an example of an instrumentally recorded large normal fault earthquake following a shallow megathrust earthquake. The 1933 Showa-Sanriku earthquake (Mw=8.6) is also considered a trench outer-slope normal fault event following the 1896 Meiji-Sanriku shallow megathrust earthquake. It has been proposed, as a general idea, that a formation process of the normal fault system is attributed to bending the subducting plate and that the normal fault earthquake occurs when extensional stress from the subducting plate is transmitted to the outer trench region. However, since the source region of the outer trench-slope earthquake is far outside the observation network and a shallow megathrust earthquake that activates outer trench-slope earthquakes is not frequently occurred, the geometry, distribution and formation processes of the fault system in the outer trench-slope has long been debated. To address the unanswered questions above, we have conducted bathymetry surveys, seismic surveys and earthquake monitoring in the outer trench-slope region of the Japan Trench, and started acquiring additional data in the southern part of the Kuril trench in the last year. In the Japan Trench region, the seismicity is mapped along graben in horst-and-graben formed by bend related normal faults, except for areas where a young igneous activity is observed. The oceanic Moho is imaged along most of the seismic reflection profiles, including newly processed PSTM sections. The lineation of seismicity seems to be aligned along areas where breaks of the lateral continuation of Moho is observed. We concluded, by comparing the seismicity and the seismic reflection images, that the normal faults activated since the 2011 Tohoku-oki earthquake extend to the upper mantle with a high dip angle of as high as 75^o, at least down to several km below Moho. To consider our observations and a focal mechanism determined by earthquake and tsunami data of the 1933 earthquake, we propose a model showing that the fault's dip angle becomes shallow toward the deep end of the fault.