Structural and chemical characterization of shear zones in the freshly activated Nojima fault, Awaji Island, southwest Japan

Behavior and role of each shear zone in a shallow fault zone of granitic origin during seismic cycles are revealed by comprehensive examinations of petrographic and chemical characterization on a fault zone in the Geological Survey of Japan (GSJ) drill core penetrating the Nojima fault which was activated during the 1995 Hyogoken-Nanbu earthquake (M = 7.2). The GSJ core consists of granodiorite and porphyritic intrusive rocks including a Nojima fault zone which involves seven thin shear zones: main shear zone (MSZ, 625 m depth), upper cataclasite zone (UCZ), upper shear zone (USZ), lower shear zones (LSZ-1 and LSZ-2), and lower cataclasite zones (LCZ-1 and LCZ-2). These shear zones are generally surrounded by weakly pulverized and altered (fault-related) rocks (WPAR) which generally show volume gain. The fault zone architecture is clarified as follows: (1) Total thickness of the Nojima fault zone is ∼70 m. (2) All shear zones except the older cataclasite zones (UCZ-1, LCZ-1, and LCZ-2) were evolved from WPAR, indicating that pulverization and alteration of recent activity were more diffused at the initial stage of faulting and gradually localized to each shear zone. (3) The MSZ (2 m thick) can be regarded as a high-velocity frictional zone with accompanying volume loss (compaction) and possibly with heat generation during coseismic periods. (4) The LSZ-1 (3.6 m thick), located just beneath the MSZ and typically showing explosion brecciation texture, is also regarded as a coseismic shear zone. This zone could function as a trap zone for fluid or gas during postseismic/interseismic periods. (5) The LSZ-2 (2.7 m thick), located around 710 m depth, contains foliated fault gouge enriched with clay minerals and characterized by a large degree of volume gain, possibly a result of slow velocity motion or creep during the interseismic and/or postseismic periods.