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

Citation
H. Tanaka et al., Structural and chemical characterization of shear zones in the freshly activated Nojima fault, Awaji Island, southwest Japan, J GEO R-SOL, 106(B5), 2001, pp. 8789-8810
Citations number
47
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
ISSN journal
2169-9313 → ACNP
Volume
106
Issue
B5
Year of publication
2001
Pages
8789 - 8810
Database
ISI
SICI code
0148-0227(20010510)106:B5<8789:SACCOS>2.0.ZU;2-M
Abstract
Behavior and role of each shear zone in a shallow fault zone of granitic or igin during seismic cycles are revealed by comprehensive examinations of pe trographic and chemical characterization on a fault zone in the Geological Survey of Japan (GSJ) drill core penetrating the Nojima fault which was act ivated during the 1995 Hyogoken-Nanbu earthquake (M = 7.2). The GSJ core co nsists of granodiorite and porphyritic intrusive rocks including a Nojima f ault 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). Th ese shear zones are generally surrounded by weakly pulverized and altered ( fault-related) rocks (WPAR) which generally show volume gain. The fault zon e architecture is clarified as follows: (1) Total thickness of the Nojima f ault zone is similar to 70 m. (2) All shear zones except the older cataclas ite 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) T he MSZ (2 m thick) can be regarded as a high-velocity frictional zone with accompanying volume loss (compaction) and possibly with heat generation dur ing coseismic periods. (4) The LSZ-1 (3.6 m thick), located just beneath th e MSZ and typically showing explosion brecciation texture, is also regarded as a coseismic shear zone. This zone could function as a trap zone for flu id or gas during postseismic/interseismic periods. (5) The LSZ-2 (2.7 m thi ck), located around 710 m depth, contains foliated fault gouge enriched wit h clay minerals and characterized by a large degree of volume gain, possibl y a result of slow velocity motion or creep during the interseismic and/or postseismic periods.