田中 秀実 氏 （地球惑星科学専攻）

2005年4月13日

Title
An extremely thin slip zone activated by 1995 Kobe earthquake
detected at 1140 m depth in the drilled core penetrating the Nojima
fault, and its relation to fault zone architecture

Abstract
An extremely thin slip surface (7 mm) activated by 1995
Kobe-earthquake (M=7.2) is detected at 1140 m depth by comprehensive
examinations of petrological, geophysical, and geochemical
characterizations on a fault zone in the National Research Institute
for Earth Science and Disaster Prevention (NIED) drill core
penetrating the Nojima fault. The NIED core consists mainly of
granodiorite including a Nojima fault zone (Ca. 100 m in thickness).
Three evolution stages of the fault zone were distinguished (Stage I
to III). The Nojima fault zone initiated as thin slip layers of
pseudotachylyte (10 cm in thickness) at 10 to 15 km in the crust
(Stage I). Fundamental framework of the fault zone was established at
stage II (6 to 9 km depth) in which thick fracture zone was generated
associated typically with zeolite precipitation. Shear zones formed
at Stage III (1 to 4 km depth) are recognized by a combination of
thin (0.1 to 3 m thick) ultracataclasite layer(s) at the core of
shear zone characterized by precipitation of iron oxyhydroxide and
mafic carbonates, coupled with thicker (1 to 3 m thick) weakly
pulverized and altered rocks (WPAR) characterized by carbonate
precipitation in the hanging wall. A thin ultracataclasite layer (7
mm thick) overprinted main shear zone of stage III (MSZ; 1141 m
depth, 12.1 m thick), which is activated during 1995 Kobe earthquake
as evidenced by conspicuous geological and geophysical anomalies.
Dynamic slip occurred under low resolved shear stress because neither
microscopic evidence of melting nor positive temperature anomaly by
geophysical logging are observed in this layer, nevertheless
co-seismic strain in the layer is estimated to be around 320.
Geochemical examination clarifies that volume-loss associated with
frictional slip did not occur in the slip zone at the depth of 1 km
by dynamic slips of 1995 Kobe-earthquake, but occurred at stage II,
associated with zeolite precipitation, which are overprinted by those
associated with mass-increase and carbonate precipitation occurred in
younger stages at shallower depths.