「地震及活斷層研究」－地震地質─車籠埔斷層沿線同震單斜褶皺構造幾何形態與運動機制之初步研究
Other Title
Preliminary geometric and kinematic analysis of monoclinal
structures formed during the 1999 Chichi earthquake in central
Taiwan
structures formed during the 1999 Chichi earthquake in central
Taiwan
Date Issued
2003
Date
2003
Author(s)
楊昭男
DOI
912119M002025
Abstract
From observations in the field and photo published, it is revealed that only a
few of the escarps, formed during the 921 Chichi earthquake in 1999, is truly fault
escarp which only occurs at the river bed without or with very thin sediments, and
most of the escarps are monoclinal fold, because the dipping surface of the escarps
is still the original ground surface other than newly-produced fault surface. This fact
suggests that most part of the fault that caused the earthquake is still a blind fault
and the monoclines are fault-propagation folds. From the variation of the escarp
height along the Chelongpu shear zone, it is deduced that the zone can be divided
into several doubly-plunging folds that produce culmination and salient structure along the shear zone. In trench excavations cross the monoclines, it is shown that the layering of
recent sediments such as clay, sand and conglomerate under the ground surface have
been severely deformed into folds and/or faults. Although the detailed structures are
complex, but in general, the monoclinal folds have some characteristic feature in
common. The overall monoclinal fold is tightening downward, and broadens upward
toward ground. The forelimb becomes steeping and thickens near the synclinal hinge,
thinning between the anticlinal and synclinal hinge. In the case where the forelimb
of monoclinal fold was truncated by thrust fault, the displacements (fault separation)
along the fault are decreasing toward the ground surface. All such characteristics
strongly suggest that the monoclinal fold is resulted from trishear fault-propagation
folding (Erslev, 1991) during fault event(s). It is also noted that in a monocline, the
higher the escarp height, the steepness of the forelimb. This also indicates that the
monocline is a plunging fold, plunging from the portion where the height of the
escarp is high toward the portion where the height of the scarp is low. It seems that the formation of individual doubly plunging monocline may due
to the lateral propagation of the tip-line of the fault. The saddle structure may result
from the linkage of two separate propagating faults in the soft sediments underneath
ground surface. The factors that controlling the formation of the regional
culmination and salient structure along the Chelongpu shear zone may include the
variation along strike of the fault in magnitude of slip, dip of the fault plane, epical
angle of the triangular shear, the ratio between the rate of tip line of propagating
fault and the amount of slip on the fault (i.e. P/S ratio) as suggested by Cristallini &
Allmendinger (2001). In the present case, it seems that the change of slip and the dip of the fault plane along strike maybe more important than others. This imply that the
higher uplift height in the Choulan and Shihkang area the dip angle of the fault is
steeper than that in the area from Shihkang to Chushan where uplift height is lower.
Publisher
臺北市：國立臺灣大學地質科學系暨研究所
Type
report
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