The surface trace of Luyeh fault is not so clear. It always shows a
antiform scarp when the fault cutting through the terraces. This study
we trench the fault scarp and drilling boreholes near the trench site in
Yungan alluvial fan. (figure.1) In the trench profile there are 11
stratigraphic units defined.(figure.2,3) We only observed 2 very tiny
shear plane in the sandy layers, F1 shear plane offset the O2 unit about
6.5-14 cm and the F2 offset it about 3-4.5 cm. Stratigraphic units in
the trench profile display a monoclino folding. The trishear folding
growth strata may present onlapped structure, wedge deposits, and
downward-steepening structure in the profile. We define 3
paleoearthquake events based on the characteristics. These 3 events are
dated to occur in 4240-4430 yr BP (N1), 2120-2340 yr BP (N2), and <
1890-2110 yr BP (N3). And we convinced the event cycle is about 2100
yrs, so the N3 event might have occurred in recent time.
estimating of restoration of the onlapping strata, we get the vertical
deformation of 3 events: 2.24/2.47 m (N wall / S wall) of N1 event,
1.36/1.02 m of N2 event, and 3.07/2.92 of N3 event. The total vertical
deformation is 6.67 m and 6.41 respectively. Long-term vertical
deformation rate estimate from the time after N1 event is about 1.5
mm/yr. And the long-term net slip rate estimated by the shear plane dip
angle 20 º is about 4.4 mm/yr. But if we use the offset 8 m of the top
of G1 unit, the estimation of long-term uplift rate is 1.85 mm/yr and
the long-term net slip rate is 5.4 mm/yr.
borehole no.1 there is a strongly sheared thick mud of lacustrine
environment from 0-77.1 m underground.(figure.4) There are 2 dating
points of 13060-12940 yr BP in 10.2m, and 18510-17690 yr BP in 69.6 m.
Beneath the mud stara there is a 46 m thick alluvial fan gravel
deposits. The sedimentary environment change may cause by the activity
of the Luyeh fault, and we estimate the timing of the dammed lake by
extrapolating the age to the bottom of the mud strata, it is about 18870
yr BP. If the top surface of the gravel bed in the Mabei BH-1 can be
correlated to the Terrace 12 or 14, the vertical offset is 92 m and 116
m respectively.(figure.4) The long-term uplift rate of Luyeh fault could
be 4.9 mm/yr or 6.1 mm/yr.
Figure.1 The loacation of the Yungan trench and core-boring.
Figure.2 North wall profile of Yungan trench.
Figre.3 South wall profile of Yungan trench.
Figure.4 Geological profile of Luyeh area.