Regional Planning Report on Geological Hazard Management
for Southern Longitudinal Valley along Rye-long, Taitung
Tien-Huei Wang ¤ý¤Ñ¼z

Report Summary
This document is a geological survey of area along peinan river from rye-long community, kaotai, long-tein, across the luye river to peinan mountain.(See Fig.1) Based on observation in the field, former geological documentation, GPS data, and seismic events, we provide reliable locations of fault system and deformation history in recent years, and suggest their future activities. There are some recommendations for facilities and construction plans or possible modification to residents and government associated, in relation to hazard management. Hazards which we mentioned are landslide, surface deformation caused by earthquakes and by tectonic activities, and earthquake events. The analytical limit bound us to give only plausible land deformation details on condition of continuous tectonic activity, or possible earthquake events.

Fig.1 Report area: main structural raised area includes Pingding, Kaotai, Longtien, Painanshan, Lichi village, and Taitung. Two major thrust fault goes through southern longitudinal valley, one tends to west side forming kaotai area and folding uplift of peinanshan. The other, part of coastal range fault, is at east side of the valley. Red stars with id marked as occur time are earthquake events magnitude greater than 4.

Introduction

Previous understanding of geological tectonic motion and local lithologic unit help us to clarify cause and effect of some hazard. Taking precaution about possible future hazards, their locations and properties are thus applicable.

1. Tectonic setting of Taiwan, Taitung
Region along Rye-long, to Taitung in the longitudinal valley is under constant north-west trending pressure from the Philippine sea plate, which directed toward Eurasian plate. (See Fig.2) Under a relative velocity 82 mm/yr moving northwestward at 310¢X(Yu, Chen and Kuo, 1997), north of report area is accumulating suture force of thrusting and transforming to force parallel contact direction. At the same time, The report region accommodates compression rate increasing from north to south, which is theoretically logic since more compression force locked or transformed in the north, and southern part is catching-up with suturing. In these circumstances, coastal range is uplifting with approximately 13 mm/yr (controlled by terrace C14 dating) within 1000yr. Also, Long-range uplift vertical rate is around 20 mm/yr. These tectonic continuous motion offer great effort to form Kaotai and Peinanshan area, together with several flat stages of river terraces like Longtein, and river fan tilting as north of Kaotai.

Fig.2 (C. P. Chang, J. Angelier, C.Y. Huang, C. S. Liu, 2001)
Tectonic structure of plates under Taiwan: East of Taiwan is composed of Luzon volcanic arc collision with the Central Range, and Logitudinal Valley in between as material deposited from either Central Range or Coastal Range, deep marine mudstone, and fluvial peinanshan conglomerate.

2. Lithological Units
On Geological map (Fig.3) there are four basic lithical sets: Lichi m Álange( mudstone and mixture of exotic lithology blocks), peinanshan conglomerates(well cemented, sub angular, not well-sorted metamorphic pebbles), modern fluvial deposite conglomerates ( layered, not well cemented, clear with imbrication, slightly graded), and metamorphic slates of Central Range. Distribution of them integrate whole picture of this area. In relation to different properties of rock type, we might expect at edges of mountain range or scarps, slumping type would be different. Also, where Lichi m Álange locates attribute to easy erosion or river cut.

Fig.3 Geological map regional 0

3. Historical Seismic events
There were not much large seismic events during 1973-2003 occurring in northern part of survey region, besides Chengkung earthquake in December ,2003, which ruptured along a east-dipping, trending north 10 degree east plane (Yi-Min Wu, Y.G. Chen, J. C. Wu, AGU, 2004), mostly referred as Chihshang fault. Chihshang fault include only part of Coastal range fault, coseismic movement focused on north of Pingding, but there is some uplift at Kaotai and southern tip of Coastal Range for almost 5cm vertical and 3~5cm horizontal toward east to south east(GPS data by Liwei Chen, 2005). Through these latest events, we might earn a concept of recent activities that the fault system mainly migrate to thrust, plus right-lateral faulting of Coastal Range fault. With several substructures and partitions in northern and southern tips of the region, we would expect deformation increase at Pingding, and eastern edge along Peinanshan.
Read table.1 compare with Fig.1 to see historical events distribution.
date x y depth magnitude

Table.1 Green background mark three events rupture near surface, all of them are south of Peinanshan and not surely related to fault system.

Hazards Summary

Observation in the field, 40 , and 5-m DEM mapping suggests three main type of hazard existed and will possibly occur. Those are landslide, fault scarp deformation, earthquake events. We would discuss in the following sections:

1. Surface deformation:
Mainly deformations are due to blind-thrust or faulting, folding of land. Field investigations provide detail mapping of fault system and former surface deformation. In the report area there are two main thrust fault at west and east sides of Peinanshan and Kaotai, Coastal range. (Fig.4) During field work days we observed only at northwestern tip of Peinanshan the contact between slate and Peinanshan conglomerate. This contact shows no clear sense of shear motion and shear zone, comparing to eastern side the coastal range fault, which we can find large range of shear zone at one stream near chunye bridge, also the obvious contact and scarp in the Lichi m Álange at northeast tip of Peinanshan. This confirmed the idea that coastal range fault is more activated than western side in recent years. Considering horizontal GPS data (Fig.5) in 3 years, right side of eastern fault moves with a speed almost 1/3 times faster on suturing. If we consider elevation change, there is about 25 mm /yr uplift in 2004. With such a high speed each bridge across Peinan river suffered 2 to 3 cm uplift deformation causing extension on the bridge floor connection points and even left-lateral shear tear on the beam at Taitung Main bridge.(Fig.6) Likewise, fault contact between Peinanshan conglomerate and Lichi m Álange passing through Shanli forms a small scarp through the river terrace, and noteworthy, across the railway.

Fig.4
Several anticline at Kaotai area formed as substructures along with thrust fault. At the southern part there is a syncline extending to the middle of Peinanshan, forming high-angle bedding of strata at the east limb. These strata produced steep mountains at the northeast side. If the well-cemented conglomerates are washed-off and clastic supported material is left, area would exposed to landslide danger. Several anticlines appears at the west limb, and the western fault cease activity further down south, defined by the south side tilting terrace.

Fig.5 Gps data: reference point S105, across structure line severe changes of deformation rate appeared. Further down south, Peinanshan appears to be extruding southward combined with left-lateral component slipping, thus we see damage on the Taitung Main Bridge shows similar sense.

   
Fig.6-1 Double Yellow line bends leftward on Taitung Main Bridge.
Fig.6-2 Damage on the handrail shows clearly deformation trend.

2. Landslide
Landslide previously happened here is at central part of Peinanshan along a old drainage, mapped on Fig.1. The size is about 1 square kilometer dimension. In field investigation we observed some normal faulting at the rim of the old landslide edge, cutting through by river besides Ding-Yengwang. Edges besides the river are much steeper than those at south and west, therefore easier to slump.
Since the contribution factors for landslide at this region are:
„« erosion by rivers create oversteepened slopes
„« heavy rain, loading from typhoon or spring plum rain season.
„« Weak strata created by washed Peinanshan conglomerate or earthquake loosen Lichi formation.
„« Large earthquake of magnitude 4.0 and greater
We might expect either side of Peinan river to take caution for severe erosion on
Lichi m Álange and steep strata at eastern side of Peinanshan. Along Peinan river, fault sheared weak zone is very much erosed by the river. Even bridges¡¦ base are under a large cutting rate. To the east side, such as Lichi village formed as modern river terrace upon deformed fault scarp, Lichi formation at the right is still a weak zone possible to be cut-through. Other weak locations are steeply turned eastern limb of syncline strata along east of Peinanshan, but drainage developed at several sites are small and lack of erosion.

3. Seismic hazard
As a consequence, the 2003 Chengkung earthquake triggered upper part of the eastern fault , besides that no clustering or major fault-motion triggered events occurred as a consequence. It is consistent with the seismic history given in table.1.
Before Chengkung earthquake, most events with magnitude greater than 4 in recent 5 years appears at the south. But no obvious energy release on certain fault plane. So we can only be ware of the active coastal range fault at the east side for further motion. Possible earthquake magnitude is very close to 6, if we calculate earthquake moment by simplified fault plane geometry only for Chihshang Fault . Regarding fault plane geometry constructed by GPS coseismic deformation inversion (Liwei Chen, 2005), we simplified the fault as in figure 7:

Fig.7 simplified fault plane for Chihshang fault geometry.