Regional Setting

2.2.1 Geological background

The Taipei region can be divided into four geological domains (Fig. 2-2b): (1) the Foothills, hilly terrains north, east, and south of the Taipei Basin, consisting of fold-and-thrust belt of Miocene continental margin sedimentary packages; (2) the Tatun and Kuanyinshan volcanoes, north and northwest of the Taipei Basin, piled up of late Quaternary andesitic volcanic formations; (3) the Linkou Tableland, west of the Taipei Basin, covered by Quaternary thick lateritic conglomerates as an ancient fan-delta which rests above Miocene/Pliocene sedimentary rocks (Chen and Teng, 1990); (4) the Taipei Basin, a triangular-shaped half-graben filled with late-Quaternary fluvial deposits since about 0.4 Ma (Wei et al., 1998; Teng et al., 2001), which lie uncomformably over the deformed Miocene sedimentary packages.

The late-Quaternary deposits of the Taipei basin form an asymmetric wedge shape in thickness: reaching a maximum depth of about 700 m in the western margin and gradually becoming thinner toward the east and south (Fig. 2-2c). These unconsolidated deposits in the Taipei basin are divided into four lithostratigraphic units (Teng et al., 1999), from bottom to top (Fig. 2-3b): (1) the lowest Banchiao formation, consisting of fluvial sand, mud and conglomerates, with minor pyroclastic debris and thick varved mud in the upper section; (2) the Wuku formation, consisting of fluvial sand and conglomerates with minor mud and lateritic conglomerates; (3) the Jingmei formation, comprising of lateritic alluvial-fan conglomerates; (4) the Sungshan formation, composed of estuary interbedded sand-mud deposits. The basin deposits are marked by prominent facies changes and most of the sedimentary layers laterally pinch out rapidly. However, the widespread lateritic gravel of the Jingmei formation and the varved layers in upper Banchiao formation serve as basin-wide marker beds (Teng et al., 1999).

Brief geological evolution of the Taipei Basin is summarized in the following, based on previous studies. In Pliocene, the Taiwan Orogeny has initiated as the Luzon Arc of the Philippine Sea plate approaching to continental margin of Eurasia (e.g.

Suppe, 1981). The Miocene shallow marine sedimentary rocks of the Taipei area were deformed into imbricated fold and thrust sheets with several major faults including the Hsinchuang, Kanjiao, and Taipei Faults (Fig. 2-2b, c) as mountainous ranges (Ho, 1975; Wang-Lee et al., 1978). While the orogeny reached climax in northern Taiwan about 2 Ma, the Paleo-Tanshui River, the major river in the Taipei Basin, produced the

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Linkou fan-delta around the ancient mountain front thrust (Chen and Teng, 1990), the Hsinchuang Fault which runs approximately parallel to the western margin of the Taipei basin (Teng et al., 2001). The compressive stress regime of the northernmost Taiwan appeared to cease during the middle to late Quaternary (Lee and Wang, 1988).

The eruption of the Tatun volcanism to the north of the Taipei Basin might reflect the onset of regional extension which was interpreted to be related to the Okinawa trough back-arc opening and/or lateral extraction in the corner of plate convergence (Lee and Wang, 1988; Lu et al., 1995; Hu et al., 2002). Subsidence along western margin of the Taipei basin is interpreted to result from the repeated normal faulting on the Shanchiao Fault as tectonic inversion on the Hsinchuang thrust fault (Chiu, 1968;

Hsieh et al., 1992). It turned the Taipei area from rugged mountains into a sediment-receiving basin. The accumulation of fluvial and lacustrine sediments was estimated to be started at about 0.4 Ma (Wei et al., 1998; Teng et al., 2001). Since then the Taipei Basin has kept expanding due to continual asymmetric subsidence along the Shanchiao fault in the western edge of the basin (Wang-Lee et al., 1978). Under the combining influences of sea level fluctuations, volcanic activities, drainage system changes, and tectonic processes, the basin was filled with various types of sediments, including alluvial, lacustrine, marine and pyroclastic deposits, as mentioned above.

As a consequence, the Shanchiao Fault, which separates the Linkou Tableland from the Taipei Basin, is considered the major active structure responsible for accommodating the extension across the Taipei region and thus for the formation of the half-graben Taipei Basin (Teng et al., 2001). Shallow seismic reflections across the Shanchiao Fault imaged an offset of Holocene sediments at shallow tens of meters depth (Wang and Sun, 1999). GPS surveys of the Taipei area showed extension with a rate of 0.08 ± 0.02 µstrain/yr in the direction of SEE - NWW across the fault (Yu et al., 1999a). Huang et al. (2007) correlated stratigraphy of three sets of boreholes across the Shanchiao Fault, and identified three paleoseismic events within the Holocene time (8400-8600, 9000-9300, and 11100 years b.p.). Geomorphology analysis also exhibits series of scarps closely related to the development of the Shanchiao Fault (Chen et al., 2006). Thus the Shanchiao Fault is considered active (as stated in reports of Central Geological Survey, Chang et al., 1998 and Lin et al., 2000).

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Fig. 2-2. (a) Tectonic framework of Taiwan. The converging rate and direction of Philippine Sea Plate relative to Eurasia plate is adapted from Yu et al. (1999b) and Zang et al. (2002). (b) Four geological domains of Taipei area (see text for details).

The thin grey lines within the Taipei Basin are the basement depth contour of 100 m interval (after Teng et al., 2001). The red line along the western margin of the Taipei Basin is the Shanchiao Fault (after Chen et al., 2006). The black dots mark local district names in the Taipei Basin: 1. Guandu, 2. Wuku, 3. Shulin, 4.

Zhonghe, 5. Jingmei, 6. Sungshan, 7. Dazhi, 8. central Taipei, 9. Banchiao. (c) Simplified geological cross section of the Taipei Basin (modified from Teng et al., 1999). The Hsinchuang, Kanjiao, and Taipei Faults are inactive thrust faults (denoted with grey arrows) which slipped during the collision phase in the Taipei area.

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2.2.2 Hydrogeologic framework and utilization of groundwater

Four major aquifers, which generally correspond to the stratigraphic formations, have been defined in the late-Quaternary sediments of the Taipei Basin (Fig. 2-3b; Wu, 1987). The topmost free aquifer (Aquifer 0), which extends from ground surface to depth around 50 m (except in the western edge of the basin being deeper to about 120 m in maximum), is composed of interbedded clay, sand and pebbles. Aquifer 0 is thus considered to correspond to the Sungshan formation. The confined Aquifer 1 sits at depth between 50 to around 100 m (again except in the western edge of basin, about 115-140 m) in the conglomerate layer covered by laterite. This conglomerate layer is interpreted to correspond to the Jingmei formation, with hydraulic diffusivity around 0.12 to 0.18 m2/s and storage coefficient ranging from 0.001 to 0.004 (Chia et al., 1999). The lower aquifers (Aquifer 2, 3) are present approximately 100 to 130 m and 140 to 160 m underground (depths for the central portion of the basin), as two layers of conglomerates within the Wuku and Banchiao formations, respectively (Fig. 2-3b).

Among them Aquifer 1 appeared to be the major groundwater source of the pumping wells in the Taipei Basin. The groundwater of the lower three aquifers comes mainly from two of the major rivers in the Taipei basin, the Xindian River and the Dahan River (Fig. 2-3a). On the other hand, rainfall is the major water source for Aquifer 0 (Chia et al., 1999).

First massive utilization of groundwater source in the Taipei Basin (primarily Aquifer 1) was initiated by an English engineer W. K. Bardon in 1895. Under his advisory the government set up 150 wells (Wu, 1987). Since then the number of wells was rapidly increasing and the groundwater resource drained twice in 1906 and 1960 to early 1970s. The second event was accompanied by severe land subsidence.

Piezometric head of Aquifer 1 decreased more than 40 m and reached the lowest point around 1975 (Wu, 1987; Chia et al., 1999; Fig. 2-3c). The basin-wide subsidence resulted from over-pumping was particularly concentrated in the central portion of the Taipei Basin where maximum land subsidence exceeded 2 m (Fig. 2-3a), corresponding to the site of major pumping at that time. Severe restriction taken by the government on groundwater pumping in the early 1970s had successfully stopped the fast decline of groundwater table which immediately began to rise as the piezometric head having gradually recovered 30-40 m in 30 years (Chia et al., 1999;

Cho, 2006; Fig. 2-3c). Water resource of the Taipei metropolitan since the 1970s has been supplied by surface water mainly from two reservoirs in upstream Dahan and Xindian Rivers.

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Fig. 2-3. (a) Cumulated subsidence in the Taipei Basin from 1955 to 1991 (after Lin et al., 1999). Contour interval is 0.25 m. Borehole sites: A: Wuku (WK-1), B:

Sanchung (SC-1), C: Banchiao (PC-2), D: Shingongyuan (Shingongyuan No.1).

Groundwater monitoring wells: i: Wuku, ii: Sanchung, iii: Shingtien Palace, iv:

Sungshan. (b) Stratigraphic architecture of late Quaternary deposits within the basin, along with the four aquifers (modified from Teng et al., 1999). (c) Piezometric head records of Aquifer 1 at four monitoring wells since 1972 to 2003 (Chia et al., 1999; Cho, 2006).

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