Tectonophysics 578 (2012) 126–135 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Assessments of serious anthropogenic land subsidence in Yunlin County of central Taiwan from 1996 to 1999 by Persistent Scatterers InSAR Hsin Tung, Jyr-Ching Hu ⁎ Department of Geosciences, National Taiwan University, Taipei, Taiwan article info abstract Article history: Anthropogenic ground subsidence due to massive pumping of groundwater is one of the severe environmen- Received 30 November 2010 tal hazards in Taiwan. The Yunlin County located in the southwestern coastal region of Taiwan is one of the Received in revised form 9 August 2012 most counties with serious land subsidence because of the agricultural needs. In certain areas of the region, Accepted 9 August 2012 the subsidence rate reaches as much as 14.3 cm/yr. The severe land subsidence gives rise to the risk of flood Available online 18 August 2012 hazard and damage of infrastructures in this area. We represented a Persistent Scatterers InSAR (PSInSAR) results deduced from 1996 to 1999 time span for monitoring of land subsidence in this area. The PSInSAR Keywords: Land subsidence results show that Baojhou, Tuku and Yuanchang Townships reveal a maximum subsidence rate of about PSInSAR 7.8 cm/yr along the LOS and Lunbei Township located on the northern Yunlin reveals a subsidence rate of Precise leveling 3.5 cm/yr, which is quite coincident with the precise leveling result. This result has proven that the effective Groundwater pumping reduction of labor and cost could be achieved by using this technique on monitoring land subsidence in Natural hazards Yunlin County. © 2012 Elsevier B.V. All rights reserved. 1. Introduction underground water has resulted in environmental hazard and potential risk in Taiwan which the severe land subsidence of about 1 to 10 cm/yr The anthropogenic activities of excessive groundwater utilization was observed in several counties from 2002 to 2006 (Fig. 1b, data from in agriculture, aquaculture, industry and urban area give rise to Water Resource Agency, Department of Economics). Particularly in the serious land subsidence. Consequently, land subsidence could result in Choshui River alluvial fan where the Yunlin section of the Taiwan High environmental hazards such as exhaustion of groundwater resources, Speed Rail (THSR) had been constructed through the central of subsi- damage of infrastructures, increase of risks of inundation and inland dence area which might pose a serious threat of its operation (Chang sea water intrusion (Abidin et al., 2001; Amelung et al., 1999; Chen and Wang, 2006; Hwang et al., 2008). From 1992 to 2007, a severe et al., 2007; Hou et al., 2005; Hsieh et al., 2011; Hung et al., 2011; cumulative land subsidence larger than 110 cm was observed at Motagh et al., 2007; Osmanoglu et al., 2010; Phien-wej et al., 2006; Mailiao, Lunbei and Baojhon Townships (Fig. 2). From 1996 to 1998, Teatini et al., 2005; Wang et al., 2011). In Taiwan, groundwater has the center of land subsidence was located in Lunbei and Baojhon been abundantly used as an alternative to surface water, especially in Townships with a subsidence rate of 7–8 cm/yr (Fig. 3a). However the the southwestern coastal region where the deficiency of surface water center of land subsidence changed to Tuku and Yuanchang counties resources is severe due to the high water demand from aquacultural and almost covered the THSR (Fig. 3b). It is believed that the land sub- and industrial utilization in Taiwan (Hsu, 1998). From a tectonic view- sidence is caused by a deformation of clay or sand layers by compres- point, Taiwan is situated along the ongoing collision boundary sion, accompanied by heavy withdrawal of underground water near between the Eurasian Plate and the Philippine Sea Plate (Fig. 1a) with the coastal regions of Yunlin County (Liu et al., 2004). A lot of efforts a convergence rate of about 8 cm/yr (Hu et al., 2001; Lin et al., 2010). have been done in managing groundwater over-extraction and land Most of the western coastal plains are considered to be in the foreland subsidence in the coastal area (Hsu, 1998; Tang and Tang, 2006). Sea- basin of the Taiwan orogen (Lin and Watts, 2002). Although the mor- sonal effects of land subsidence occurring in the study area had been es- phology of western Taiwan is flat, the subsurface deformation is charac- timated using a regression analysis of a series of weekly GPS height terized by various subsiding or uplift rate in response to the incipient solutions. The average rate of ground subsidence in this area over the development of fold-and-thrust belt (Huang et al., 2006a). However, period of 1995–2001 was 3 cm/yr (Chang and Wang, 2006). Based on anthropogenic ground subsidence induced by heavy withdrawal of the data collected at the piezometer, the variation of land subsidence rate appeared to be associated with an unstable underground water level, which drops gradually during winter and either remains constant ⁎ Corresponding author. Tel.: +886 2 23634860; fax: +886 2 23636095. or rises during summer time. Consequently, land subsidence rates vary E-mail address: [email protected] (J.-C. Hu). considerably from 1.5 cm/yr for the summer time to 9.0 cm/yr for the 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.08.009 H. Tung, J.-C. Hu / Tectonophysics 578 (2012) 126 – 135 Fig. 1. (a) Topography, bathymetry and main geological units in Taiwan. Rectangle indicates the study area. The large red indicator arrow notes direction and convergence rate of Philippine Sea Plate relative to Eurasia Plate. Major thrust faults with triangles are on the upthrust side. (b) Average land subsidence area in Taiwan during 2002 to 2006. Black lines are the administrative boundaries. Data from Water Resource Agency, Department of Economics. 127 128 H. Tung, J.-C. Hu / Tectonophysics 578 (2012) 126–135 Fig. 2. Cumulative land subsidence in Yunlin County by precise leveling from 1992 to 2007. Maximum cumulative land subsidence >110 cm occurred in Baojhon, Tuku and Yuanchang Townships. Red line indicates Taiwan High Speed Rail (TSHR). Data from Water Resource Agency, Department of Economics. winter time (Chang and Wang, 2006). In addition, the annual subsi- from SAR interferograms (e.g., Buckley et al., 2003; Ding et al., 2004, dence rate is deeply influenced by annual rainfall. An annual subsidence 2008). Thus persistent scatterer (PS) technique has been proposed to rate of greater than 3 cm/yr is considered significant in the Choshoui improve the applicability of radar interferometry when applied to River alluvial fan. Based on the six leveling surveys carried out in detect long-term ground deformation with tracking the signals of dis- 2000, 2002, 2003, 2005, 2006, and 2007, the maximum annual subsi- crete point-wise targets (Berardino et al., 2002; Ferretti et al., 2000, dence rate was 9.5 cm/yr in 2002, 12.2 cm/yr in 2003, 11.6 cm/yr in 2001; Hooper et al., 2004; Kampes and Hanssen, 2004; Liu et al., 2005, 10.1 cm/yr in 2006 and 8.2 cm/yr in 2007, respectively (Hung 2008; Mora et al., 2003). In this paper, we use PSInSAR technique to et al., 2010). The subsidence rate was highest in 2003 due to a serious deduce the perturbation and obtain land subsidence motion around a drought that occurred in Taiwan which led to excessive pumping of section of THSR in Yunlin County in central Taiwan by the natural tar- groundwater. According to the report of Hung and Liu (2007),the gets recognized from a time series of SAR interferograms. Thus 33 total area with a subsidence rate >3 cm/yr was 1600 km2 in 2001. ERS-1 and ERS-2 images (Table 1) acquired from 1996 to 1999 are The total area with a subsidence rate >3 cm/yr was reduced to employed to improve the monitoring density and to further character- 803 km2 in 2007 after the efforts of mitigations in land subsidence ize the land subsidence in the area of interest. since 2007. However, the Changhua and Yunlin areas still suffer with the problems of heavy land subsidence. Recent study using multiple 2. Geological setting and data sensor of multi-level compaction monitoring well demonstrates that the aquifer-system compaction occurs mostly below depths >200 m 2.1. Geological and hydrological background and could occur at the depth greater than 300 m (Hung et al., 2010). Although the observations from GPS measurements, precise leveling The study area is located in central Taiwan island (Fig. 1), which is a and monitoring wells provided robust monitoring of land subsidence zone of active continental deformation located at the plate boundary hazard, however spatial coverage is quite sparse due to these point zone of the Eurasian Plate (EUP) and the Philippine Sea Plate (PSP) observations, thus the low spatial density over large areas could be (Lin et al., 2010). The PSP moves towards the northwest with respect improved by the detection of surface deformation revealed by differen- to the stable EUP (Penghu Islands) at a rate of 82 mm/yr and the polar- tial SAR interferometry (D-InSAR). ity of subduction between the EUP and PSP is flipped near the central The InSAR technique has proven to be capable of measuring topo- part of Taiwan (Lin et al., 2010). Yunlin County is one of the important graphic and crustal deformation at fine space resolution of tens of agricultural production regions located in the southwestern coastal re- meters over wide coverage (e.g., Buckley et al., 2003; Bürgmann et al., gion of Taiwan where the irrigated area is up to 123,000 ha and the 2000; Ding et al., 2004; Huang et al., 2006b, 2009; Massonnet and agricultural water consumption reaches approximately 90% of all avail- Feigl, 1998; Pritchard and Simons, 2002; Wright, 2002; Yen et al., able water resources in the Choshui River Basin (Zhang, 2005).