applied sciences Article Subsidence Evolution of the Leizhou Peninsula, China, Based on InSAR Observation from 1992 to 2010 Yanan Du 1,2, Guangcai Feng 1,2,*, Xing Peng 1,2 and Zhiwei Li 1,2 1 School of Geosciences and Info-Physics, Central South University, Changsha 410083, China; [email protected] (Y.D.); [email protected] (X.P.); [email protected] (Z.L.) 2 Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education, Changsha 410012, China * Correspondence: [email protected]; Tel.: +86-182-7486-7449 Academic Editor: Saro Lee Received: 2 March 2017; Accepted: 26 April 2017; Published: 29 April 2017 Abstract: Over the past two decades, the Leizhou Peninsula has suffered from many geological hazards and great property losses caused by land subsidence. However, the absence of a deformation map of the whole peninsula has impeded the government in making the necessary decisions concerning hazard prevention and mitigation. This study aims to provide the evolution of land deformation (subsidence and uplift) in the whole peninsula from 1992 to 2010. A modified stacking procedure is proposed to map the surface deformation with JERS, ENVISAT, and ALOS1 images. The map shows that the land subsidence mainly occurs along the coastline with a maximum velocity of 32 mm/year and in a wide range of inland arable lands with a velocity between 10 and 19 mm/year. Our study suggests that there is a direct correlation between the subsidence and the surface geology. Besides, the observed subsidence in urban areas, caused by groundwater overexploitation for domestic and industrial use, is moving from urban areas to suburban areas. In nonurban areas, groundwater extraction for aquaculture and arable land irrigation are the main reason for land subsidence, which accelerates saltwater intrusion and coastline erosion if regular surface deformation measurements and appropriate management measures are not taken. Keywords: The Leizhou Peninsula; land subsidence; stacking Interferometric SAR; groundwater pumping 1. Introduction The Leizhou Peninsula (LZP), one of the three largest peninsulas in China, is located in the southwest of Guangdong province, China, covering an area of 8500 km2 [1] (see Figure1). It plays an important role in the economic development of Guangdong province with its considerable agriculture contributions. However, over the past two decades the fast growing economy has accelerated groundwater over-pumping, causing large scale land subsidence and other geological hazards. Until 2001, the subsidence area in Zhanjiang city reached 220 km2, with the maximum cumulative subsidence up to 17.9 cm [2]. Subsidence induced geological hazards have resulted in substantial property damage every year. For example, in 2006 only, the loss in Zhanjiang city was greater than 90 million RMB [3]. However, large-scale land subsidence measurement is still scarce in the LZP apart from a few leveling measurements carried out in the urban area of Zhanjiang city in 1984, 1989, 1999, and 2001, whose results indicated that the surface subsidence increased by >10 mm/year [2,4]. A surface deformation map presenting the temporal-spatial pattern of land subsidence of the whole peninsula is urgently needed to help the government make the right decisions in geological disaster prevention and mitigation, as well as to understand the mechanism of land subsidence caused by over-pumping groundwater. Appl. Sci. 2017, 7, 466; doi:10.3390/app7050466 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 466 2 of 18 decisionsAppl. Sci. 2017 in, 7geological, 466 disaster prevention and mitigation, as well as to understand the mechanism2 of 18 of land subsidence caused by over‐pumping groundwater. Figure 1. BackgroundBackground setting setting (left) (left) and and Synthetic Synthetic Aperture Aperture Radar Radar (SAR) (SAR) coverages (right) (right) of the Leizhou Peninsula. Peninsula. The The yellow, yellow, red, red, and and black black rectangles rectangles represent represent the the coverage coverage of of JERS, JERS, ENVISAT, ENVISAT, and ALOS1 respectively. Interferometric Synthetic Aperture Radar (InSAR), a satellite based technique, offers high Interferometric Synthetic Aperture Radar (InSAR), a satellite based technique, offers high precision precision (cm even to mm), high spatial resolution, and wide coverage measurements of surface (cm even to mm), high spatial resolution, and wide coverage measurements of surface deformation, deformation, reducing the use of in‐situ traditional techniques [5,6]. Furthermore, with the reducing the use of in-situ traditional techniques [5,6]. Furthermore, with the development of the SAR development of the SAR satellite missions, more and more newly acquired TerraSAR, satellite missions, more and more newly acquired TerraSAR, COSMO-SkyMed, Sentinel-1, ALOS-2 and COSMO‐SkyMed, Sentinel‐1, ALOS‐2 and archived SAR data (ERS1/2, JERS‐1, ENVISAT, ALOS1) archived SAR data (ERS1/2, JERS-1, ENVISAT, ALOS1) can be used to analyze the spatial-temporal can be used to analyze the spatial‐temporal characteristics of large area surface deformation with the characteristics of large area surface deformation with the temporal InSAR technique [7]. Therefore, temporal InSAR technique [7]. Therefore, the InSAR technique can serve as a good option in the InSAR technique can serve as a good option in mapping the surface deformation of a large-scale area. mapping the surface deformation of a large‐scale area. Actually, it has been widely used and its Actually, it has been widely used and its superiorities shown in detecting time-series surface superiorities shown in detecting time‐series surface deformation over many peninsulas and delta deformation over many peninsulas and delta areas, such as the Mexico City [8], Shanghai [9], areas, such as the Mexico City [8], Shanghai [9], and Indonesia [10]. Traditional temporal InSAR and Indonesia [10]. Traditional temporal InSAR methods, such as Persistent Scatterer Interferometry methods, such as Persistent Scatterer Interferometry (PS‐InSAR), IPTA (Interferometric Point Target (PS-InSAR), IPTA (Interferometric Point Target Analysis), and Small Baseline Subset (SBAS-InSAR) Analysis), and Small Baseline Subset (SBAS‐InSAR) have been applied in detecting surface have been applied in detecting surface deformation, but they need abundant SAR images in data deformation, but they need abundant SAR images in data processing [11–13]. As the datasets are processing [11–13]. As the datasets are very limited in our study area, we used a modified stacking very limited in our study area, we used a modified stacking method to measure the surface method to measure the surface deformation evolution over the whole LZP with three kinds of SAR deformation evolution over the whole LZP with three kinds of SAR images (JERS, ENVISAT and images (JERS, ENVISAT and ALOS1) acquired from 1992 to 2010. Based on those results, the reasons ALOS1) acquired from 1992 to 2010. Based on those results, the reasons and mechanisms of land and mechanisms of land subsidence can be explained and are investigated in this study. subsidence can be explained and are investigated in this study. Here, we map the surface deformation of three different sensor platforms and illustrate Here, we map the surface deformation of three different sensor platforms and illustrate the the spatial-temporal evolution patterns in urban and agriculture areas. Based on the surface spatial‐temporal evolution patterns in urban and agriculture areas. Based on the surface hydrogeological and land use, we analyze the reason for land subsidence. The consequences and hydrogeological and land use, we analyze the reason for land subsidence. The consequences and implications of our land subsidence measurement are discussed in the last section. implications of our land subsidence measurement are discussed in the last section. 2. Hydrogeological Setting 2. Hydrogeological Setting The study area is the most southerly tip of China, with Qiongzhou Strait to the south, separating the peninsulaThe study from area Hainan is the Island.most southerly The elevation tip of of China, this area with ranges Qiongzhou from a fewStrait meters to the along south, the separatingcoast to 275 the m peninsula in the south from of thisHainan peninsula, Island. seeThe Figure elevation1. The of this Suixi area Fault ranges borders from the a areafew meters in the alongnorth, the see coast Figure to2 275a. Terranes m in the ofsouth Cambrian of this peninsula, age and Cretaceous see Figure age 1. The constitute Suixi Fault the basementborders the rocks area inof thisthe north, peninsula. see Figure Tertiary 2a. semi-consolidated Terranes of Cambrian sandy age clay and and Cretaceous Quaternary age unconsolidated constitute the sediments, basement rockssuch as of alluvial this peninsula. and lacustrine, Tertiary overlie semi the‐consolidated basement rocks. sandy The clay unconsolidated and Quaternary sediments unconsolidated include (1) Appl. Sci. 20172017,, 77,, 466 466 3 of 18 sediments, such as alluvial and lacustrine, overlie the basement rocks. The unconsolidated sedimentsZhanjiang Groupinclude of (1) Lower Zhanjiang Pleistocene Group age of Lower with an Pleistocene upper layer age of claywith and an upper a lower layer layer of of clay scattered and a lowerlenses layer of clay of andscattered coarse lenses sand of with clay gravel and coarse (Qlz), sand (2) Beihaiwith gravel group (Qlz), of middle (2) Beihai Pleistocene