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MONITORING OF RECENT LAND SUBSIDENCE AND GROUND 
FISSURES IN XIAN WITH SAR INTERFEROMETRY 
Chaoying Zhao 1 ’ 2 , Xiaoli Ding 1 , Qin Zhang 2 , Zhong Lu 3 Zhiwei Li 1 
1 Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, Hung Horn, Hong Kong, 
China - (zhaochaoying@163.com, lsxlding@inet.polyu.edu.hk, zwli@mail.csu.edu.cn) 
2 School of Geological Engineering and Geomatics, Chang’an University, Xian Shaanxi, China -zhangqin@263.net.cn 
3 U.S. Geological Survey, Vancouver, Washington, USA - (lu@usgs.gov) 
KEY WORDS: Land subsidence, Ground fissure, Differential InSAR, Monitoring 
ABSTRACT: 
The City of Xian, China, has been experiencing significant land subsidence and ground fissure activities since 1960s that have brought 
various severe geohazards including damages to buildings, bridges and other facilities. Monitoring of land subsidence and ground 
fissure activities can provide useful information for assessing the extent of, and mitigating such geohazards. We study the land 
subsidence and ground fissure activities during 1992 - 2006 using ERS and Envisat SAR data. Three main land subsidence stages in 
1992, 1996 and post 2004 are identified. The maximum land subsidence rates increased from 16cm/a in 1992 to 22 cm/a in 1996 and 
then decreased to 7.5cm/a by 2004, while the area affected by the subsidence expanded from the first stage to the second and then 
decreased from the second to the third. Some of the ground fissures that were active during 1990s became inactive after 2004 while 
some new fissures appeared in the new subsidence areas. It is also found that the land subsidence and ground fissure activities were 
related to ground water withdrawal and city development. 
1. BACKGROUND 
Xian, the Capital City of Shaanxi Province, lies in the middle 
of Wei River Basin, in northwestern China. Four main rivers, 
Wei River, Ba River, Chan River and Zao River surround the 
city in the northern, eastern and western suburbs respectively 
(Figure 1). The Chang’an-Lintong fault (CAF hereinafter), one 
of the main active faults controling the geological activities of 
the region (Peng et al., 1992), lies in the southern side of the 
city. Xian has been undergoing serious land subsidence and 
ground fissure activities since 1960s mainly due to ground 
water withdrawal and the CAF (Zhang, 1990; Yi et al., 1997; 
Wang, 2000; Suo et al., 2005). Levelling measurements started 
in 1959 have shown that by earlier 1990s the areas with 
cumulative subsidence of more than 200 mm were over 145 
km 2 ; the maximum subsidence was 2322 mm; the average 
ground subsidence rate was 50-100 mm/a; the maximum 
subsidence rate was up to 300 mm/a; and there existed eight 
subsidence funnels in the southern, eastern and southwestern 
suburbs of the city (Tao, 1999). The number of ground fissures 
has further increased since then. The ground fissures and CAF 
are approximately in an east-north easterly (ENE) direction, 
which resulted in ellipsoidal subsidence cones with their major 
axes being approximately parallel to the directions of the 
ground fissures (Lee et al.,1996; Yan, 1999; Xian City 
Planning Bureau, 2006). A contour map of subsidence from 
levelling measurements of 1959 to 1995 is shown in Figure 1 
with the known ground fissures superimposed onto the map. 
Unfortunately, regular levelling measurements have not been 
conducted since 1995. Not much information is therefore 
available for assessing the recent ground subsidence and fissure 
activities. 
We will in this work analyze 15 ERS 1/2 and Envisat SAR 
scenes to study the evolution of land subsidence during 1992 - 
2007. Some GPS data will also be used to validate the InSAR 
results. 
Figure 1. Location map of Xian City, where 3 rivers, the CAF, 
13 ground fissures, contour map of subsidence from levelling 
measurements of 1959 to 1995, confined water contour and 
GPS stations are plotted on a SAR magnitude map. The green 
rectangle is the ancient Xian city wall. The inset is the map of 
Shaanxi Province, where the black rectangle represents the 
study area. 
2. DATA SETS AND PROCESSING 
Fifteen ERS and Envisat SAR images acquired during 1992 - 
2007 (see Table 1) are selected for this study. Six 
interferometric pairs with time intervals from 140 to 595 days 
and baselines of less than 200 m (Table 2) are formed and 
analyzed in detail.