# Ground Subsidence Analysis in Tianjin (China) Based on Sentinel-1A Data Using MT-InSAR Methods
## 4 Discussion
### Recent Ground Subsidence Characteristics and Its Influencing Factors in Tianjin  
This paper proves that recently the ground subsidence velocity in most of the urban area of Tianjin is under the threshold of \(-\)10 mm/year, while there are still obvious serious settlement funnels in the suburban area (over \(-\)50 mm/year in local regions). This concurs with the findings of [PERSON] et al. [12], who derived the land subsidence of Tianjin from 2015 to 2016 based on Sentinel-1A using the method of SBAS, showing that the subsidence rate of the central urban area varies from \(-\)6.1 to 3.9 mm/year and the subsidence rate of the high settlement area in the suburbs ranges between \(-\)126.3 and \(-\)65.7 mm/year. Due to Tianjin and the central government adopting a variety of measures to control subsidence [3; 6; 26] since the 1980s (such as deriving alternative surface water, restriction on groundwater extraction, manual recharge of aquifers, and establishment of a settlement monitoring network), the urban settlement in Tianjin is presently under control [27; 29]. However, subsidence in the suburbs of Tianjin has not been effectively managed and the issue is becoming serious in recent years. For example, during the period from 1992 to 2000, land subsidence occurred in almost all the outskirts around the Tianjin urban center, consisting of the Wuqing, Beichen and Xiqing districts, and most subsidence rates were between \(-\)30 and \(-\)60 mm/year [44]. Since the 21 st century, the subsidence in the western suburbs of Tianjin has continued to increase, and many researchers have performed inversion analysis of the area using In-SAR technology. [PERSON] et al. [45] showed that, between 2003 and 2004, in Tianjin, the western zone is most severely affected, being Xiqing district the worst-hit area. [PERSON] [46] pointed out that the maximum subsidence rate of the settlement funnel in the western suburbs of Tianjin exceeded \(-\)80 mm/year from 2007 to 2010. This has been further confirmed by [PERSON] et al. [6], who found the south-western area of Wuqing district and western part of Beichen district displaying subsidence rates of up to \(-\)136 mm/year from 2016 to 2017. In our research, the latest land subsidence in Tianjin is basically consistent with previous studies. The land subsidence in the suburbs is still not under control, and some areas are even getting worse. As a typical geological hazard in Tianjin, ground subsidence seriously threatens the safety of people's life and property. The subsidence has a long history and its settlement mechanism is very complicated. Based on the previous results, the causes of Tianjin subsidence can be summarized as natural and human factors. Natural factors consist of tectonic sinking and loose soil consolidation. [PERSON] et al. [47] studied the background of tectonic subsidence in the Tianjin area, finding that the tectonic subsidence velocity is 1.7 mm/year approximately. The soil in most parts of Tianjin is under-consolidation, often with the natural water content greater than the liquid limit. Even without external load, this type of formation is still slowly consolidating under self-weight stress, causing a small amount of ground subsidence over a long period [28]. The Tianjin Land Subsidence Controlling Office showed that the natural average deformation value of the unconsolidated strata in Tianjin is about 1.0 mm/year [48]. In addition to natural factors, for many years, human activities, especially the excessive exploitation of groundwater resources, have always been the main influencing factors for Tianjin's land subsidence. Taking Wangqingto town (TJ10 and TJ11 in Figure 4b) as an example,its land subsidence rate has been high for many years [12; 28]. As the largest bicycle production base in northern China, Wangqingtuo town has a total of 268 bicycle manufacturers with an annual output of 8 million bicycles. Many production workshops are densely distributed in the town's industrial park (Figure 5a).