# Study of Subsidence and Earthquake Swarms in the Western Pakistan
## 1 Introduction  
The Quetta Valley is located in western Pakistan, in a 560 km-long and 150 km-wide complex belt of north-south-oriented mountain ranges and intervening valleys [1]. A groundwater-level decline of about 1 m/year was observed in the Quetta Valley by the Water and Power Development Authority (WAPDA) in 1989 and has been suggested as a driver of subsidence in this region [2, 3]. Four ground fissures appeared in February 2011 near the Quetta Valley that are oriented at \(\sim\)25\({}^{\circ}\) and \(-\)25\({}^{\circ}\) to the north, with a \(\sim\)6 m depth and \(\sim\)2 m maximum width. Although subsidence in urban areas is generally attributed to groundwater depletion, it is not clear whether ground fissures in the Quetta Valley are caused by water withdrawal or related to the tectonics of the region. The appearance of these fissures has also prompted fears of earthquakes. Western Pakistan is tectonically active and bounded by several regional strike-slip faults, including the north-south striking left-lateral Chaman Fault System, which slips at a rate of \(\sim\)1-2 cm/year [4; 5] (Figure 1). This region has experienced several deadly earthquakes: An Mw 6.4 earthquake was recorded in October 2008 northeast of the Quetta Valley, followed by smaller magnitude earthquakes through December 2008. In addition, more recently, an Mw 5.4 earthquake was recorded near Qilla Abdullah, a district along the Chaman Fault, on 13 May 2016, followed by an Mw 4.4 earthquake on 17 August 2016. The two most recent earthquakes caused at least six deaths and damaged 20 homes. The Quetta Valley is an elongated depression flanked by the Murdar Mountain Range on its eastern side and the Chiltan Mountain Range on its western side. Takatu Mountain is located toward the northern end of the depression (Figure 1) [2]. The Chiltan Mountain Range contains high-angle thrust faults trending almost north-south. The Quetta Valley has several composite folds, including the Quetta Syncline, which is a north-south-trending fold up to 10 km wide and covered by recent alluvial deposits. The Murdar Anticline is another north-south-trending fold, with a width of more than 12 km in several places. Most of the exposed rocks in this area belong to the Chiltan Limestone. A number of strike-slip faults were observed to the east of Quetta, such as the Harnai and Karahi Faults, both of which are located near the epicenter of the 2008 earthquake sequence [6]. There are two main causes of subsidence: human activities and tectonics. Land subsidence caused by human activities is affecting many cities around the world, such as Houston (Texas) [7; 8]; Springfield (Missouri) [9]; Shanghai, Tianjin, Beijing, Jiangsu, Zhejiang, and Hebei (China) [10]; and several cities in California and Arizona [11]. Many scholars have studied land subsidence caused by tectonics, such as post-glacial sediment load-caused subsidence in Louisiana [12] and extensional rifting-caused subsidence in eastern Canada [13]. Human activities and tectonics that cause land deformation have been studied separately before [14; 15; 16]. However, the combined influence of anthropogenic and geological processes on surface deformation has rarely been studied due to the complexity involved in combining the different datasets needed to separate them. The main objective of this study was to evaluate the potential of combined surface deformation observed from InSAR with subsurface interpretation from seismic reflection profiles to find the ultimate causes of surface deformation. Both anthropogenic and tectonic mechanisms were evaluated and the relative contributions of these different processes to subsidence in the Quetta region were assessed. Both associated with and integral to this study are the swarms of earthquakes that occurred in 2008; thus, this study investigated pre-, co-, and post-seismic patterns for these earthquake events. To achieve our objectives, we focused on three research questions:  
1. What caused the fissures in the Quetta Valley?