# Study of Subsidence and Earthquake Swarms in the Western Pakistan
## 4 Discussion  
### Anthropogenic- versus Tectonic-Induced Subsidence  
The Quetta Valley is a highly populated area (over 1.1 million residents as of 2016) and experiencing rapid population growth. Several earlier studies suggested that the excessive use of groundwater has caused subsidence in the valley [2; 3]. The Quetta Valley is also located in an active tectonic region, along the boundary of the western Indian Plate. In order to assess the role of anthropogenic and tectonic processes in subsidence in the Quetta Valley, we adopted a novel technique that utilized a median filter commonly used in seismic data processing. A median filter operates by selecting the middle value of numbers within a window, which is then used as a geophysical tool for regional-residual separation [36; 37]. Generally, tectonic deformation shows relatively slow movement rates over large regions [12; 13] and can be considered as a long-wavelength background signal. In contrast, anthropogenic deformation has relative faster movement rates concentrated in smaller regions [3]; as such, this type of deformation can be considered as a short-wavelength localized signal. The terminology of long/short wavelengths is adopted here for the SBAS-InSAR distance-velocity profiles, not for the SBAS-InSAR time series profile. In other words, to separate the effects of tectonic processes from those of anthropogenic processes, only a distance profile on a regional scale can be used--a localized time series is not suitable. A five-point median filter was applied to separate the short- and long-wavelength signals in order to assess the role of anthropogenic and tectonic processes on subsidence (Figure 6C). The filtered signal is the long wavelength, suggesting regional tectonics; whereas the residual signal is the short wavelength for the Quetta Valley, which is mainly caused by water withdrawal. The residual signal was calculated by subtracting the filtered signal from the original signal. In Figure 6C, the InSAR velocity profile shows uplift (1.4 mm/year) at zone A and may be subsidence at zone B, separated on the filtered curve (red-colored line). The displacements at zones A and B are relatively small--they could be noise. Because these zones are located along the transmission zone of the Chaman Fault and there are no urban areas, we considered that zones A and B are only affected by tectonic processes. On the other hand, zone C and zone D show subsidence and uplift on both the residual curve (green-colored line) and filtered curve (red-colored line) in the Quetta Valley (Figure 6C). This means that both tectonic and anthropogenic processes cause subsidence in zone C and uplift in zone D. Zone D covers the hanging wall of blind thrust fault 1 and 2 mapped by seismic profiles (Figure 6A); the creep along the thrust faults may cause accelerated uplift at zone D. There are studies showing that faults can also be reactivated and accelerated by water withdrawal in California, Arizona, and Houston [38; 39; 40]. The median filtered SBAS-InSAR distance-velocity profile revealed that the uplift and subsidence is mainly caused by tectonic processes in rural areas (zones A and B). In urban areas, both tectonic and anthropogenic processes contribute to the subsidence in the Quetta Valley (zones C and D). Anthropogenic processes include extensive water withdrawal and accelerated thrust fault movement due to water withdrawal. In zones A and B, we assume that the only contributions are from tectonic thrust faults, and that these regional contributions are homogeneous across the approximately 100 km span of profile CC. A recent InSAR study along the Chaman Fault [41] found a relative thrust fault of movement around 2 mm/year further south toward the Nushki area. Hence, the tectonic contributions of zones A and B are the same as those in the Quetta Valley in zones C and D. After removing the regional tectonic movement, we can see that the dominant subsidence in the Quetta Valley (zone C) is caused by water withdrawal.