# Study of Subsidence and Earthquake Swarms in the Western Pakistan
## 4 Discussion
### Earthquake Displacement Interpretation  
The Quetta-Ziarat 2008 earthquake swarms caused 215 deaths and left 200 injured. Previous studies have come up with different models for these earthquake swarms. Based on GPS data, previous work suggested a co-seismic slip of the northwest-southeast basement faulting in the earthquake sequence, but no surface rupture was found [22]. Using focal mechanisms analysis, other studies theorized that the northwest-southeast-oriented Urghargai Fault triggered the earthquakes [42; 43]. [PERSON] et al. [44] and [PERSON] et al. [45] provided a better explanation based on a two-image DInSAR technique and a forward modeling result. They proposed that the 2008 earthquake sequence was generated by two conjugate faults: a main northwest-southeast right-lateral strike-slip fault and a secondary northeast-southwest left-lateral strike-slip fault forming a \"book shelf\" structure.  
However, using the SBAS-InSAR time series, gravity result, and the location of the two earlier mapped Harnai and Karahi Faults [46], we suggest a different explanation for the 2008 earthquake sequence. Compared with the two image DInSAR technique, the more comprehensive SBAS-InSAR time series results in this study show a right-lateral strike-slip movement of ~50 mm/year (including the co-seismic deformation) for the Harnai Fault and a left-lateral strike-slip movement of ~10 mm/year for the Karahi Fault (including the co-seismic deformation). Previous studies failed to take into consideration the known geological structures when developing their models, interpreted the focal mechanism fault plane into two northwest-southeast right-lateral and northeast-southwest left-lateral striking conjugate faults, and proposed the \"book shelf\" structure. Based on SBAS-InSAR result from this study and the northwest-southeast orientation of the Harnai and Karahi Faults, the northwest-southeast right-lateral focal mechanism fault plane is a more plausible explanation, since it is parallel to the Harnai and Karahi Faults.  
Using these results and the gravity map, three zones were chosen at the earthquake locations along the Karahi and Harnai Faults for time series analysis. Figure 8C shows the time series for zone 1, with a significant increase in pre-seismic slip rates. Zone 2 and zone 3 had sinusoidal displacement patterns during the earthquake cycle. The positive velocity peak correspond to the 2008 earthquake swarms, where all three zones had sharp velocity drops and indicate a sharp release of strain. Zone 2 had a faster westward movement compared to zone 1, which indicates the Karahi Fault may have switched into a right-lateral movement during the earthquake swarms. This explains the right-lateral focal mechanism (EQ 3, 4 and 5, Figure 1) around the Karahi Fault. The Harnai and Karahi Faults correspond to the earthquake locations and may be easily extended further north to match the observed two right-lateral earthquakes from the earthquake sequence in that area (EQ 1 and 2, Figure 1). Zones 2 and 3 have sinusoidal displacement patterns that demonstrate the accumulation and release of strain on the Harnai and Karahi Faults during the earthquake cycle. The Harnai and Karahi Faults both had right-lateral movement during the 2008 earthquake swarms. [PERSON] et al. [47] found similar results in their laboratory experiments while simulating slow-slip behaviors for faults. Although due to different tectonic scenarios and rock friction properties, the inter-seismic strain patterns are different and vary by location [48]. Even at the same location, the amplitude and period of the pattern varies, which is a major obstacle to implementing the pre-seismic displacement pattern in seismic hazard prediction.  
However, the sinusoidal displacement patterns at the Quetta-Ziarat earthquake location may suggest an association between tectonic displacement and earthquakes in the Quetta Valley.