Characterizing seismic and aseismic deformation along the Chaman fault system with InSAR

Chaman fault system, the transform boundary between the Indian and Eurasian plates, is a group of sinistral faults including the Chaman, Ghazaband and Ornach Nal faults. This fault system accommodates ~3 cm/yr relative motion between the Indian and Eurasian plates. Previous studies based on seismic data have suggested a significant slip deficit on this fault system, likely accommodated by aseismic creep; however no clear evidence for the suggested creep has yet been provided. Here, we use several tracks of Envisat ASAR data acquired over ascending and descending orbits during the years 2003-2011 to study the tectonic deformation across the Chaman fault system and resolve how deformation is accommodated along this plate boundary.

Our InSAR analysis yield a coherent velocity field, composed of multiple tracks, which are consistent in overlapping areas. The velocity map shows left-lateral relative motion across the Chaman fault system with strain partitioning among different faults. One of our key findings is a ~50 km long creeping segment of the Chaman fault with a maximum creeping rate of ~7 mm/yr during 2003-2011. This creeping segment is located between Nushki and Chaman cities, north of the rupture area of Mw 7.7 2013 Balouchistan earthquake. This observation of fault creep is the first geodetic evidence for steady aseismic slip on Chaman fault. Additionally, our InSAR velocity field reveals typical (arctangent) long wavelength crustal displacement across different segments of the fault system, suggesting interseismic strain accumulation, likely to be released seismically in future earthquakes. We also detect deformation induced by a Mw 5 earthquake on the Ghazaband fault, south of the Mw 7.5, 1935 Quetta earthquake, with long-lasting (~2 years) aseismic afterslip, similar to a recent observation reported by Furuya and Satyabal, [2008].

Our analysis shows that the deformation in the western Indian plate boundary is accommodated in three different forms: aseismic slip along creeping segments, interseismic strain accumulation along locked fault segments, and aseismic afterslip following moderate earthquakes. The observed creep and afterslip are consistent with the lack of seismic activity on parts of the Chaman fault system, and could ultimately provide new constraints to refine seismic hazard in the area.