GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.3 141 of the source model parameters, corresponding to Mw 5.7. The centroid depth of 6.7 km is similar to past events in the Eastern SFT region (Reynolds et al. , 2015). This simple model indicates that the rupture took place on a 40˚ dipping thrust fault located east of the exposed SFT, buried deep below the Miocene or younger sediment of the Indus River floodplain. We use this result to construct a three-dimensional model of coseismic deformation compatible with the regional tectonic setting. Based on the fold-and-thrust tectonic environment, we investigate two relevant end- member of fault-bend geometries of crustal deformation that involve fault slip at the base of the thrust sheet and folding in the overlying sedimentary strata. Using structural models compatible with long-term deformation, we seek to determine the distribution of fault slip and flexural slip that took place during the InSAR observation period. We model surface deformation due to slip on the ramp and the decollement using analytic solutions (Okada, 1985). In simple models of folding, flexural slip accommodates the advection of thrusted sediments through the active axial surface as they start or stop climbing the ramp. This localized plastic deformation can be represented by dislocation theory, whereby the nucleus of strain is defined by the normal direction of the axial surface and the direction of long-term motion (Sathiakumar et al. , 2020). The direction of long-term motion along the axial surfaces is the difference between the long-term velocity vectors for slip on the ramp and slip on the decollement (Daout et al. , 2016). Discussions. We first consider the case of the double fault-bend fold system (Fig. 3a). We determine the distribution of slip on the ramp and on the two decollements, and of flexural slip over the two axial surfaces through a non-negative least-squares inversion where motion is aligned with the direction of long-term motion (e.g., Barbot et al. , 2013). We discretize the model into square patches of 800 m length allowing non-zero along-strike and down-dip slip. Our preferred model explains the observations well, with residuals less than 3.2 mm (Fig. 3e). The coseismic rupture is much elongated, almost entirely confined to the ramp, with along strike rupture propagation from north to south, parallel to the BT. Virtually no slip takes place on the shallower or deeper decollements (Fig. 3a). The model produces up to 15 cm of flexural slip along the deeper axial surface (axial surface 1), concentrated near the nucleation area to the north. There is virtually no flexural slip on the shallow axial surface (axial surface 2). For second case, we consider the case of the fault-propagation fold end-member (Fig. 3f), which we model as a ramp-decollement system with a single active axial surface that follows the hinge of the fault bend. We resolve the spatial distribution of fault slip and flexural slip using the same approach as above. The model reproduces the observations well, with residuals around 3.3 mm (Fig. 3h, j). The coseismic slip is entirely contained on the frontal ramp, with the maximum slip of 50 cm found at 6.5 km depth (Fig. 3f). The inversion reveals around 24 cm of flexural slip extending laterally following the pattern of coseismic slip (Fig. 3f). The two-models perform almost equally well in terms of data reduction, but coseismic slip seems entirely confined to the ramp. As only the ramp and the lower axial surface exhibit much deformation, Occam’s razor and the principle of parsimony suggest that the fault-propagation fold model, with much fewer degrees of freedom, is more meritorious. The amount of flexural slip in the fault-propagation fold model represents 70% of the expected value for coseismic folding. The remaining fraction may occur later in the postseismic period, when folding may also propagate up-dip of the axial surface through the entire sedimentary stack. The 2015 Mw 5.7 Dajal earthquake likely represents the rupture of the frontal section of the BT at the easternmost boundary of the SFT. The rupture can be understood as the seismic activity of the blind ramp of a fault-propagation fold. We suggest that the Dajal earthquake occurred on the underlying ramp of an even less developed anticlinal structure formed by fault-propagation fold buried under the Miocene and