Postseismic deformation in Pakistan after the 8 October 2005 earthquake: Evidence of afterslip along a flat north of the Balakot-Bagh thrust

Abstract

22p.International audienceThe 8 October 2005 Kashmir earthquake ruptured an out-of-sequence Himalayan thrust known as the Balakot-Bagh thrust. The earthquake's hypocenter was located at a depth of 15 km on the ramp close to a possible ramp/flat transition. In the weeks following the earthquake a GPS network was installed to measure postseismic displacement. The initial measurements in November 2005 were followed by other campaigns in January and August 2006, in March and December 2007, and in August 2008 and 2009. Two hypotheses were tested: post-seismic displacements controlled by viscous relaxation of the lower crust or by afterslip along a flat north of the ramp affected by the main shock. A single Newtonian viscosity for the different periods cannot be determined by numerical simulations of viscous relaxation, which may indicate that the viscosity of the lower crust is non-Newtonian or that viscous relaxation does not control postseismic displacements. Numerical simulations using dislocations in a uniform elastic half-space indicate afterslip north of the ramp of the earthquake along a flat connected to the ramp. Slip along the northwestern portion of the flat accrued to about 285 mm between November 2005 and August 2006, while slip along the southeastern portion accrued to 130 mm over the same time period. Residual misfit of the observed and predicted displacements clearly indicated that afterslip is a better explanation for the observations than the hypothesis of viscous relaxation. The time evolution of the afterslip was found to be consistent with that predicted from rate-strengthening frictional sliding

Postseismic relaxation in Kashmir and lateral variations in crustal architecture and materials

“…For example, Wang and Fialko [2014] use a spatially and temporally varying afterslip distribution, and Jouanne et al [2011] propose aseismic slip on a secondary deep dislocation in the north.…”

“…We also processed raw observations from 18 sites previously described in Jouanne et al [2011] and three additional sites in India. Not all sites were observed at all measurement epochs, and the total amount of data at each site varies considerably ( Figure S3 in the supporting information).…”

“…To reduce the observed displacement time series, we first estimate an interseismic linear secular velocity for all sites by fitting a linear function with a total of 14 mm/yr of shortening over 520 km between the far-field sites in the Salt Range and western Pakistan and through those in Ladakh, hence spanning the epicentral region. Such a simple interpolation is justified by the observed velocity gradients in GPS measurements of interseismic deformation near the epicentral region [Jouanne et al, 2011;Kundu et al, 2014;Schiffman et al, 2013;Jouanne et al, 2011;Khan et al, 2008]. We then subtract displacement due to this estimated interseismic velocity from the displacement time series for all epicentral sites (within 100 km of the rupture).…”

“…It is also possible that the heterogeneity we describe may influence seismic hazards, as mechanical properties determine whether tectonic loading is accommodated by the earthquake cycle or other unrecoverable plastic and viscous mechanisms. At present, although the surface geology clearly indicates compositional differences, a range of applications including elastic strain accumulation for seismic hazard [e.g., Bollinger et al, 2004;Drukpa et al, 2012;Schiffman et al, 2013], orogenic continuum mechanics [e.g., Bendick and Ehlers, 2014], and even structural interpretations from geodesy [e.g., Jouanne et al, 2011;Bendick et al, 2007] all approximate Himalayan architecture as laterally homogeneous. Our data confirm Hetland and Hager's [2004] observation that interpretations of interseismic deformation are less sensitive to this lateral inhomogeneity than measurements of postseismic deformation.…”

“…Some data have been interpreted to support a second fault segment with much less slip to the north [Bendick et al, 2007] or northeast [Jouanne et al, 2011] of the main rupture, based on relatively large coseismic [Bendick et al, 2007] or postseismic [Jouanne et al, 2011] surface displacements north of the termination of the main rupture.…”