Probing fault frictional properties during afterslip up- and downdip of
the 2017 Mw 7.3 Sarpol-e Zahab earthquake with space geodesy
Abstract
We use Interferometric Synthetic Aperture Radar (InSAR) data collected
by the Sentinel-1 mission to study the co- and postseismic deformation
due to the 2017 Mw 7.3 Sarpol-e Zahab earthquake that occurred near the
Iran-Iraq border in Northwest Zagros. We find that most of the coseismic
moment release is between 15 and 21 km depth, well beneath the boundary
between the sedimentary cover and underlying basement. Data from four
satellite tracks reveal robust postseismic deformation during
~ 12 months after the mainshock (from November 2017 to
December 2018). Kinematic inversions show that the observed postseismic
InSAR LOS displacements are well explained by oblique (thrust + dextral)
afterslip both updip and downdip of the coseismic peak slip area. The
dip angle of the shallow afterslip fault plane is found to be
significantly smaller than that of the coseismic rupture, corresponding
to a shallowly dipping detachment located near the base of the sediments
or within the basement, depending on the thickness of the sedimentary
cover, which is not well constrained over the epicentral area.
Aftershocks during the same time period exhibit a similar temporal
evolution as the InSAR time series, with most of aftershocks being
located within and around the area of maximum surface deformation. The
postseismic deformation data are consistent with stress-driven afterslip
models, assuming that the afterslip evolution is governed by
rate-strengthening friction. The inferred frictional properties updip
and downdip of the coseismic rupture are significantly different, which
likely reflect differences in fault zone material at different depths
along the Zagros.