The Transient Behavior of Aseismic Slip Along the Creeping Section of the North Anatolian Fault, Turkey

Abstract:

The ongoing development of constellations of Synthetic Aperture Radar (SAR) satellites with short repeat time acquisitions allows to explore the behavior of active faults with an unprecedented temporal and spatial resolution. The improvement from monthly to daily repeat times sheds a new light on the dynamics of near-surface fault creep along continental faults, which has been shown to exhibit various temporal behaviors, from persistent slow silent slip to discrete episodes of aseismic slip. Along the North Anatolian Fault (NAF), an 80 km-long section is creeping at least since the 1944, M7.3 earthquake near Ismetpasa, Turkey. Recent geodetic measurements suggest an average creep rate of about half the total slip rate accommodated by the NAF (8±3 mm/yr vs. 22±3 mm/yr). In addition, an effective bi-modal distribution of frictional properties along fault dip (rate-strengthening from the surface to 5-7 km-depth and rate-weakening down to the locking depth) can explain the persistent creep rate and the extent of past ruptures. We take advantage of the dense set of SAR images acquired by the Cosmo-SkyMed™ (ASI) constellation over the creeping section of the NAF to quantify, with a high spatial and temporal resolution, the distribution of aseismic slip along strike and its evolution between August 2013 and June 2014. We compute 1000+ interferograms from 350+ radar acquisitions over 7 tracks using the ISCE software (JPL). We use the Generic InSAR Analysis Toolbox (GIAnT) and the PyAPS library to correct interferograms from the propagation delays due to the stratification of the troposphere, predicted using the ERA-Interim (ECMWF) re-analysis. We use the New Small Baseline (NSBAS) method to derive the spatial and temporal evolution of the near-fault displacements independently for each track. Our results suggest the fault does not creep steadily over the 2013-2014 period but rather releases strain through discrete aseismic events we refer to as bursts of creep. In particular, we identify one major burst, equivalent to a M5.0 earthquake that took place in a maximum of 8 days releasing the equivalent of 1 year of creep at that location. The observed bursts of creep suggest new exploration of the mechanical properties of the fault zone during the interseismic period is needed along the creeping section of the NAF.