The Relationship Between Coseismic Slip and Postseismic Creep

Abstract:

Shallow aseismic creep has been observed on many strike-slip faults around the world. The initiation process for shallow creep remains unclear, and no clear explanation exists for why creep persists for years/decades after some earthquakes while after others it decays completely within months. Many earthquakes exhibit a shallow coseismic slip deficit. We hypothesise that shallow creep can begin after such events to 'catch up' with the slip deficit relative to deeper sections of the fault. We test this hypothesis with persistent scatterer InSAR analysis of 3 descending and 2 ascending Envisat tracks that together span a region covering the 1999 Izmit and Duzce ruptures in Turkey. Our data covers an 8 year time window between 2003 – 2010. A section of the Izmit rupture was previously shown to be undergoing shallow creep. But the spatial and temporal nature of this creep remains unclear. We use a small baseline processing strategy using the StaMPS software, which allows for checking of unwrapping errors by summing the residuals around closed interferometric loops. We make an addition to the code to detect and fix unwrapping errors. This is done by automatically detecting and fixing pixels that are unwrapped correctly in an initial run. The procedure is iterated until unwrapping errors are corrected. By combining our InSAR results with existing GPS measurements we obtained a map of horizontal and vertical surface displacements over time. From this we calculated the variation in creep rates along the earthquake ruptures. We show that shallow creep at an average rate of 10 mm/yr is limited to the Izmit rupture and does not extend laterally to the Duzce segment. We use elastic dislocation models to determine the creep distribution with depth and compare with the coseismic slip observed in the Izmit earthquake. We find the creep rate is highest in a region of shallow coseismic slip deficit. The creep rate exponentially decays with time after the earthquake to a steady rate. We model this temporal behaviour using rate and state friction theory and determine variations in frictional properties within the creeping region.