Shallow creep on the Haiyuan Fault (Gansu, China) revealed by SAR Interferometry



[1] Interferometric synthetic aperture radar data are used to map the interseismic velocity field along the Haiyuan fault system (HFS), at the north-eastern boundary of the Tibetan plateau. Two M ∼ 8 earthquakes ruptured the HFS in 1920 and 1927, but its 260 km-long central section, known as the Tianzhu seismic gap, remains unbroken since ∼1000 years. The Envisat SAR data, spanning the 2003–2009 period, cover about 200 × 300 km2 along three descending and two ascending tracks. Interferograms are processed using an adapted version of ROI_PAC. The signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals. Mean line-of-sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise. These maps show a dominant left-lateral motion across the HFS, and reveal a narrow, 35 km-long zone of high velocity gradient across the fault in between the Tianzhu gap and the 1920 rupture. We model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion. The inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments. The average creep rate is ∼5 mm yr−1, comparable in magnitude with the estimated loading rate at depth, suggesting no strain accumulation on this segment. The modeled creep rate locally exceeds the long term rate, reaching 8 mm yr−1, suggesting transient creep episodes. The present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment.