Marie Pierre Doin

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

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.

The Burst‐Like Behavior of Aseismic Slip on a Rough Fault: The Creeping Section of the Haiyuan Fault, China

Recent observations suggesting the influence of creep on earthquakes nucleation and arrest are strong incentives to investigate the physical mechanisms controlling how active faults slip. We focus here on deriving generic characteristics of shallow creep along the Haiyuan fault, a major strike‐slip fault in China, by investigating the relationship between fault slip and geometry. We use optical images and time series of Synthetic Aperture Radar data to map the surface fault trace and the spatiotemporal distribution of surface slip along the creeping section of the Haiyuan fault. The fault trace roughness shows a power‐law behavior similar to that of the aseismic slip distribution, with a 0.8 roughness exponent, typical of a self‐affine regime. One possible interpretation is that fault geometry controls to some extent the distribution of aseismic slip, as it has been shown previously for coseismic slip along active faults. Creep is characterized by local fluctuations in rates that we define as creep bursts. The potency of creep bursts follows a power‐law behavior similar to the Gutenberg–Richter earthquake distribution, whereas the distribution of bursts velocity is non‐Gaussian, suggesting an avalanche‐like behavior of these slip events. Such similarities with earthquakes and lab experiments lead us to interpret the rich dynamics of creep bursts observed along the Haiyuan fault as resulting from long‐range elastic interactions within the heterogeneous Earth’s crust.

Dem corrections before unwrapping in a Small Baseline strategy for InSar time series analysis

Synthetic Aperture Radar interferometry allows to measure spatio-temporal patterns of deformation. However this geodetic technique is limited by unwrapping difficulties linked with temporal decorrelation and topographic errors in partially incoherent and mountainous areas. This paper presents a new algorithm to correct and remove DEM errors in order to improve the phase unwrapping step. The method consists in a mix approach between Small Baseline and Permanent Scatterers strategy using a series of wrapped interferograms. First we develop our methodology and then we apply it to a series of wrapped ENVISAT interferograms on the Tibetan plateau.