Cecile Lasserre

September 2005 Manda Hararo‐Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data

faults. The volume of the 2005 dike (1.5–2.0 km 3 ) is not balanced by sufficient volume loss at Dabbahu and Gabho volcanoes (0.42 and 0.12 km 3 , respectively). Taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment. Above the dike, faults slipped by an average 3 m, yielding an estimated geodetic moment of 3.5 � 10 19 Nm, one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm. Between Dabbahu and Ado’Ale volcanic complexes, significant opening occurred on the western side of the dike. The anomalous location of the dike at this latitude, offset to the east of the axial depression, may explain this phenomenon. A two-stage intrusion scenario is proposed, whereby rifting in the northern Manda Hararo Rift was triggered by magma upwelling in the Dabbahu area, at the northern extremity of the magmatic segment. Although vigorous dike injection occurred during the September 2005 event, the tectonic stress deficit since the previous rifting episode was not fully released, leading to further intrusions in 2006–2009.

Systematic InSAR tropospheric phase delay corrections from global meteorological reanalysis data

[1]xa0Despite remarkable successes achieved by Differential InSAR, estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated, mainly because of the topography-related atmospheric phase screen (APS). In areas of high relief, empirical removal of the stratified component of the APS may lead to biased estimations of tectonic deformation rates. Here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with SAR acquisitions using the ERA-Interim global meteorological model. The modeled phase delay is integrated along vertical profiles at the ERA-I grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel. This approach is validated on unwrapped interferograms. We show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography.

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 Pingding segment of the Altyn Tagh Fault (91 °E): Holocene slip-rate determination from cosmogenic radionuclide dating of offset fluvial terraces

Morphochronologic slip-rates on the Altyn Tagh Fault (ATF) along the southern front of the Pingding Shan at 90.5E are determined by cosmogenic radionuclide (CRN) dating of seven offset terraces at two sites. The terraces are defined based upon morphology, elevation and dating, together with fieldwork and high-resolution satellite analysis. The majority of the CRN model ages fall within narrow ranges (<2 ka) on the four main terraces (T1, T2, T3 and T3′), and allow a detailed terrace chronology. Bounds on the terrace ages and offsets of 5 independent terraces yield consistent slip-rate estimates. The long-term slip-rate of 13.9+/-1.1 mm/yr is defined at the 95% confidence level, as the joint rate probability distribution of the rate derived from each independent terrace. It falls within the bounds of all the rates defined on the central Altyn Tagh Fault between the Cherchen He (86.4E) and Akato Tagh (88E) sites. This rate is 10 mm/yr less than the upper rate determined near Tura at 87E, in keeping with the inference of an eastward decreasing rate due to progressive loss of slip to thrusts branching off the fault southwards but it is greater than the 9+/-4 mm/yr rate determined at 90E by GPS surveys and other geodetic short-term rates defined elsewhere along the ATF. Whether such disparate rates will ultimately be reconciled by a better understanding of fault mechanics, resolved transient deformations during the seismic cycle or by more accurate measurements made with either approach remains an important issue.

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.

Extraction of frequent grouped sequential patterns from Satellite Image Time Series

This paper presents an original data mining approach for extracting pixel evolutions and sub-evolutions from Satellite Image Time Series. These patterns, called frequent grouped sequential patterns, represent the (sub-)evolutions of pixels over time, and have to satisfy two constraints: firstly to correspond to at least a given minimum surface and secondly to be shared by pixels that are sufficiently connected. These spatial constraints are actively used to face large data volumes and to select evolutions making sense for end-users. Successful experiments on an optical and a radar SITS are presented.