Abstract
Interferometric synthetic aperture radar (InSAR) data from 6 Envisat ASAR descending tracks; spanning the 2003–2010 period; was used to measure interseismic strain accumulation across the Northeastern Tibetan Plateau. Mean line-of-sight (LOS) ratemaps are computed by stacking atmospheric-corrected and orbital-corrected interferograms. The ratemaps from one track with different atmospheric-corrected results or two parallel; partially overlapping tracks; show a consistent pattern of left-lateral motion across the fault; which demonstrates the MERIS and ECMWF atmospheric correction works satisfactorily for small stain measurement of this region; even with a limited number of interferograms. By combining the measurements of InSAR and GPS; a fine crustal deformation velocity and strain rate field was estimated on discrete points with irregular density depending on the fault location; which revealed that the present-day slip rate on the Haiyuan fault system varies little from west to east. A change (2–3 mm/year) in line-of-sight (LOS) deformation rate across the fault is observed from the Jinqianghe segment to its eastern end. Inversion from the cross-fault InSAR profiles gave a shallow locking depth of 3–6 km on the main rupture of the 1920 earthquake. We therefore infer that the middle-lower part of the seismogenic layer on the 1920 rupture is not yet fully locked since the 1920 large earthquake. Benefit from high spatial resolution InSAR data; a low strain accumulation zone with high strain rates on its two ends was detected; which corresponds to the creeping segment; i.e., the Laohushan fault segment. Contrary to the previous knowledge of squeezing structure; an abnormal tension zone is disclosed from the direction map of principal stress; which is consistent with the recent geological study. The distribution of principal stress also showed that the expanding frontier of the northeastern plateau has crossed the Liupan Shan fault zone; even arrived at the northeast area of the Xiaoguan Shan. This result agrees with the deep seismic reflection profile.
Highlights
Surface velocity and strain fields provide an important constraint on geodynamic models of tectonic deformation as well as in the assessment of earthquake hazard [1]
We focus on the northeastern margin of the Tibet Plateau, where the crustal shortening and lateral extrusion caused by the northeastward growth of the Tibet Plateau is accommodated by the left-lateral strike-slip Haiyuan fault and a series of arcuate structures in its north, and the thrusting Liupan Shan fault and a few folds and thrust faults in its east (Figure 1)
For each Envisat ASAR track we collected, we constructed an interferometric stack composed of several chains of small-baseline interferograms, applied the stacking Interferometric synthetic aperture radar (InSAR) method with atmospheric and orbital correction described in Section 3.1, to construct a deformation ratemap
Summary
Surface velocity and strain fields provide an important constraint on geodynamic models of tectonic deformation as well as in the assessment of earthquake hazard [1] Such velocity fields may be determined using geodetic measurements such as GPS, but GPS observations are too sparse to describe deformation spatial variations in the near-fault area. Satellite InSAR data provide us a chance to give an insight into the near-fault crustal displacement caused by the fault motion, for the shallow aseismic fault slip. It has provided important information on imaging the spatial variation of interseismic coupling on the fault plane in the upper crust. GPS data with respect to the Eurasia-fixed Reference Frame is from Gan et al [22]
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