西南天山地表三维位移场及断层位错模型

利用1992—2012年间西南天山GPS观测和2003—2009年EnviSAT卫星InSAR图像,构建西南天山与塔里木盆地间(喀什坳陷)震间变形的三维位移场,约束区域内滑脱断层运动模型.结果显示:位于喀什坳陷基底与沉积盖层间埋深为12~18km的主滑脱断层进入西南天山(迈丹—喀拉铁克断裂带以北)沿高角度断坡深入天山底部至23~33km,并北倾1°~2°延伸至天山内部,从完全闭锁到自由蠕滑,滑动速率9~10mm·a-1.依据断层位错模型,1902年阿图什M8大地震可能从铁列克断层根部23km左右开始破裂,沿高角度断坡断层扩展25~30km的距离至科克塔木背斜南翼托特拱拜孜—阿尔帕雷克断裂.1902年阿图什地震可能导致阿图什背斜下方埋深2~12km的高角度断坡断层以2~3mm·a-1速率持续蠕滑,蠕滑过程释放的应力等价于一次Mw6.7左右的中强地震,西南天山及喀什坳陷基底滑脱断层控制了西南天山及前陆地带的现今变形和地震活动.

Detachment fault model characterized for the 3D surface displacement field in the southwestern Tian Shan

GPS measurements show that a total of 20 mm·a^-1 north-south convergence across the western Tian Shan (74°E—78°E ) is distributed broadly in the interior of the Tian Shan and on its margins. But there are two competing explanations for how the strain buildup is accommodated by slip on either single fault on the edges or numerous faults throughout. Either the present-day convergence is partitioned essentially by numerous slowly-moving (at rates of a few mm·a^-1) faults with which small to moderate earthquakes were associated, or otherwise is localized primarily on the two flanks of the Tian Shan where the basal detachment faults emerge and large earthquakes occurred infrequently. In order to understand which one is better to characterize the deformation pattern as illustrated by the existing GPS and historic earthquakes, we used GPS observations taken at about 80 campaign sites in 1994—2012 and 220 ENVISAT ASAR acquisitions in 2003—2009 to determine three-dimensional displacement field of convergence deformation in the Kashi depression, southwestern Tian Shan and construct a dislocation model for strain accumulation along the basal detachment fault. We used GAMIT/GLOBK software to process the GPS data collected at the campaign sites in the study area and continuously-tracking stations nearby to calculate daily position time series on the ITRF reference frame, from which site velocities at uncertainties at 0.5~1.0 mm·a^-1 are derived by a linear approximation. The SAR images pairs with baseline length <150 meters and time span <150 days were analyzed with the ROI-PAC software to obtain a series of interferograms that quantify range changes in the satellite line-of-sight (LOS) direction. The LOS rate map at an average uncertainty of 0.7~1.5 mm·a^-1 is derived by stacking all interferograms processed with good coherence. In final, vertical signals in the LOS rate map are separated from horizontal ones by using a plane strain-rate map that is interpolated from all available horizontal GPS velocities with bicubic Bessel functions. Furthermore, we exploit two dimensional edge dislocation in an elastic half-space for two transects normal to the Tian Shan, assuming that surface deformation is caused by an aseismic creeping along a basal detachment fault. The dislocation model is defined by the creeping rate along the detachment fault, dip angle of the fault and locking depth of its updip tip. We exploited a grid-searching in model spaces to determine the optimal model that minimizes the postfit residuals of horizontal GPS velocities and vertical InSAR rates. GPS measurements show that the Kashi depression moves northerly relative to Siberia and a total of 5~7 mm·a^-1 crustal shortening is distrubibted over a 200 km distance from the northwestern Tarim Basin to the southwest Tian Shan. GPS velocities south of the Kashi Anticline show that the Tarim Basin moves at rates of 16~17 mm·a^-1 relative to Siberia. The GPS sites between the Kashi anticline and the Maidan fault, which separates the Kashi degresson from the southwest Tian Shan, slow down in rates by 2~3 mm·a^-1 and are reduced further to 10~11 mm·a^-1 in the southwest Tian Shan. Our results show that almost all InSAR vertical velocities are between -1.5 and 3.5 mm·a^-1 in the Kashi depression and southwest Tian Shan. insignificant tropospheric effects on deformation signals. However, there is a strong correlation between vertical rate and geologic province such as Tieliek range and several paralleled anticlines. Two transects normal to southwest Tian Shan show clearly upward motions at the Kashi-Atux Anticline and the Tieliek Range, between which minor subsidence is observed in. The peak uplift of 3.5 mm·a^-1 at the Tieliek Range is associated with the strain accumulation on the edge of the Tian Shan. Our preferred model shows that a detachment fault with a dip angle of 2°~3° extends down into the Tian Shan. The detachment fault starts creeping at a 9~10 mm·a^-1 rate 23~33 km below the Tieliek range. The geometry of the detachment fault inferred from geodesy is broadly consistent with that imaged by a deep seismic refection profiling for the crustal structures under the Kashi depression, which shows a series of décollement layers under the Tarim Basin and Kashigar depression at 7~17 km depths and the reverse faults that emerge in the range front are all rooted into it. According to the model geometry, the shallow décollements that are locked interseismically are linked with the creeping detachment fault by a steep ramp fault like the Maidan fault. The dislocation model for interseismic strain buildup suggests that the 1902 great Artux earthquake (M>8) may nucleate at ~23 km depth and propagated along a 30°~40° dipping ramp fault to somewhere close to the Keketamu anticline. Slip behavior on the detachment fault under the Kashi Depression and its downdip extension beneath the southern flank of the Tian Shan governs the deformation pattern and seismicity of major earthquakes there. In the interseismic period, the detachment fault and its ramp faults under the Kashi Depression are locked, instead its downdip extension under the Tian Shan is creeping at a rate comparable to the shortening rate across the Kashi depression. Large earthquakes such as the 1902 Artux earthquake released eventually all accumulated stresses thank to the locking of the detachment fault and transferred elastic strain southward along individual ramp faults onto the Keketamu and Artux anticlines. We suggest that the large earthquakes nucleate somewhere around the maximum uplift rate and the maximum gradient in horizontal velocity determined by InSAR and GPS geodesy. At the present, the Tian Shan as a whole is less deformed within its interior and instead significant deformation is occurring on its margins that is growing with the plate convergence between India and the rest of Asia.