ALOS-2和Sentinel-1形变观测揭示的青藏高原西部郭扎错断层三维运动学特征

Three-dimensional kinematic features of the Gozha Co fault in the western Tibetan Plateau revealed by ALOS-2 and Sentinel-1 deformation observations

龙木-郭扎错断裂系是青藏高原西部甜水海地块的南边界,其运动学特征研究程度较低.本文利用2014~2022年ALOS-2 PALSAR-2长条带雷达数据,采用小基线集时序处理方法,获取了该断裂系中东段的地表形变速率场,发现其现今应变积累主要集中于郭扎错断层,具有明显的左旋走滑特征,且该断层的中东段存在浅部蠕滑现象.随后,结合已发表的高精度Sentinel-1地表形变速率场所构成的雷达干涉测量数据集作为观测约束,通过使用弹性位错模型获取了郭扎错断层的三维滑动速率空间分布.结果表明:整个断层面的滑动速率具有空间异质性,在孕震层的深度范围内,由断层西段的一个长约40 km的强闭锁区和东段的一个长约100 km的无震蠕滑区所组成,其中无震蠕滑区内的平均速率从断层西侧的部分蠕滑区~2.8 mm/a向东逐渐增加至东侧的完全蠕滑区~5.6 mm/a.最后,进一步计算了郭扎错断层的地震矩积累速率,发现浅部蠕滑段内地震矩积累速率较小,约为(3.3±1.9)×10^(16)N m/a,这与地震台站记录的较低水平地震活动性相一致;而浅部蠕滑段以西的区域具有较高的地震矩积累速率,约为(1.2±0.2)×10^(17)N m/a,考虑到缺少微震记录,该段落的地震危险性需要重点关注.

Although faults are thought to be locked by friction during the intervening time between large earthquakes,a subset of continental faults that exhibit aseismic slip in the crust,known as shallow creep,have been documented with the aid of accurate geodetic data.This unusual slip behaviour may control the dynamic processes of earthquake rupture and can help illuminate fault physical properties.Therefore,it has attracted great attention in kinematic and dynamic studies of continental faults over the past decade.Previously,the Haiyuan fault is the only example in the Tibetan Plateau that has been found to exhibit steady-state shallow creep,i.e.creep rate equates to the tectonic loading rate.Our recent study has reported a new shallow creeping fault in the western Tibetan Plateau,the Gozha Co fault,and characterised its creeping rate and temporal evolution based on Sentinel-1 Interferometric Synthetic Aperture Radar(In SAR)data.In this paper,we investigate the Gozha Co fault in greater detail,including its three-dimensional kinematic features and geodetic moment accumulation using a Bayesian inversion approach.We collect JAXA's Advanced Land Observing Satellite-2(ALOS-2)PALSAR-2 data acquired from 2014 to 2022,and derive a rate map covering the central-eastern section of the Gozha Co fault using the Small Baseline Subset(SBAS)method.The ALOS-2 rate map shows that strain is concentrated on the Gozha Co fault and the Longmu Co fault,part of the commonly called Longmu-Gozha Co fault system,has no measurable motion.We then take a velocity profile from the ALOS-2 and Sentinel-1 surface deformation maps to invert fault geometry and creeping rates in order to provide prior information for the subsequent three-dimensional kinematic inversions.Profile inversions show that the Gozha Co fault is a vertical(dip angle 88.1°)left-lateral strike-slip fault(~5 mm/a)with a small thrust component(<1 mm/a).Using these parameters as prior constraints,we derive the three-dimensional slip-rate distribution,which reveals spatial heterogeneity both down-dip and along-strike.Within the seismogenic depths(0–20 km),the Gozha Co fault is composed of a~40-km-long highly locked segment in the west(slip rate less than1.5 mm/a)and a~100-km-long aseismic(creeping)segment in the east(slip rate 1.5–6 mm/a).Shallow creep rate increases from the west(partially creeping zone,~2.8 mm/a)to the east(fully creeping zone,~5.6 mm/a).The Gozha Co fault is located in a region with low P-wave velocity anomalies,which suggests decompression melting within the asthenosphere and magma upwelling,so one possible explanation for the shallow creeping behaviour is that deep fluid migration towards the surface affects crustal rheology and strength,resulting in low frictional strength of the fault.Based on the rate distribution,we estimate the seismic moment accumulation rate.The shallow creeping section has a fairly low seismic moment accumulation rate(~(3.3±1.9)×10^(16)N m/a),consistent with its low seismicity level,so it is unlikely to generate large earthquakes.On the contrary,the highly locked section has a high moment accumulation rate(~(1.2±0.2)×10^(17)N m/a),incompatible with the moment release by 1≤M<6 earthquakes,suggesting that this section is capable of producing moderate to strong earthquakes.These new findings shed light on the kinematics of the Gozha Co fault and the style of present-day deformation in the northwestern Tibetan Plateau.