海原断裂带"天祝地震空区"断裂浅层蠕滑精细特征的GPS观测研究

Tianzhu Seismic Gap, which is composed of the Laohu Shan - Maomao Shan - Jingqiang He fault segments of the Hanyuan active fault system on the northeastern margin of the Tibetan Plateau, has been paid a lot of attention by scientists at home and abroad for many years, because of its high seismic risk. Recently, a group of French scientists found that a narrow and high velocity gradient across a section of the Laohu Shan fault segment, based on their maps of mean line-of-sight velocity of crustal deformation inferred from the Envisat SAR data, spanning the 2003-2009 period, with advanced data process technology. The narrow and high velocity gradient reveals the existence of a shallow, asismic creep on the fault segment. Because the creep may reduce seismic hazard by releasing stress in the seismogenic part of the crust, this finding is of great significance. However, so far, we know very little about the details of the distribution, rate, mechanism and the time-varying feature of the creep, much less about whether some creeps also exist on the other fault segments of the seismic gap. Thus, in this project, we are planning to obtain a regional crustal deformation field with high spatial and temporal resolution,especially the near-fault crustal deformation characteristics along the fault zone, based on the observations of a dense GPS network. And then, based on the GPS results, as well as the results from other other disciplines, to study and manage to answer the above questions about the creep. At the same time, we are also planning to add the our crustal deformation models to the analysis of the seismic risk of Tianzhu seismic gap.

青藏高原东北缘海原断裂带的老虎山-毛毛山-金强河区段一直是中外学者多年密切关注的"天祝地震空区"。近年来法国学者基于2003-2009时段的Envisat InSAR资料和先进的微构造形变提取技术，发现老虎山区段有显著的形变梯度出现在紧邻断裂的两侧，并由此推测了"断裂浅层无震蠕滑"的存在。鉴于这种断裂浅层蠕滑可释放地壳孕震层的部分应力积累而降低区域地震危险性，因而该发现具有重要意义。但目前对这一蠕滑的具体分布、确切量值、产生机理和时变特征等问题尚不清楚，更不知天祝地震空区其它区段是否存在类似蠕滑。为此，本项目拟通过高精度、高密度的GPS观测，获取天祝地震空区高时-空分辨率的地壳形变场图像，特别是断裂沿线"紧邻断裂两侧"地壳差异运动的时-空变化特征。在此基础上，结合多学科成果资料，研究解答上述蠕滑问题。同时，还将通过加入区域精细地壳形变模型的约束，对天祝地震空区的地震危险进行研究分析。

青藏高原东北缘海原断裂带的老虎山-毛毛山-金强河区段一直是中外学者多年密切关注的"天祝地震空区"。近年来法国学者基于InSAR资料，发现老虎山区段有显著的形变梯度出现在紧邻断裂的两侧，并由此推测了"断裂浅层无震蠕滑"的存在。鉴于这种断裂浅层蠕滑可释放地壳孕震层的部分应力积累而降低区域地震危险性，因而该发现具有重要意义。但目前对这一蠕滑的具体分布、确切量值、产生机理和时变特征等问题尚不清楚，更不知天祝地震空区其它区段是否存在类似蠕滑。为此，本项目通过高精度、高密度的GPS观测和研究，获得了以下重要的观测资料和研究成果：a)在“天祝地震空区”及其周边，针对性地进行了30个GPS观测站的加密布设和观测，由此获得了该区域高精度、高密度的GPS观测资料，为分析研究区域地壳形变特征、断裂运动方式和地震危险性潜势提供了至关重要的地壳形变约束资料;b)创新性地构建了“天祝地震空区”断裂运动与地壳形变关系模型并由此获得各主要活动断裂的现今运动速率及高分辨率的区域“三维”地壳运动速度场，进而获得了比以往传统方法更加合理、可靠的地壳应变率场、最大剪应变率场、面膨胀率场和地震矩累积率场;c)对“天祝地震空区”老虎山断裂可能存在浅层蠕滑的InSAR观测结果进行了近断裂连续GPS观测的独立核实。结果表明：在老虎山断裂西段的近断裂两侧，我们并未观测到InSAR所声称的显著的左旋走滑。因此，老虎山断裂的所谓浅层蠕滑，即便存在，也仅仅局限在东段的局部区域，并不存在整个老虎山断裂的浅层蠕滑;d) 我们的GPS观测分析表明，在“天祝地震空区”西端的2006年青海门源Ms6.4地震之前，孕震区域的皇城-双塔断裂，可能发生了“瞬态预滑移”，以无震方式释放了相当于M6.0的地震矩能量。这一重要研究成果表明，今后在区域的地震危险性分析中，需要充分考虑无震滑移的可能存在及其对潜在发震断层的应力加/卸载作用。