Collaborative Research: Deformation and stress modeling of the 2001 Kokoxili earthquake, western China

这是美国和中国科学家合作研究2001年11月14日中国西部7.8级科克西里地震的变形过程的项目。大地震为了解断裂带和地壳地幔的流变性提供了一个独特的机会。大地震引起的应力变化在断裂带和周围物质中触发了各种过程，随着时间的推移，这些过程反过来又放松了应力。这些过程的时间依赖性与断裂带的应力演化和材料性质直接相关。以往的研究讨论了哪种机制控制了震后变形：断层面上的余滑，特别是脆韧性层之间的过渡深度，还是下地壳和上地幔的粘弹性松弛。已经收集了地震前后的GPS数据。初步震后结果显示出昆仑断裂上强烈的不对称性、时间上的快衰减率和空间上的慢衰减率等较为独特的形变特征，可以直接用模型来区分上述两种形变机制。初步模型结果表明，需要两种机制的结合才能解释数据。详细的研究需要多学科的方法来解决驱动震后变形的同震破裂问题。它们还需要先进的建模方法和程序，以恰当地模拟具有复杂流变性的地壳和上地幔中的应力演化和变形。在本项目中，开发了一个边界元程序作为反演工具。为了更好地了解藏北断裂带和壳幔的流变性，对GPS、InSAR、地质和地震资料进行了震源和构造的分析和反演。对2002年11月3日阿拉斯加科克西里地震和7.9级德纳利断裂地震进行了对比研究。地震规模和断裂机制的相似性以及地球结构的差异揭示了两次地震的孕震过程以及西藏和阿拉斯加的流变性。这项研究也有助于解决关于青藏高原长期变形的长达数十年的争论：与印度碰撞产生的变形是广泛分布的还是块状的。争论的主要焦点是沿着主要走滑断层的断层滑移率，以及断层及其周围地壳和地幔的流变性。震前和震后滑动速率的对比有助于更好地了解地震周期内的应力/应变演化和断层滑动速率变化。断裂带流变学的知识也有帮助，因为被强大的环境地壳包围的弱断裂带通常以块状方式变形，而高韧性的下地壳往往导致广泛分布的变形。

This is a collaborative project between US and Chinese scientists to study the deformation process of the November 14, 2001 MW 7.8 Kokoxili earthquake in western China. Large earthquakes provide a unique opportunity to learn about the rheology of fault zones and the crust and mantle. The stress changes that result from large earthquakes trigger a variety of processes in the fault zone and surrounding material that in turn relax the stress over time. The time dependence of these processes is related directly to the stress evolution in the fault zone and the material properties. Previous studies have debated which mechanism dominates the postseismic deformation: afterslip on the fault plane, particularly in the transition depth between the brittle and ductile layers, or visco-elastic relaxation in the lower crust and upper mantle. GPS data have been collected before and after the earthquake. The initial postseismic result shows rather unique deformation features, such as strong asymmetry across the Kunlun fault, fast temporal and slow spatial decaying rate, which can be directly modeled to differentiate the two deformation mechanisms mentioned above. Preliminary model result shows that it requires a combination of the two mechanisms to explain the data. Detailed studies require a multi-disciplinary approach to solve for the coseismic rupture that drives the postseismic deformation. They also require advanced modeling methods and programs to properly model the stress evolution and deformation in the crust and upper mantle with complex rheology. In this project a boundary element code is developed as the inversion tool. GPS, InSAR, geological, and seismological data are analyzed and inverted for both the source and structure, to better understand the rheology of the fault zone and the crust and mantle in north Tibet. A comparative study is performed between the Kokoxili earthquake and the November 3, 2002 MW 7.9 Denali fault earthquake in Alaska. Similarities in the earthquake sizes and faulting mechanisms and differences in the Earth structures shed light on the seismogenic processes for both earthquakes, and the rheologies of Tibet and Alaska. This study also helps solve a decades long debate about secular deformation of the Tibetan plateau: Is the deformation from the collision with India broadly distributed or block-like. Much of the debate has been focused on the fault slip rates along major strike slip faults and the rheology of the fault and surrounding crust and mantle. Comparison between the pre-quake and post-quake slip rates helps better understand the stress/strain evolution and fault slip rate change over an earthquake cycle. Knowledge of fault zone rheology also helps, because a weak fault zone surrounded by strong ambient crust usually deforms in a block-like manner, while a highly ductile lower crust tends to result in broadly distributed deformation.