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

这是中美两国科学家的合作项目，旨在研究2001年11月14日发生在中国西部科西利的里氏7.8级地震的变形过程。大地震为了解断裂带、地壳和地幔的流变学提供了一个独特的机会。大地震引起的应力变化触发了断层带和周围物质的各种过程，这些过程反过来又随着时间的推移而放松了应力。这些过程的时间依赖性与断裂带应力演化和材料性质直接相关。以前的研究一直在争论哪一种机制主导了震后变形：是断层面的余震，特别是在脆性层和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。在地震前后都收集了GPS数据。最初的震后结果显示出昆仑断裂上强烈的不对称性、时间衰减速度快而空间衰减速度慢等较为独特的变形特征，可以直接建立模型来区分上述两种变形机制。初步的模型结果表明，需要两种机制的结合来解释数据。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。它们还需要先进的建模方法和程序来正确地模拟具有复杂流变的地壳和上地幔的应力演化和变形。在这个项目中，开发了一个边界元代码作为反演工具。利用GPS、InSAR、地质和地震资料对震源和构造进行了分析和反演，以更好地了解藏北断裂带和壳幔的流变学。对科科西利地震和2002年11月3日发生在阿拉斯加的7.9级地震进行了对比研究。地震规模和断层机制的相似性以及地球结构的差异，揭示了这两次地震的发震过程，以及西藏和阿拉斯加的流变学。这项研究还有助于解决长达数十年的关于青藏高原长期变形的争论：与印度碰撞产生的变形是广泛分布的还是块状的？大部分争论都集中在沿主要走滑断层的断层滑动率和断层及其周围地壳和地幔的流变学上。对比震前和震后滑动率有助于更好地理解一个地震周期内的应力应变演化和断层滑动率变化。对断裂带流变学的了解也有帮助，因为被强周围地壳包围的弱断裂带通常以块状方式变形，而高度延展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.