Collaborative Research: Deformation and Stress Modeling of the 2001 Kokoxili Earthquake, Western China

这是中美科学家合作研究2001年11月14日可可西里7.8级地震形变过程的项目。 大地震提供了一个独特的机会，了解断层带和地壳和地幔的流变学。由大地震引起的应力变化在断层带和周围物质中引发了各种过程，这些过程反过来又随着时间的推移而放松应力。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。以往的研究一直争论的机制占主导地位的震后变形：断层面上的后滑，特别是在脆性和韧性层之间的过渡深度，或粘弹性松弛在下地壳和上地幔。 地震前后的GPS数据已经收集完毕。 震后初步结果显示，昆仑断裂带具有较强的不对称性、时间衰减快、空间衰减慢等独特的形变特征，可以直接模拟以区分上述两种形变机制。 初步的模型结果表明，它需要两种机制的组合来解释数据。 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。 它们还需要先进的模拟方法和程序来正确模拟具有复杂流变学的地壳和上地幔的应力演化和变形。 在这个项目中，边界元程序作为反演工具。 通过对GPS、干涉合成孔径雷达、地质和地震资料的分析和反演，从物源和构造两个方面，更好地了解藏北断裂带和壳幔的流变性。 对比研究了2002年11月3日美国阿拉斯加州的Kokoxili地震和Denali断层7.9 MW地震。 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两次地震的孕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几十年来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。 大部分的争论都集中在沿着主要走滑断层的断层滑动速率和断层及其周围地壳和地幔的流变学上。 对比震前和震后的滑动速率有助于更好地理解应力/应变演化和断层滑动速率在地震周期中的变化。 断裂带流变学的知识也有帮助，因为被强环境地壳包围的弱断裂带通常以块状方式变形，而高韧性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.