Collaborative Research: Deformation Processes in the Andaman Islands

通过捕捉2004年苏门答腊-安达曼大地震后的震后变形，该项目继续了六年的努力，以了解瞬态应变和应力的演变。几十项研究已经对地震后的瞬态变形进行了大地测量，但涉及近场变形的主要机制和过程仍然不清楚。这些研究大多得出这样的结论：在地震发生后的最初几周或几个月里发生了少量加速的下倾滑动，但随后的震后变形和隆升主要是由更深的粘弹性流动控制的。然而，最近两年来自安达曼地区的近场数据显示了隆起和右侧剪切，这在现有的粘弹性流动模型中是无法复制的。网络的扩展将优化更大的网络，通过以下方式隔离非线性粘弹性流响应：(1)将网络覆盖范围扩展到时变大地面响应中最大地震后信号的沿走向区域；(2)将间隔较近的站点配对，使我们能够识别叠加在所需流信号上的断层滑动信号。孟菲斯大学的合作者将与班加罗尔印度科学研究所地球科学中心合作，在连续和活动地点进行维护和测量。所有数据将存档于联阿援助团设施，并可从该设施公开获得，该设施也为本项目借出若干接收器。犹他州立大学将把这些测量结果与其他本地和区域GPS网络数据以及GRACE时变重力数据结合起来，模拟该地区瞬态应力和应变的完整演变。我们将改进现有的同震滑动估计，并（单独或一起）检查断层滑动、粘弹性流动和孔隙弹性流动模型，以确定哪些变形过程是观测到的瞬变最可能的机制。地震周期是由地震带内缓慢积累的浅层应力驱动的。这些浅层应力变化是由于断层的滑动和温度较高的深层岩石流动造成的。我们对深层断层滑动和流动过程的理解受到测量深层微小变化的困难的限制，但重要的是，这些过程在大地震之后立即被短暂地加速到可测量的速率。2004年的苏门答腊-安达曼地震是有记录以来的第三大地震，GPS测量表明，它使安达曼和尼科巴群岛的地表向印度方向移动了16英尺，向上或向下移动了3英尺。从那以后，这些岛屿继续快速移动，总共向印度移动了一英尺，向上移动了一英尺。传统的科学观点认为，在地震发生后的最初几周到几个月里，这种运动可能主要是由断层滑动驱动的，但在接下来的几年里，这种运动应该由更深的流动主导。然而，最近两年的测量显示，异常运动与现有的岩石流动模型不一致，但与断层滑动的预测一致。该项目将把GPS和重力数据与新的建模工具结合起来，试图理解这一令人惊讶的观察结果，特别关注深层断层滑动引起的应力变化如何影响驱动深层岩石流动的应力，反之亦然。这个项目吗?S的科学目标对我们对地震物理、地震危险性和整个地震周期应力演变的基本理解具有潜在的深远影响。

By capturing the postseismic deformation following the 2004 Sumatra-Andaman great earthquake, this project continues a six-year effort to understand the evolution of transient strain and stress. Several dozen studies have examined geodetic measurements of the transient deformation that followed the event, but the dominant mechanisms and processes involved in near-field deformation remain murky. Most of these studies conclude that a small amount of accelerated downdip slip occurred in the first few weeks or months following the earthquake, but that subsequent postseismic deformation and uplift are dominated by deeper viscoelastic flow. The most recent two years of near-field data from the Andaman region however exhibit uplift and right-lateral shear that is not replicated in existing models of viscoelastic flow. Expansion of the network would optimize the greater network to isolate nonlinear viscoelastic flow response by (1) extending the network coverage across the along-strike region of largest postseismic signal present in the time-variable geoid response, and (2) pairing up closely-spaced sites, enabling us to fingerprint fault slip signals superimposed on the desired flow signals.U Memphis collaborators will perform the maintenance and measurements at both continuous and campaign sites, in partnership with Center for Earth Sciences, Indian Institute of Science, Bangalore. All data will be archived at, and openly available from, the UNAVCO Facility which is also loaning several receivers for this project. Utah State University will combine these measurements with other local and regional GPS network data, and with GRACE time-variable gravity data, to model the full evolution of transient stress and strain in the region. We will refine existing estimates of the coseismic slip and examine (separately and together) models of fault slip, viscoelastic flow and poroelastic flow to determine which deformation processes are the most likely mechanisms for observed transients.The earthquake cycle is driven by a slow build-up of shallow stress within the earthquake zone. These shallow stress changes result from quiet slip on faults and flow of rocks at greater depths where temperatures are higher. Our understanding of the deeper fault slip and flow processes is limited by the difficulty of measuring small changes at great depths, but importantly these processes are briefly accelerated to measurable rates immediately following large earthquakes. The 2004 Sumatra-Andaman earthquake was the third-largest ever recorded, and GPS measurements indicate that it moved the ground surface in the Andaman and Nicobar Islands by up to sixteen feet toward India, and as much as three feet upward or downward. Since then, the islands have continued moving rapidly, totaling an additional foot toward India and a foot upward. Conventional scientific wisdom holds that the motion may be largely fault-slip driven in the first few weeks to months after an earthquake, but should be dominated by deeper flow in the years that follow. However, the most recent two years of measurements show anomalous motion that is not consistent with existing rock flow models, but would be consistent with predictions for fault slip. This project will combine GPS and gravity data with new modeling tools to try to understand this surprising observation, with special attention to how stress changes resulting from deep fault slip may influence the stress that drives deep rock flow, and vice-versa. The project?s scientific objectives have potentially far-reaching implications for our fundamental understanding of earthquake physics, seismic hazard, and the evolution of stress throughout the earthquake cycle.