Collaborative Research: High-Precision Teleseismic Relocation and Tomography for the M 9 and M 8.7 Sumatra Great Earthquake Sequences

大俯冲带地震造成的地震和海啸危险早已得到认识，但鉴于2004年12月26日苏门答腊岛-安达曼群岛发生9兆瓦地震、由此引发的海啸、基础设施受损和巨大的生命损失，显然这类事件造成的危险仍然被低估。苏门答腊岛大地震是仪器记录中的四次最大地震之一，在许多方面可与1960年智利9.5兆瓦和1964年9.2阿拉斯加俯冲大地震相提并论，后者在太平洋盆地内造成了重大破坏和破坏。苏门答腊岛-安达曼群岛事件直接向南引发了第二次大地震，2005年3月28日发生的8.7Nias地震，加剧了人们对近期内在苏门答腊岛南部海域发生第三次大地震的担忧。这项研究包括对2004年和2005年苏门答腊岛余震序列和先前区域地震活动的一系列高精度地震重新定位，以及苏门答腊岛地区三维纵横向地震波速的计算，以便更好地限制大地震期间沿俯冲大断裂的热、力学、成分、水文和破裂过程的相互作用。2004年和2005年的主震和余震序列产生了全球1700公里俯冲系统地震台网记录的5000多次地震。然而，在苏门答腊岛地区，俯冲巨型逆冲的孕震部分主要位于海洋环境中，这最终限制了将可获得的当地和区域地震数据的数量和覆盖范围，并强调需要对现有的全球数据进行最先进的分析。提高精度的地震目录被用来改进俯冲带几何形状，并探索余震系列内部和之间的时空变化。余震和先前区域地震活动之间的比较提供了对俯冲带内导致这些大地震的过程的洞察，而纵波和横波速度模型与区域地震活动模式之间的比较可能与沿俯冲巨型逆冲的破裂行为的变化有关。该研究分三个阶段进行：1)利用Engdahl、van der Hilst和Buland方法进行概率震相重新确定、波形互相关以减少拾取误差和单次地震定位；2)利用远震双差方法进行高精度相对地震定位；3)利用远震DD层析成像方法对三维区域纵波和横波速度模型进行反演。更广泛的影响包括：1)开发的方法大大推进了使用全球目录和波形数据来研究区域规模的问题，在这些问题中，高空间密度的本地地震仪的部署是不可行的，所有的技术都与公众共享；2)数据分析和解释涉及本科生、研究生和青年研究员与地震定位和层析成像领域的知名专家的合作；以及3)该项目涉及包括万隆技术研究所在内的5个机构，并加强了与印度尼西亚科学界的联系。

The seismic and tsunami hazard posed by great subduction zone earthquakes has long been recognized, but in light of the December 26, 2004 Mw 9+ Sumatra-Andaman Islands earthquake, the resulting tsunami, infrastructure damage and huge loss of life, it is clear that the danger posed by such events remains underestimated. The Sumatra great earthquake is one of the four largest events in the instrumental record, comparable in many respects to the 1960 Mw 9.5 Chilean and 1964 Mw 9.2 Alaska subduction megathrust events that generated significant damage and tsumani within the Pacific ocean basin. The Sumatra-Andaman Islands event triggered a second great earthquake, the March 28, 2005 Mw 8.7 Nias event, directly to the south, increasing concerns about the occurrence of a third great earthquake offshore southern Sumatra in the near-term. This study consists of a series of high-precision earthquake relocation of the 2004 and 2005 Sumatra aftershock sequences and prior regional seismicity and computation of compressional and shear seismic wave speeds in three-dimensions for the Sumatra region in order to better constrain how thermal, mechanical, compositional, hydrological, and rupture processes interact along subduction megathrusts during great earthquakes. The 2004 and 2005 main shock and aftershock sequences generated over 5000 earthquakes recorded globally by seismic networks along 1700 km of subduction system. In the Sumatra region, however, the seismogenic portion of the subduction megathrust lies primarily within an oceanic environment, which ultimately limits the amount and coverage of local and regional seismic data that will become available, and emphasizes the need for state-of-the-art analysis of the available global data. The increased precision earthquake catalog is used to refine subduction zone geometry and to explore spatio-temporal variability within and between the aftershock series. Comparisons between aftershocks and prior regional seismicity provide insight into processes within the subduction zone leading up to these great earthquakes, while comparisons between compressional and shear wave speed models and regional seismicity patterns can be related to variations in rupture behavior along the subduction megathrust. The study builds over three stages: 1) probabilistic phase re-determination, waveform cross-correlation of phases to reduce picking error, and single-event earthquake relocations using the Engdahl, van der Hilst, and Buland method, 2) high-precision relative earthquake locations using teleseismic double-difference methods, and 3) inversion for regional compressional and shear wave speed models in three dimensions using teleseismic DD tomography methods. Among the broader impacts are: 1) The methods developed significantly advance the use of global catalog and waveform data to study regional-scale problems where high spatial density local seismometer deployments are not feasible, and all techniques are shared with the public; 2) Data analysis and interpretation involves undergraduate, graduate, and young investigator collaboration with well-established experts in the fields of earthquake location and tomography; and 3) The project involves five institutions, including the Institut Teknologi Bandung, and strengthens ties with the scientific community in Indonesia.