RAPID: Deep Shear Wave Velocity Profiling for Seismic Characterization of Christchurch, NZ - Reliably Merging Large Active-Source and Passive-Wavefield Surface Wave Methods

2010-2011年，新西兰克赖斯特彻奇市遭受了一系列强烈地震的破坏，其中最具破坏性的是2011年2月22日的Mw6.2克赖斯特彻奇地震。在这次活动中，城市许多地方的建筑环境对地震的要求高于工程设计水平，造成了严重的结构破坏和倒塌，特别是在中央商务区（CBD）。最终，克赖斯特彻奇地震造成181人伤亡，数千人受伤，大面积的土壤液化给建筑物、房屋和基础设施造成了数十亿美元的损失。事件发生后，整个CBD被封锁，直到今天（2012年10月）仍不对公众开放，同时估计有1000座建筑被拆除。在大克赖斯特彻奇地区，一个由19个地震记录站组成的网络在2010-2011年地震期间捕获了大量独特的地面运动（GM）。潜在地，这些GM可以用于反向分析，旨在了解地面震动的空间变异性（特别是场地和盆地效应），然后进行精确的前向估计，旨在量化未来设计GM的幅度和频率内容。由于没有关于克赖斯特彻奇地下超过400米深的层间砂和砾石沉积物的横波速度(v)结构的信息，目前还无法进行详细的GM分析。这项快速响应研究（Rapid）拨款的重点是在新西兰基督城的12-15个关键地点进行深层（400米）Vs剖面分析，以帮助进行重要的地震GM反应分析。这些信息是急需的，因为生物多样性公约的重建计划正在迅速进行，一旦2013年初至中期重建正式开始，拟议的测试将非常复杂（如果不被禁止的话）。经济而快速地获得这些大深度的v值的唯一方法是通过非侵入式表面波测试。然而，目前最常用于深v剖面分析的无源波场技术存在很大的不确定性。因此，将进行一项独特的研究，在扩展的频率/波长范围内比较和合并来自大型有源和无源波场表面波方法的数据，这将允许可靠地确定数据的不确定性和相对偏差。主动源表面波测量将使用一个大型且独特的NEES@UTexas移动伺服液压振动筛和高达48,1 hz的检波器进行，而无源波场数据将使用由10个宽带地震仪组成的中直径和大直径圆形传感器阵列收集。这项研究将使用于深v剖面的大有源和无源波场表面波方法之间的可用比较增加两倍。在获得独立使用无源波场方法的信心之前，需要进行这些比较。因此，这项工作的智能优点包括：(a)收集和解释一个独一无二的数据集，可用于评估合并大型主动源和被动波场表面波方法进行深v剖面分析的可靠性；(b)通过分析来自多个地震事件的一组独特的破坏性GM记录，这些深v剖面使深沉积盆地的精确地震动预测成为可能。在这两个问题上取得的进展将直接影响到美国、新西兰和全世界的地震工程研究。这项工作的广泛影响是多方面的，因为这些研究工作将通过开发强大的地震设计GM来影响基督城社会，这将有助于减轻未来地震中的极端损失。此外，通过分析坎特伯雷地震序列中记录的强GM，对深部沉积物的现场响应有了更多的了解，这将有利于美国深部沉积盆地下的地震活跃地区，如洛杉矶和西雅图。这项工作还将有助于加强美国和新西兰之间的国际研究合作，并将为美国研究生提供有益的国际旅行经验，这将有助于平衡他们的技术教育，并使他们接触到地震工程中仍然存在的全球联系问题。

In 2010-2011, the city of Christchurch, New Zealand was devastated by a series of powerful earthquakes, the most destructive being the 22 February 2011 Mw6.2 Christchurch Earthquake. During this event, the seismic demands imposed on the built environment at many locations in the city were higher than engineering design levels, causing severe structural damage and collapse, especially within the central business district (CBD). Ultimately, the Christchurch Earthquake resulted in 181 casualties, thousands of injuries, and widespread soil liquefaction that caused billions of dollars in damage to buildings, homes and infrastructure. The entire CBD was cordoned-off following this event and remains closed to the public today (October 2012), while an estimated 1000 structures are being demolished. A network of 19 seismic recording stations in the greater Christchurch area captured an extensive and unique set of ground motions (GM) during the 2010-2011 earthquakes. Potentially, these GM can be used for back-analyses aimed at understanding the spatial variability of the ground shaking (particularly site and basin effects), followed by accurate forward-estimates aimed at quantifying the amplitude and frequency content of future design GM. However, detailed GM analyses cannot presently be conducted because no information exists on the shear wave velocity (Vs) structure of the greater-than-400-m deep interlayered sand and gravel deposits that underlie Christchurch. The thrust of this Rapid Response Research (RAPID) grant is to conduct deep (400 m) Vs profiling at 12-15 key sites in Christchurch, New Zealand to aid in important seismic GM response analyses. This information is needed rapidly, as plans for reconstruction of the CBD are proceeding quickly and the proposed testing will be significantly complicated (if not prohibited) once reconstruction begins in earnest in early-to-mid 2013. The only way to economically and rapidly obtain Vs estimates to these great depths is through non-intrusive surface wave testing. However, there is currently a great deal of uncertainty involved in the passive-wavefield techniques most commonly utilized for deep Vs profiling. Therefore, a unique study will be conducted to compare and merge data from large active-source and passive-wavefield surface wave methods over an extended frequency/wavelength range, which will allow robust determination of data uncertainty and relative bias. The active-source surface wave measurements will be conducted using one of the large and unique NEES@UTexas mobile, servo-hydraulic shakers and up to 48, 1-Hz geophones, while passive-wavefield data will be collected using intermediate- and large-diameter circular sensor arrays composed of 10 broadband seismometers. This research will triple the available comparisons between large active-source and passive-wavefield surface wave methods utilized for deep Vs profiling. These comparisons are needed before confidence in utilizing passive-wavefield methods independently can be achieved. Therefore, the intellectual merits of this work include: (a) the collection and interpretation of a one-of-a-kind dataset that can be used for evaluating the reliability involved with merging large active-source and passive-wavefield surface wave methods for deep Vs profiling, and (b) the advancement in accurate ground motion prediction for deep sedimentary basins made possible by these deep Vs profiles through analysis of a unique set of damaging GM records from multiple seismic events. Progress made on both of these issues will directly impact earthquake engineering studies in the US, New Zealand, and throughout the world. The broader impacts of this work are many, as these research efforts will impact Christchurch society at large through development of robust seismic design GM that will help to mitigate such extreme losses in future earthquakes. Furthermore, the additional understanding of site response in deep sediments garnered from analyzing the strong GM recorded in the Canterbury Earthquake Sequence will benefit seismically active areas of the U.S. underlain by deep sedimentary basins, such as Los Angeles and Seattle. This work will also serve to strengthen international research collaborations between the U.S. and New Zealand, and will provide U.S. graduate students with rewarding international travel experiences that will serve to balance their technical education and expose them to the globally-connected problems that still exist in earthquake engineering.