Seismic Trenching for the Pulse of the Earthquake Engine

美国犹他大学地质与地球物理系工程师ronald L. Bruhn and Gerard schuster项目摘要：本研究项目旨在研究地震是如何在正断层上发生的。地震在空间和时间上的重复模式提供了地震引擎的“脉冲”。研究这些模式揭示了地球内部相互作用产生地震的复杂过程。成功地理解过去心跳的本质是预测未来产生地震的大断层行为的关键。到目前为止，在断裂带上挖沟一直是揭示过去地震历史的主要工具。通过仔细记录海沟中的沉积层，地质学家已经成功地推断出过去地震的大小和复发间隔。然而，挖沟法有三个缺点：成本昂贵且对环境有害，只能开采30英尺左右的深度，而且只能得到断层的垂直剖面。这些限制意味着地震心跳在空间和时间上的采样都很差，这可能导致关于地震性质的错误结论。为了克服这些问题，并回答关于正常断层的心跳的基本问题，犹他大学的地球科学家正在奥奎尔、斯坦斯伯里和瓦萨奇断层的断层陡坡上进行二十多个浅层地震实验。这是位于犹他州瓦萨奇前缘的三个大型正常断层带。记录到的地震波第一次到达的时间产生了速度层析图，或图像，描绘了沉积“崩塌”楔形的浅层和深层模式，这些楔形是在过去的地震中由于地球表面的偏移而形成的。当确定后，用一台强大的Odex钻头在深度为50米左右的崩塌楔中取芯，并通过各种测年方法对楔周围和楔内的沉积物样本进行测年。通过这一过程，科学小组将我们对地震行为的认识扩展到了10万年以后，并显著延长了瓦萨奇断层和东部盆地和山脉省其他断层的地震历史。这种地震层析成像、钻井和测年方法的结合被称为地震挖沟。本研究确定的扩展地震历史为预测该地区未来的地震历史提供了新的信息，并为建立新一代断裂过程的力学模型提供了新的信息。该研究对地震灾害应用领域和地震力学基础科学具有重要影响。

EAR-0207372: SEISMIC TRENCHING FOR THE PULSE OF THE EARTHQUAKEENGINERonald L. Bruhn and Gerard SchusterDepartment of Geology and GeophysicsUniversity of UtahSalt Lake City, UT 84112-01 11PROJECT ABSTRACTThe purpose of this research project is to study how earthquakes occur on normal faults. The patterns of earthquake recurrence in space and time provide the 'pulse' of the earthquake engine. Investigating these patterns is revealing the complex processes that interact to generate earthquakes within the earth. Successfully understanding the nature of past heartbeats is the key to predicting the future behavior of large faults that generate earthquakes.Until now, trenching across fault scarps has been the primary tool for revealing the past history of earthquakes. By carefully logging the sedimentary deposits in trenches, geologists have been successful in deducing the sizes and recurrence intervals of past earthquakes. However, there are three shortcomings with the trenching method: it is expensive and environmentally intrusive, it is limited to depths no deeper than about 30 feet, and yields only a vei7tical cross-section of the fault. These limitations mean that the earthquake heartbeat is poorly sampled in both space and time, which can lead to erroneous conclusions about the nature of earthquakes. To overcome these problems, and to answer fundamental questions about the heartbeat of normal faulting, University of Utah earth scientists are conducting more than two dozen shallow seismic experiments over fault scarps in the Oquirrh, Stansbury and Wasatch faults. These are three large normal fault zones located in the Wasatch Front of Utah. The first arrival times of the recorded seismic waves yield velocity tomograms, or images, that delineate both the shallow and deep patterns of sedimentary 'colluvial' wedges that formed in response to the offset of the earth's surface during past earthquakes. When identified, the colluvial wedges to a depth of 50 m or so are cored by a powerful Odex drill, and samples of the deposits surrounding and within the wedges are dated by a variety of dating methods. By this procedure the science team is extending our knowledge of earthquake behavior beyond 100,000 years, and significantly extending the history of earthquakes along the Wasatch Fault and other faults in the eastern Basin and Range Province. This combined use of seismic tomography, drilling and dating methods is referred to as seismic trenching.The extended earthquake history being determined during this study provides new information for predicting the future history of earthquakes in the region, and for creating a new generation of mechanical models of processes of faulting. The research has significant impact on the applied field of earthquake hazards and the basic science of earthquake mechanics.