Imaging faults at depth: the seismic transport properties of fault zones

地震和断层滑动仍然是相对鲜为人知的现象。造成这种情况的主要原因之一是，地震成核的地壳水平上的断裂带很难观察到。地表大断层的野外测绘可以提供有价值的信息，但它们经常不完全暴露和/或在挖掘过程中遭受持续滑动，从而产生叠印。因此，地震学是研究深部断裂带结构和性质的重要工具之一。它有可能显示断裂带的结构和尺寸、滑动分布、裂缝损害、应力方向和断层流体压力。然而，为了破译断裂带的结构和性质，必须对地震数据进行反演，而这些反演往往会产生不唯一的答案。在这项研究中，我们的目标是将野外测绘、实验室测量和地震实验结合起来，在西班牙南部一个特别暴露和具有特征的断裂带上，以了解地震信号对通过详细测绘和实验室地震测量确定的断层岩石的观测表面结构和物理性质的敏感性。作为联合学生的一部分，我们还将测量从加利福尼亚州圣安德烈亚斯断层3公里深处恢复的岩石的地震性质，这是圣安德烈亚斯深度断层观测站(SAFOD)项目的一部分，该项目最近在帕克菲尔德附近的断层上钻了一个科学钻孔。在钻孔仪器上记录的自然地震活动将提供与实验室测量的比较，并使我们能够通过对另一个主要断裂带的详细分析来扩大工作范围。所有这些数据的结合将大大改善对断裂带地震学控制的理解，从而更清楚地了解深部断裂带的情况。具体地说，我们将详细绘制西班牙东南部卡博内拉斯断层的一部分；一个错开40公里的主要走滑断层，已经从4到6公里深的地方挖掘出来。来自断裂带的样品将在实验室中测量其地震性质，包括P波和S波的速度，地震波的衰减，以及S波的偏振程度。现场数据和实验室数据将结合在一起，创建一个断层带的合成3D模型，在该模型中可能会发生地震事件，并预测产生的地震信号。我们还将在断裂带上进行“活跃”的地震实验，在那里，爆炸产生的受控震源将激发地震波，然后我们可以在断裂带及其周围仔细布设地震台网进行测量。这些实验的信号将有助于表征断层的地下结构，并可与详细测绘和实验室测量计划中的预测信号进行比较。该项目将通过对主要断层的直接观测、对一系列断裂带材料的物理性质的测量以及对断裂带的直接地震测量来提供有关断裂带结构的信息，这些测量可以直接与地表结构进行比较。这些数据不仅将提供从地震数据中了解断裂带结构和性质的关键见解，而且还将对碳氢化合物和采矿行业产生重大兴趣，因为断层控制着地下流体的运动，导致石油和天然气的回收以及热液/输运的断层赋存矿床的分布。

Earthquakes and fault slip are still relatively poorly understood phenomena. One of the principal reasons for this is that fault zones, at the crustal levels where earthquakes nucleate, are very difficult to observe. Field mapping of large faults at the surface can provide valuable information, but they are often incompletely exposed and/or have suffered continued slip and hence overprinting during exhumation. As a consequence seismology is one of the key tools used to investigate fault zone structure and properties at depth. It has the potential to show fault zone structure and dimensions, slip distributions, fracture damage, stress orientations and fault fluid pressures. However the seismic data have to be inverted to decipher fault zone structure and properties and these inversions often yield non-unique answers. In this research we aim to combine field mapping, laboratory measurements and seismic experiments on an exceptionally well-exposed and characterized fault zone in southern Spain in order to understand the sensitivity of the seismic signals to the observed surface structure and physical properties of the fault rocks as determined from detailed mapping and laboratory seismic measurements. As part of a tied studentship, we will also measure the seismic properties of rocks recovered from 3km depth on the San Andreas fault in California as part of the San Andreas Fault Observatory at Depth (SAFOD) project that recently drilled a scientific borehole through the fault near Parkfield. Natural seismicity recorded on borehole instruments will provide comparison with laboratory measurements and allow us to broaden the scope of the work by detailed analysis of another major fault zone. The combination of all these data will provide a greatly improved understanding of the controls on fault zone seismology leading to a clearer picture of fault zones at depth. Specifically, we will map in detail part of the Carboneras fault in southeastern Spain; a major strike-slip fault with 40km offset that has been exhumed from 4 to 6km depth. Samples from the fault zone will have their seismic properties measured in the laboratory, including the P and S wave velocity, the attenuation of the seismic waves, and the degree of the polarization the S waves. The field and laboratory data will be combined to create a synthetic 3D model of the fault zone in which earthquake events may be 'created' and the resultant seismic signals predicted. We will additionally conduct 'active' seismic experiments on the fault zone where controlled seismic sources from explosions will excite seismic waves that we can then measure with a carefully positioned seismic network within and around the fault zone. The signals from these experiments will help characterize the subsurface structure of the fault and can be compared with predicted signals from the detailed mapping and laboratory measurement program. The project will provide information on fault zone structure from direct observation of a major fault, measurements of the physical properties of a range of fault zone materials and direct seismic measurements of the fault zone that can be directly compared with the surface structure. These data will not only provide key insights in understanding fault zone structure and properties from seismic data, but they will also be of significant interest to the hydrocarbon and mining industries, as faults control the movement of subsurface fluids, leading to problems in the recovery of oil and gas, and also distribution of hydrothermally /transported, fault hosted ore deposits.