Core to regional scale synthesis of fault zone properties and fluids at subduction zones: Drivers of seismogenic behaviour

世界上大多数的大地震都发生在两个板块交汇的俯冲带板块边界断层上（例如2004年、2005年和2007年的苏门答腊地震；2010年的智利地震）。因为这些断层在地震中移动的部分位于水下，它们也可能是大海啸的源头。然而，不同的俯冲带发生的地震规模不同，地震破裂方式也不同，因此其危险性差别很大。在大多数情况下，板块边界断层的破裂仅限于断层位于海床下~30-40公里至~5-15公里的区域，但在某些情况下，断层破裂被认为范围更广，可能已经到达海床。在其他情况下，有时可以看到断层的移动更加缓慢，没有地震。在其他情况下（例如，日本近海的南开边缘），主板块边界断层的运动受到增生棱镜内断层的影响，这些断层是由于沉积物从下行板块上刮下来而形成的，这些断层可能会滑动，影响所产生的海啸波的大小。在已知的俯冲带了解这些危险的最后一个主要问题是，最大的地震通常每几百年才发生一次，因此我们对地震影响的记录是非常不完整的。这些不同的断层行为取决于断层本身的物理性质，由俯冲带附近的海底沉积物和断层内流体的存在等因素控制。确定这些性质和流体存在的一种方法是钻入断裂带，并使用“测井”技术直接取样或测量岩石性质；这种方法已经在世界各地的几个地方进行过，但即使使用最先进的技术（立管钻探），也只能在断层的较浅部分进行，并且只能在一个地点产生一组有效的观察结果。在俯冲带的许多不同地方进行的钻探以及相关的地震实验（从地球内部的结构上反射声波）表明，这些特性在一个区域内和不同区域之间都是非常多变的。这加强了钻井（提供局部详细信息）和地震数据（提供区域信息）的结合应该是评估断层性质及其潜在危害的主要方法：该项目采用的技术。我们的目标是通过结合来自世界各地几个俯冲带的地震和钻井数据，更好地了解俯冲带断层的行为。我们选择了沉积物厚度不同的地区，在那里，已知的断层活动以及由此产生的地震的类型和大小各不相同。我们将利用钻井数据来提高我们从地震数据中解释同一位置断裂带性质和流体含量的能力。然后，利用地震解释将我们对断裂带的了解扩展到更广泛的区域，我们将研究板块边界断层的变化。我们将使用相同的方法来研究主板块边界断层与增生棱镜内较小断层之间的关系。然后，我们将把我们的分析扩展到已收集地震数据但尚未钻探的地区，包括苏门答腊岛和新西兰的近海边缘。该项目产生的结果将使这些新的边缘（苏门答腊和新西兰）的钻探更加有效，从而从这些俯冲系统的剖面中获得新的样本和测量结果，这对地震的发生具有最重要的意义。最终，我们将把对板块边界断层状态的解释与已知的地震行为和海啸历史联系起来，旨在改进对其他不知道长期行为的地方的危害的评估。

Most of the world's large earthquakes happen on the plate boundary faults at subduction zones where two plates converge (e.g. Sumatra in 2004, 2005, and 2007; Chile in 2010). Because the parts of these faults that move during the earthquake lie underwater, they can also be the source of major tsunami. However, different subduction zones are subject to different sizes of earthquakes, and different patterns of earthquake rupture, so that the hazards vary significantly. In most cases rupture on the plate boundary faults is limited to a zone where the fault lies from ~30-40km up to ~5-15km beneath the seabed, but in some cases the fault rupture is thought to have been much more extensive and potentially to have reached the seabed. In other cases the faults are sometimes seen to move more gradually, without an earthquake. In other cases (e.g., Nankai margin offshore Japan), movement on the main plate boundary fault is affected by faults within the accretionary prism, that forms as sediment is scraped off the downgoing plate, and these faults may slip affecting the size of the tsunami waves generated. A final major problem with knowing these hazards at a given subduction zone is that the biggest earthquakes normally only occur every few hundred years, so that our records of the effects are very incomplete. These different fault behaviours depend on the physical properties of the faults themselves, controlled by the seabed sediments adjacent to the subduction zone, and factors such as the presence of fluids within the fault. One way to determine these properties, and presence of fluids, is to drill into the fault zone and directly take samples or measurements of the rock properties using 'logging' technology; this has been done in several places round the world, but even with the most modern technology (riser drilling), it is only possible in the shallower parts of faults, and generates a set of observations effectively at a single location. Drilling at a number of different places on subduction zones together with associated seismic experiments (that bounce sound waves off structures within the earth) show that these properties are very variable, within a single region, and between regions. This reinforces that the combination of drilling (providing local detailed information) and seismic data (providing regional information) should be the primary method for assessing fault properties and their hazard potential: the technique employed in this project. We aim to better understand the behaviour of subduction zone faults by combining seismic and drilling data from several subduction zones around the world. We have chosen regions which have contrasting thicknesses of sediments, and where known fault activity and type and size of resulting earthquakes vary. We will use the drilling data to increase our ability to interpret the properties and fluid content of the fault zones from seismic data at the same location. Then using the seismic interpretations to extend our knowledge of the fault zones over much broader regions, we will investigate variations both down and along the plate boundary fault. We will use the same methods to investigate the relationship between the main plate boundary fault and smaller faults within the accretionary prism. We will then extend our analysis to regions where seismic data have been collected, but which have not yet been drilled, including margins offshore Sumatra and New Zealand. The results generated by the project will allow drilling on these new margins (Sumatra and New Zealand) to be targeted more effectively, thus obtaining new samples and measurements from the sections of these subduction systems with greatest significance for earthquake generation. Ultimately we will relate the interpretations of the state of the plate boundary faults to the known earthquake behaviour and tsunami history, aiming to improve assessments of the hazards at other locations where the long-term behaviour is not known.