Fabrics and mechanics of forearc deformation at the Costa Rican erosive convergent margin and implications for seismogenic fault zone behavior - Follow-up of IODP expedition 334 (Costa Rica Seismogenesis Project, CRISP)

世界上90%的地震都发生在活跃大陆边缘的俯冲带，这些俯冲带要么是增生的，要么是侵蚀的。在增生边缘，来自下行大洋板块的物质，主要是海洋沉积物，在俯冲过程中被构造分离并增生到上部大陆板块。在侵蚀边缘，来自上覆大陆板块的物质，主要是大陆基底和斜坡沉积物，被构造侵蚀并附着于较低的板块。尽管这两种不同类型的俯冲在世界范围内的发生频率大致相同，但迄今为止，bbbb8.5级的大逆冲地震几乎都是在沉积边缘记录的（例如，2010年智利Maule 8.8级地震；2004年印度尼西亚苏门答腊9.0级地震；1960年智利瓦尔迪维亚9.5级地震）。为了更好地了解相关的地震机制，国际海洋钻探计划（IODP）的两个主要项目首次在海底以下约5000-6000米的增积带（日本南开海沟项目NanTroSEIZE）和侵蚀边缘（哥斯达黎加项目CRISP）的发震带进行钻探。陆上洋下板块之间的俯冲通道主要由侵蚀边缘的上板块物质和增生边缘的下板块物质填充。该填料的性能是控制其变形行为的最重要因素之一。如果变形是分布的，它通常是连续的，而如果变形是局部的，它可能是不连续的。在以局部变形为特征的主要逆冲断层上，高摩擦和高凸起可能会锁住断层，从而阻止断层的持续滑动和应变能的释放。当应力积累达到临界阈值时，摩擦力和凹凸不平就会被克服，地震就会形成核。然而，我们假设，只有当断层物质容易发生速度减弱时，大地震和特大地震才会发生，即随着滑动速率的加快，它变得更弱。侵蚀性俯冲通道物质在岩性上变化较大，在大陆前弧楔体中经历了较长的压实和变形历史，而增生性俯冲通道物质主要由较均匀的上层和海沟填充物组成，并不断向下运至发震带。在我们的研究中，我们将通过微观结构分析和岩石力学测试来研究来自哥斯达黎加近海的弧前楔和来自南开海沟以及日本近海的传入板块的输入物质。这些结果应该有助于澄清这些物质输入的差异是否显著，以及它们是否对变形、地震成核和传播的差异至关重要。

Subduction zones at active continental margins, where 90% of the worldwide earthquakes occur, are either accretionary or erosive. At accretionary margins, material from the downgoing oceanic plate, mostly marine sediments, is tectonically detached during subduction and accreted to the upper continental plate. At erosive margins, material from the overriding continental plate, mostly continental basement and slope sediments, is tectonically eroded and attached to the lower plate. Although these two different types of subduction have approximately the same worldwide frequency of occurrence, the Megathrust earthquakes with a magnitude > 8.5 have so far almost exclusively been recorded at accretionary margins (e.g., 8.8 Maule, Chile, 2010; 9.0 Sumatra, Indonesia, 2004; 9.5 Valdivia, Chile, 1960). To better understand the related earthquake mechanisms two major projects of the International Ocean Drilling Program (IODP) endeavor to drill for the first time into the seismogenic zone of an accretionary (Nankai trench, Japan; project NanTroSEIZE) and an erosive margin (Costa Rica; project CRISP) at approximately 5000-6000 meters below sea floor. The subduction channel between the continental upper and the oceanic lower plate is mainly filled with upper plate material at the erosive margin and lower plate material at the accretionary margin. The properties of this infill are among the most important factors controlling the deformation behavior. If deformation is distributed it is usually continuous, whereas if deformation localizes it can be discontinuous. At major thrust faults characteristic of localized deformation, high friction and asperities might lock the fault and hence prevent continuous sliding and strain energy release. When a critical threshold in stress build-up is reached, friction and asperities are overcome and earthquakes nucleate. It is assumed, however, that large and very large earthquakes can only develop if the fault material is prone to velocity weakening, i.e. it becomes weaker with faster sliding rate. The erosive subduction channel material is lithologically rather variable and characterized by a long history of compaction and deformation in the continental forearc wedge, whereas the accretionary subduction channel material mainly consists of more homogeneous pelagic and trench fill deposits which is continuously transported down into the seismogenic zone. In our study we will investigate this input material from the forearc wedge offshore Costa Rica and from the Nankai trench and incoming plate offshore Japan by microstructural analysis and rock mechanial testing. The results should help to clarify if these differences of the material input are significant and if they are crucial for differences in deformation, earthquake nucleation and propagation.