Mechanical properties of shallow fault rock in erosive margins: Influence on earthquake rupture propagation and slow slip events

汇聚边缘的板块边界逆冲断层会产生最大、最具破坏性的地震和海啸。世界上超过一半的俯冲带是侵蚀边缘，包括日本海沟和哥斯达黎加边缘。在日本海沟发生的Mw = 9.0级的东北地震表明，侵蚀性俯冲带能够产生大地震和海啸。俯冲带的浅层部分之所以重要，有两个原因：(1)它们可能容易发生地震破裂和同震滑动传播；(2)它们可能产生慢震。对俯冲带发震深度的断裂带的直接观测、监测和采样尚未实现，但在主要逆冲断层内部和附近的浅层（~1 km或以下）已经恢复了岩心样品。从钻孔应变计和GPS数据中也有证据表明哥斯达黎加近海存在浅层慢滑事件。缓慢地震也发生在日本海沟的浅层近海沟区域。因此，从描述良好的俯冲带环境中获得的可用自然样本，如日本海沟和哥斯达黎加，非常适合研究浅层地震破裂传播和慢滑事件。描述主要断裂带的滑动行为在很大程度上依赖于实验室摩擦实验的结果。拟议的研究计划使用IODP远征343 （Jfast）在日本海沟和ODP Leg 170和205在哥斯达黎加近海尼科亚半岛地区回收的岩心样本，以测量摩擦强度和滑动稳定性。测量将涵盖从上覆沉积物棱镜到滑脱和日本海沟逆冲沉积物的垂直样带的岩性变化。进一步的实验将检验空间变化的岩性对哥斯达黎加进入板块的影响。将使用低滑动速度（<1厘米/秒），这对于预测断层带对地震传播的反应至关重要。测试计划的一个重要部分将是测量断层泥和围岩的内聚强度，这是地球物理学界尚未探索的，但可能是一个关键参数。将包括精确模拟自然界缓慢地震的其他实验。通过x射线衍射（XRD）和WLI对剪切试样进行详细的矿物学成分分析和物理表征，本研究将得到一套全面的摩擦特性测量结果，揭示侵蚀俯冲带浅层部分地震传播和慢滑事件发生的机制。

Plate boundary thrust faults at convergent margins generate the largest and most destructive earthquakes, and tsunamis. Over half of worldwide subduction zones are erosional margins, inlcuding the Japan Trench and Costa Rica margins. The Mw = 9.0 Tohoku event in the Japan Trench demonstrates that erosive subduction zones are capable of generating great earthquakes and tsunamis. The shallow portions of subduction zones are of great importance for two reasons: (1) they may be prone to earthquake rupture and coseismic slip propagation and (2) they may generate slow earthquakes. Direct observation, monitoring and sampling of fault zones at seismogenic depths in subduction zones has yet to be achieved, but core samples from within and adjacent to major thrust faults have been recovered from shallow depths (~1 km or less). There is also evidence of shallow slow slip events offshore Costa Rica from borehole strainmeters and GPS data. Slow earthquakes also occur in the shallow near-trench region of the Japan Trench. Therefore, available natural samples obtained from well-described subduction zone settings, such as the Japan Trench and Costa Rica, are ideally suited for studying shallow earthquake rupture propagation and slow slip events. Describing the slip behavior of major fault zones relies heavily on the results of laboratory friction experiments. The proposed research intends to use core samples recovered during IODP Expedition 343 (Jfast) to the Japan Trench and ODP Leg 170 and 205 to the Nicoya Peninsula region offshore Costa Rica, in order to measure frictional strength and sliding stability. Measurements will cover lithologic variations in a vertical transect from the overlying sediment prism through the decollement and underthrust sediments in the Japan Trench. Further experiments will examine the effect of spatially varying lithology on the incoming plate in Costa Rica. Low slip velocities (<1 cm/s) will be used, which are critical for predicting the response of a fault zone to earthquake propagation. An important part of the testing program will be measurements of cohesive strength in both fault gouge and wall rock, which remains unexplored by the geophysical community but is a potentially key parameter. Additional experiments will be included which accurately mimic slow earthquakes in nature. Supplemented by detailed analyses of mineralogic composition by XRD and physical characterization of the sheared specimens by WLI, the proposed research will result in a comprehensive set of frictional property measurements revealing the mechanisms behind earthquake propagation and the occurrence of slow slip events in the shallow portions of erosive subduction zones.