A high-resolution controlled-source seismic experiment to elucidate geologic controls on megathrust slip: the 2014 Pisagua, Chile earthquake sequence as a natural laboratory

俯冲带是一个板块俯冲到另一个板块下方的地方，是地球上许多破坏性更强的地震和火山的发源地。大量研究表明，在这些地震中，滑动的分布呈斑块状；滑动最大的斑块可以出现在与以无地震变形为特征的斑块沿断层发生的模式相反的模式。2014年4月1日，智利近海皮萨瓜镇附近发生8.2级地震。该地区于1991年被确定为智利的两个主要地震空区之一。由于预计未来将发生大地震，智利国际综合板块边界观测站项目于2007年开始在皮萨瓜/伊基克缺口安装陆上地震和大地测量仪器。由于断层在震前、震中和震后的滑动历史被很好地记录下来，因此其破裂方式可以与地壳结构进行对比。地震前最大的前震和主震期间最大的滑动斑块与局部地下密度的增加有关。此次皮萨瓜余震活动的边界与南部边界范围较广、密度较低的区域重合。在皮萨瓜地震中没有滑动的地震“空隙”部分有一个明显的高密度标志，这表明地壳结构发生了突变。这个项目的目的是收集海洋地球物理数据，以阐明地壳结构和地震滑动之间这种相关性的地质意义。该项目还有一个重要的教育组成部分。来自美国、智利、德国的学生将参加船上的地震反射数据处理课程。大地测量技术的进步和高分辨率地震台网数量的增加，在记录大地震之前、期间和之后俯冲带板块边界断层上的滑动分布方面取得了显著的进展。主震前一个月发生了两次截然不同的前震序列，随后又发生了多次余震，最大余震为7.8级。根据板块构造和大地测量锁定模型，该序列只破裂了缺口的北半部，留下了大量未释放的应变积累。这项研究将采集和分析海洋地震数据，以确定皮萨瓜震源区的特征。由美国海底地震仪仪器池和德国海洋研究所GEOMAR操作的大容量调谐气枪阵列和多通道拖缆(MCS)和90个短周期海底地震仪(OBS)。将得到三维层析P波速度模型和二维深部地震反射剖面的网格。反射边界的速度模型和位置将与高分辨率测深数据相结合，以生成地壳地质模型，该模型可与地震前、地震中和地震后由地震学和大地测量确定的滑动历史相比较。该计划将分两个阶段完成。在第一阶段，成像将针对2014年地震期间滑动的区域，并从剩余空隙获取侦察数据，以优化第二阶段的外业计划。由该地震台阵和陆上台阵记录的地震将被纳入3D层析成像的数据集中，以增加成像的深度范围，并改进S波速度和泊松比的测定。该项目的合作伙伴包括俄勒冈州立大学、GEOMAR和智利大学。

Subduction zones, where one tectonic plate plunges beneath another, are the birthplace of many of Earth's more destructive earthquakes and volcanoes. Numerous studies reveal patchiness in the distribution of slip during these earthquakes; patches with the greatest slip can occur in pattern that is the opposite of where patches characterized by aseismic deformation occur along the fault. On April 1, 2014, a magnitude 8.2 earthquake occurred offshore Chile near the town of Pisagua. This region was identified in 1991 as one of two major seismic gaps in Chile. In anticipation of a future great quake, the international Integrated Plate Boundary Observatory Chile project started installing onshore seismic and geodetic instrumentation in the Pisagua/Iquique gap in 2007. Because the slip history of the fault prior to, during, and after the earthquake was well recorded, the rupture pattern can be compared with crustal structure. The largest foreshock preceding the earthquake and the patch of greatest slip during the mainshock are associated with a local increase in subsurface density. The boundary of the Pisagua aftershock activity coincided with the southern boundary of the broader, regional density low. The portion of the seismic 'gap' that did not slip during the Pisagua quake is marked by a distinct density high, indicating an abrupt change in crustal structure. The objective this project is to collect marine geophysical data to elucidate the geologic significance of this correlation between a crustal structure and earthquake slip. This project also has an important educational component. Students from the US, Chile, Germany will participate in an shipboard course in seismic reflection data processing. Advances in geodetic techniques and an increase in the number of high-resolution seismic networks have brought remarkable advances in documenting the distribution of slip on subduction zone plate boundary faults prior to, during, and after great earthquakes. The main Pisagua shock was preceded by two distinct foreshock sequences that occurred in the month prior to the main shock, and it was followed by many aftershocks, the largest of which had Mw 7.8. The sequence only ruptured the northern half of the gap, leaving significant unreleased strain accumulation according to plate tectonic and geodetic locking models. This study will acquire and analyze marine seismic data to characterize the Pisagua earthquake source region. A large-volume tuned airgun array and multichannel streamer (MCS) and 90 short-period ocean bottom seismometers (OBS) operated by the US Ocean Bottom Seismometer Instrumentation Pool and the German marine research institute GEOMAR. A 3D tomographic P-wave velocity model and a grid of 2D deep-crustal seismic reflection profiles will be obtained. The velocity model and positions of reflective boundaries will be integrated with high-resolution bathymetric data to generate a geologic model of the crust that can be compared to the seismologically and geodetically-determined slip history prior to, during and after the earthquake. The program will be accomplished in two phases. During phase 1, imaging will target the region that slipped during the 2014 earthquake and acquire reconnaissance data from the remaining gap to optimize the field program for phase 2. Earthquakes recorded by this and other OBS and onshore arrays will be incorporated into the data set for 3D tomography to increase the depth extent of imaging and improve determination of S-wave velocities and Poisson's ratio. Partners in this project include Oregon State University, GEOMAR, and the Universidad de Chile.