Collaborative Research: Imaging the Cascadia Subduction Zone: A Ship-to-shore Opportunity

技术摘要：大洋板块通过洋中脊和洋盆地的蚀变将水以含水矿物的形式储存起来，当这些矿物在俯冲带分解时释放出来。相对较小的胡安·德·富卡板块使我们有机会完整地描绘从山脊到俯冲带逆冲的整个过程。该项目增强了R/V Langseth的主动式地震巡航，该巡航将在2012年夏季穿越Juan de Fuca板块，将主动式地震采集扩展到海岸，并部署了6个海底地震仪和48个陆上地震仪来记录海上震源。这将使我们能够将速度模型向海沟的陆地方向扩展，覆盖以偶发性震颤和滑动为特征的板块边界的锁定带和上倾边缘。记录的数据将在空间上与俯冲大逆冲断层上两个最密集的接收函数横断面重叠，这将使我们能够对逆冲断层带结构的短周期和宽带地震反应进行综合分析。通过对华盛顿州和俄勒冈州两条走廊的采样，我们可以研究由于大洋板块水化作用的变化而导致俯冲带逆冲性质沿走向变化的假设，并探索可能受俯冲板块水化作用变化影响的锁断带和慢滑区域的关系。主动源数据与已有被动源数据的综合分析，将对冲断带以大量超孔隙压力或厚的变质沉积俯冲通道为特征的程度施加多重波长限制。此外，在俄勒冈州附近，我们建议拍摄5条较短的线，以增强和扩大现有的射线路径的3D覆盖范围，该边缘段显示出沿走向非均质性的有力证据，包括影响板间耦合的俯冲海山的证据。古地震资料表明，这一段是板块边界地震复发期的一次重大转变。这些假设与2010-2020年地球视野科学计划中的许多“未决问题”密切相关，并解决了地球地质系统俯冲旋回和动力学计划的几个激励问题。地球上最大的地震来自于俯冲带的逆冲断层，在美国48个相邻的地区，最有可能发生9.0级地震的地方位于卡斯卡迪亚俯冲带，就在俄勒冈州、华盛顿州和加利福尼亚州北部的近海。尽管如此，人们对这种断层带的行为知之甚少，因为它们的物理特征不能在地震发生的深度直接取样。卡斯卡迪亚逆冲带历史上地震的稀少进一步蒙蔽了我们对它的理解。地震波为检查这些断层的性质提供了主要工具，可以在类似于对人体进行CAT扫描和超声波联合扫描的过程中构建这些区域的图像。先前的研究表明，水和沉积物在允许或调节大地震中起着重要作用，并且已经解释表明逆冲断裂带要么是孔隙压力非常高的厚区，要么是由沉积物润滑的区域。在这项研究中，我们利用之前计划的Cascadia俯冲带近海的主动震源地震调查，重点对进入的Juan de Fuca板块进行成像，以收集额外的数据来对冲断带的结构进行采样。该项目包括海上和陆地地震仪的部署。我们将把我们在这里获得的断层带结构信息与离岸更远的地方获得的进入板块的图像进行比较，看看板块在进入俯冲带时是如何变化的。我们还将把我们的结果与以前使用频率低得多的震源获得的图像以及最近经历过非常大地震的其他俯冲带（例如日本东北部和智利中部）的图像进行比较。本文的研究将进一步加深我们对该地区断裂带构造、来盘构造和上盘构造与地震危险性之间关系的认识。该项目还强调对学生进行广泛的现场采集和地震分析技术培训。

Technical AbstractOceanic plates store water as hydrous minerals through alteration at mid-ocean ridges and ocean basins, and release that water as these minerals break down in subduction zones. The relatively small Juan de Fuca plate allows an opportunity to completely image this process, from ridge to subduction zone thrust. This project enhances a major active-source seismic cruise of the R/V Langseth that will traverse the Juan de Fuca plate during the summer of 2012, extending the active seismic acquisition towards the shore and deploying 6 additional ocean bottom seismometers and 48 onshore seismometers to record the offshore sources. This will allow us to extend the velocity models landward of the trench, covering the locked zone and the up-dip edge of the part of the plate boundary that is characterized by episodic tremor and slip. The recorded data will overlap spatially with two of the densest receiver function transects across a subduction megathrust and will allow us to conduct an integrated analysis of the short-period and broad-band seismic response of thrust zone structure. By sampling along two corridors, one coming ashore in Washington and one in Oregon, we can investigate the hypothesis that subduction zone thrust properties vary along strike due to variable hydration of the oceanic plate, and explore relationships both in the locked zone and in the region of slow slip that may be affected by variations in hydration of the subducting plate. The integrated analysis of active-source with coincident existing passive-source data will place multiple-wavelength constraints on the extent to which thrust zones are characterized by substantial excess pore pressure or thick metasedimentary subduction channels. In addition, off Oregon we propose to shoot 5 shorter lines to enhance and extend the existing 3D coverage of ray paths in a segment of the margin that displays strong evidence of along-strike heterogeneity, including evidence for subducted seamounts that impact interplate coupling. Paleo-seismic data indicate that this segment is a major transition in the recurrence interval of plate boundary earthquakes. These hypotheses are closely related to many of the "Outstanding Questions" in the EarthScope Science Plan for 2010-2020, and address several motivating questions of the GeoPRISMS Subduction Cycles and Dynamics Initiative. Non-technical Summary The largest earthquakes on earth arise from the thrust faults of subduction zones, and the most likely place for a magnitude 9.0 earthquake in the 48 contiguous United States lies in the Cascadia subduction zone, just offshore Oregon, Washington, and northern California. Still, little remains known about the behavior of such fault zones, because their physical characteristics cannot be sampled directly at the depths that earthquakes occur. The scarcity of historical earthquakes on the Cascadia thrust zone further clouds our understanding of it. Seismic waves provide the main tool for examining the nature of these faults, allowing to construct images of these regions in a process that is analogous to a combined CAT scan and ultrasound of the human body. Prior studies have indicated an important role for water and sediment in allowing or regulating large earthquakes, and have been interpreted to show that thrust fault zones are either thick regions of very high pore pressure or regions lubricated by sediment. In this study, we take advantage of a previously-scheduled active-source seismic survey just offshore of the Cascadia subduction zone, focused on imaging the incoming Juan de Fuca plate, to gather additional data to sample the structure of the thrust zone. The project includes both offshore and on land seismograph deployments. We will compare the information we obtain on the structure of the fault zone here with images of the incoming plate being obtained farther offshore to see how the plate changes as it enters the subduction zone. We will also compare our results to images obtained previously using much lower frequency earthquake sources and to images from other subduction zones that have recently experienced very large earthquakes (e.g. NE Japan and central Chile). This study will improve our understanding of the relationship between fault zone structure, incoming and upper-plate structure, and earthquake hazards in the region. The project also emphasizes student training across a broad spectrum of field acquisition and seismic analysis techniques.