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

合作研究：卡斯卡迪亚俯冲带成像：船到岸的机会

• 批准号: 1147975
• 负责人: Anne Tréhu
• 金额: $ 19.06万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-05-01 至 2015-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1147975&HistoricalAwards=false
• 关键词: Collaborative; Research; Imaging; Cascadia; Subduction

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.

技术摘要曲板通过在中山脊和海盆中的改变将水作为含水矿物存储，并在俯冲带中释放水。 相对较小的Juan de Fuca板允许有机会完全对此过程进行成像，从山脊到俯冲带推力。 该项目增强了R/V Langseth的主要主动源地震巡游，该巡航将在2012年夏季穿越Juan de Fuca板块，将主动地震采集扩展到海岸，并部署了6个额外的海洋底部地震尺和48个陆上地震仪，以记录离岸资料。这将使我们能够延长沟槽的速度模型，覆盖锁定区域的锁定区域和板边界部分的上浸边缘，其特征是以情节性震颤和滑动为特征。记录的数据将在俯冲巨像上与两个最密集的接收器函数在空间上重叠，并使我们能够对推力区域结构的短期和宽带地震响应进行综合分析。通过沿着两个走廊进行采样，华盛顿的一个岸上，一个在俄勒冈州进行采样，我们可以研究以下假设：俯冲带推力特性因大洋板的水合变化而变化，并探索锁定区域和慢速滑移区域的关系，这可能会受到覆盖板的变化的影响。与一致的现有被动源数据进行的活动源的综合分析将在推力区域以大量过大的孔隙压力或较厚的沉积物俯冲通道为特征的程度上对多波长约束。此外，在俄勒冈州，我们提议射击5个较短的线路，以增强和扩展边缘一段的现有3D覆盖范围，以显示出沿撞击异质性的有力证据，其中包括影响撞击板偶联的俯冲封口的证据。 古理性数据表明，该部分是板边界地震复发区间的主要过渡。这些假设与2010 - 2020年《地球科普科学计划》中的许多“杰出问题”密切相关，并解决了地理上主义俯冲周期和动态计划的几个激励问题。非技术总结地球上最大的地震源于俯冲带的推力断层，也是48个连续美国发生9.0次地震的最有可能的地方。 尽管如此，对于此类断层区域的行为，尚不了解，因为它们的物理特征不能直接在地震发生的深处进行采样。卡斯卡迪亚推力区域上历史地震的稀缺性进一步掩盖了我们对它的理解。 地震波为检查这些断层的性质提供了主要工具，从而使这些区域的图像在类似于猫扫描和人体超声波的过程中构造了这些区域的图像。先前的研究表明，水和沉积物在允许或调节大地震方面起着重要作用，并被解释为表明推力断层区是孔隙压力很高的厚区域，或者是被沉积物润滑的区域。在这项研究中，我们利用了先前安排的活跃源地震调查，仅在卡斯卡迪亚俯冲带的海上，侧重于成像传入的Juan de Fuca板，以收集其他数据，以采样推力区域的结构。该项目包括离岸和土地地震仪的部署。 我们将在此处比较有关断层区结构的信息与在近海中获得的传入板的图像，以查看板进入俯冲带时的变化。 我们还将将我们的结果与先前使用较低频率地震来源获得的图像进行比较，以及来自最近经历过非常大地震的其他俯冲带的图像（例如日本和中央智利）。这项研究将提高我们对该地区断层区结构，进入板结构和地震危害之间关系之间关系的理解。该项目还强调了各种现场获取和地震分析技术的学生培训。