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.

技术摘要大洋板块通过大洋中脊和洋盆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，水就释放出来。 相对较小的胡安德富卡板块允许有机会完全成像这一过程，从山脊到俯冲带逆冲。 该项目加强了Langseth号研究船的一次主要有源地震巡航，将于2012年夏季穿越Juan de Fuca板块，将有源地震采集扩展到海岸，并部署了另外6个海底地震仪和48个陆上地震仪，以记录海上震源。这将使我们能够将速度模型向海沟的陆地延伸，覆盖锁定区和板块边界部分的上倾边缘，其特征是幕式震颤和滑动。记录的数据将在空间上重叠的两个dennial接收器功能横断面横跨俯冲大逆冲断层，并将使我们能够进行综合分析的短周期和宽带的地震响应的逆冲带结构。通过沿沿着两个走廊取样，一个在华盛顿州上岸，一个在俄勒冈州，我们可以调查的假设，即俯冲带逆冲性质变化沿着走向由于海洋板块的可变水合作用，并探讨在锁定区和在该地区的缓慢滑动，可能会受到影响的俯冲板块水合作用的变化的关系。主动源与现有被动源数据的综合分析将对逆冲带的特征在于大量超孔隙压力或厚变沉积岩俯冲通道的程度施加多波长约束。此外，在俄勒冈州外，我们建议拍摄5条较短的线，以增强和扩大现有的3D覆盖范围的射线路径在一个部分的边缘，显示出强有力的证据，沿走向的异质性，包括证据的俯冲海山影响板间耦合。 古地震资料表明，该段是板块边界地震复发间隔的一个重要转折点。这些假设与2010-2020年地球镜科学计划中的许多“未决问题”密切相关，并解决了GeoPRISMS俯冲周期和动力学倡议的几个激励问题。地球上最大的地震是由俯冲带的逆冲断层引起的，在美国48个相邻的地区，最有可能发生9. 0级地震的地方是卡斯卡迪亚俯冲带，就在俄勒冈州、华盛顿和北方加州的近海。 尽管如此，人们对这些断层带的行为仍然知之甚少，因为它们的物理特征无法在地震发生的深度直接采样。卡斯卡迪亚逆冲带历史地震的稀少进一步模糊了我们对它的理解。地震波提供了检查这些断层性质的主要工具，允许在类似于人体CAT扫描和超声波组合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，并被解释为逆冲断层带要么是非常高孔隙压力的厚区域，要么是由沉积物润滑的区域。在这项研究中，我们利用先前计划的主动源地震勘探卡斯卡迪亚俯冲带近海，重点是成像传入胡安德富卡板块，收集额外的数据，以采样的逆冲带的结构。该项目包括海上和陆地地震仪部署。 我们将把我们在这里获得的断层带结构的信息与在更远的海上获得的传入板块的图像进行比较，以了解板块在进入俯冲带时如何变化。 我们还将把我们的结果与以前使用频率低得多的地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。这一研究将加深我们对断裂带结构、传入和上板块结构与该地区地震危险性之间关系的认识。该项目还强调学生在广泛的现场采集和地震分析技术的培训。