Imaging the Northern Costa Rica Subducted Slab With Broadband Receiver Functions

使用宽带接收器功能对哥斯达黎加北部俯冲板块进行成像

• 批准号: 0440396
• 负责人: Susan Schwartz
• 金额: $ 6.31万
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-06-15 至 2007-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0440396&HistoricalAwards=false
• 关键词: Imaging; Northern; Costa; Rica; Subducted

This award supports investigation of the hydration state of subducted lithosphere at the Costa Rica convergent margin. Previous studies have suggested above average slab hydration in this region, which implies anomalous dehydration at depth. This is important because slab dehydration plays a critical role in producing arc lavas and in generating intermediate depth earthquakes within the downgoing slab. Slab dehydration at shallower depth may also be important in influencing seismogenic behavior along the plate interface. Metamorphic dehydration reactions in oceanic crust have been implicated in the serpentinization of the forearc mantle wedge and the termination of interplate earthquakes in some subduction zones. The Costa Rica Seismogenic Zone Experiment imaged a distinct pattern of geodetic locking and microseismicity along the plate boundary in northern Costa Rica. The onset of interplate microseismicity occurs down-dip of the region of geodetic locking, coincident with thermal models of the 200-250 degrees Centigrade isotherms. This has been interpreted as a down-dip weakening of the plate interface due to fluid production from low-grade metamorphic reactions in basaltic crust coupled with a decrease in permeability around 250 degrees Centigrade producing elevated pore pressure and sufficient weakening of the thrust interface to permit earthquake failure. The implication of slab dehydration in the initiation of interplate earthquakes has thus increased the importance of determining the state of hydration of subducting lithosphere. P-to-S converted seismic phases, generated at the crust-mantle (Moho) interface of the subducting Cocos slab, recorded by the Costa Rica Seismogenic Zone Experiment broadband seismometers positioned directly above the slab are being analyzed. Teleseismic converted phases provide a direct method to image the structure of the subducted slab, and are being used to determine velocity structure across the oceanic Moho and relating it to the extent of oceanic crustal hydration. In addition, the velocity structure in the vicinity of the plate boundary is being investigated and related to the onset of seismicity and the inferred weakening of the plate interface. The possible role of a large normal fault outer rise earthquake in hydrating the oceanic crust near the Nicoya Peninsula is being assessed. This work is enhancing the understanding of the velocity structure of the subducting lithosphere and plate interface and its material properties.

该奖项支持对哥斯达黎加辐合边缘俯冲岩石圈水化状态的研究。以前的研究表明，该地区的平板水化作用高于平均水平，这意味着在深度处存在异常脱水。这一点很重要，因为板块脱水在产生弧熔岩和在下沉板块内产生中深度地震中起着关键作用。在较浅深度的板块脱水也可能是影响沿板块界面的孕震行为的重要因素。洋壳的变质脱水反应与弧前地幔楔的蛇纹石化和某些俯冲带板块间地震的终止有关。哥斯达黎加发震带实验成像了哥斯达黎加北部沿板块边界的明显的大地测量锁定和微震活动模式。板块间微震活动发生在大地测量锁定区域的下倾，与200 ~ 250℃等温线的热模式一致。这被解释为板块界面的下倾减弱，这是由于玄武岩地壳中低级变质反应产生的流体，再加上渗透率在250摄氏度左右的降低，导致孔隙压力升高，冲断界面的减弱足以导致地震破坏。板块间地震发生时的板块脱水作用，增加了确定俯冲岩石圈水化状态的重要性。在俯冲Cocos板块的壳幔界面（Moho）产生的p - s转换地震相，由位于板块正上方的哥斯达黎加地震带实验宽带地震仪记录，目前正在进行分析。远震转换相提供了一种直接的方法来成像俯冲板块的结构，并被用于确定跨洋莫霍的速度结构，并将其与海洋地壳水化程度联系起来。此外，还研究了板块边界附近的速度结构，并将其与地震活动的开始和推断的板块界面减弱联系起来。正在评估大型正断层外升地震在尼科亚半岛附近海洋地壳水化中的可能作用。这项工作增强了对俯冲岩石圈和板块界面速度结构及其物质性质的认识。