Magnetotelluric Investigation of Subduction Zone Hydrology and Megathrust Slip Behavior at the Nicoya Peninsula

中美洲西海岸是一个会聚的板块边界，经常经历大的俯冲带地震。在哥斯达黎加的尼科亚半岛，观测到了具有显著地面震动的大破坏性地震以及几乎没有地面震动的缓慢滑动事件（SSE）。确定为什么板块边界的某些部分经历了大规模逆冲断裂，而其他部分经历了SSE，这在地震危险性评估中很重要。据推测，断层滑动的风格可能是由地下流体含量的变化，因为流体丰富的区域可以润滑断层，而流体贫乏的区域促进粘滑行为。然而，目前还不清楚究竟是什么因素的组合决定了断层滑动行为或水文和流体含量是否单独负责。为了解决这些关键的未知数，该项目将收集电磁成像数据，以限制地下水文和尼科亚半岛下的流体分布。新的数据将与哥斯达黎加和哥斯达黎加电力研究所的哥斯达黎加合作伙伴合作收集。这些数据将有助于区分造成破坏性地震的条件和促进地震滑动的条件。该项目将支持一名早期职业调查员和一名博士后研究员，他们将领导实地工作和数据分析。来自美国和哥斯达黎加的几名学生志愿者将被招募参加实地考察。学生参与者将在实地考察期间参加一个短期课程，他们将：审查俯冲带过程和尼科西亚半岛的地质情况，了解该项目的目标，并接受培训，以处理和分析他们帮助收集的数据与免费提供的开源软件包。断层破裂类型的物理控制是评估地震危险性的基础，特别是在俯冲带。哥斯达黎加的尼科亚半岛是研究流体如何调节地震行为的理想之地，这是由于地震活动性、大型逆冲断层滑动类型和板块耦合方面的有据可查的变化，此外，鉴于勘测区靠近海沟，板块界面的深度相对较浅。该项目将在大约80个地点部署一组大地电磁仪器，对半岛下面的三维电阻率结构进行成像，并限制流体的分布，从而限制地震带的地质水文学。该项目的数据将与地震活动和断层滑动观测相结合，长期地震和大地测量仪器网络可以很好地分辨这些数据。例如，组合数据将确定导电区域是否与SSE区域相关，电阻区域是否与易于发生破坏性地震的大地锁定区域相关。MT结果还将与先前在勘测区域内取样的地表渗漏的地球化学数据相结合，以确定渗漏处排放的流体的来源和迁移途径，以及这可能如何影响俯冲带前弧的长期挥发性循环。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查进行评估，被认为值得支持的搜索.

The west coast of Central America is a convergent plate boundary that regularly experiences large subduction zone earthquakes. In Nicoya Peninsula, Costa Rica, both large damaging earthquakes with significant ground shaking as well aseismic slow-slip events (SSEs) with little to no ground shaking have been observed. Determining why some parts of the plate boundary experience megathrust rupture, while others experience SSEs is important in seismic hazard assessment. It is hypothesized that the style of fault slip may be driven by variations in subsurface fluid content, since fluid-rich zones can lubricate faults while fluid-poor zones promote stick-slip behavior. However, it is unclear precisely what combination of factors dictate the fault slip behavior or whether the hydrology and fluid content are alone responsible. To address these critical unknowns, this project will collect electromagnetic imaging data to constrain the subsurface hydrology and the distribution of fluids beneath the Nicoya Peninsula. The new data will be collected in collaboration with Costa Rican partners at the National University of Costa Rica and the Instituto Costarricense de Electricidad. The data will help distinguish the conditions that enable damaging earthquakes from conditions which promote aseismic slip. This project will support an early-career investigator and a postdoctoral researcher, who will lead the field work and data analysis. Several student volunteers from the U.S. and Costa Rica will be recruited for the fieldwork. Student participants will attend a short-course during the fieldwork where they will: review subduction zone processes and the geology of Nicoya Peninsula specifically, learn about the goals of the project, and receive training to process and analyze the data they helped to collect with freely available open-source software packages. The physical control(s) on the style of fault rupture is fundamental for evaluating seismic hazards, particularly at subduction zones. The Nicoya Peninsula, Costa Rica, is ideal for studying how fluids modulate earthquake behavior due to well-documented variations in seismicity, style of megathrust slip, and plate coupling, in addition to the relatively shallow depth to the plate interface given the survey area’s proximity to the trench. This project will deploy an array of magnetotelluric (MT) instruments at approximately 80 sites to image the three-dimensional electrical resistivity structure beneath the peninsula and constrain the distribution of fluids and thus the geohydrology of the seismogenic zone. The data from this project will be integrated with seismicity and fault slip observations, which are well-resolved by long-term seismic and geodetic instrument networks. The combined data will determine, for example, if electrically conductive regions correlate with zones of SSEs and resistive regions correlate with geodetically-locked zones prone to damaging earthquakes. The MT results will also be combined with geochemical data from surface seeps previously sampled in the survey area in order to determine the sources and migration pathways of fluids emitted at the seeps and how this may impact long term volatile cycling in subduction zone forearcs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.