Microearthquakes on the Endeavour Segment, Juan de Fuca Ridge: A Near Real Time Earthquake Catalog for the ONC Cabled Observatory and a Reanalysis of Historical Data

大洋中脊地震虽然没有俯冲带地震那么大，也不像俯冲带地震那样具有破坏性，但对于理解形成洋壳的地质过程仍然很重要。这些地震是由与构造板块伸展有关的断层上的运动或与海底火山过程有关的岩浆活动引起的。地震也是由洋壳热液冷却引起的地壳破裂引起的。自2011年以来，加拿大海洋网络公司在太平洋西北海岸胡安德富卡山脊的奋进中洋扩张中心段运营了一个有线天文台。最初，有线天文台支持两个热液喷口领域的各种仪器，但只有一个地震台。2016年，地震台网扩大到四个台站，2018年夏季还将增加几个台站，以及监测热液喷口气田的更多仪器。这项研究将为奋进有线天文台创建一个近乎实时更新的地震目录。来自自主海底地震仪先前两次实验的地震活动将使用最近的层析成像的三维地震速度模型进行重新定位。科学目标是描述自该地段20年前火山上次破裂以来，随着伸展应力的积累，地震活动是如何演变的，改进对2005年两个复杂震群的解释，限制热液地震活动与岩浆室上方热量吸收之间的联系，并提供一个公开的地震目录，研究人员可利用该目录来调查地震活动与热液场变化之间的时间联系。在更广泛的影响方面，该项目将培训一名博士后研究人员，并促进与研究奋进热液领域的加拿大科学家的国际合作。长期观测站是捕捉形成洋壳上部的海底扩张事件并了解热液系统及其所支持的化学合成生物群落演化的重要工具。自1999年奋进段中央段上一次非喷发式堤防事件以来，已累积了约1m的伸展应变，地震活动速率和潮汐触发水平可能随着当前扩散周期接近顶峰而演变。由于较大的系统定位误差，2005年在奋进号北段和西谷传播者上发生的两个震群很难解释。使用精确的三维模型和双差技术重新定位地震，将使地震与地貌特征相关联，并有助于更好地了解岩浆和构造对这些震群的贡献。轴向岩浆房上方现有地震位置的绝对误差为数百米，用准确的三维速度模型重新定位这些地震，将有助于通过约束地震相对于轴向岩浆室的位置来了解热液吸收。它还可能解决由于从轴向岩浆室开采热量和/或注入新岩浆而导致热液系统加深而导致的热液系统配置的变化。该裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，认为值得支持。

Midocean ridge earthquakes, while not as large and damaging as subduction zone earthquakes, are nevertheless important for understanding the geological processes that form ocean crust. These earthquakes arise due to motion on faults associated with tectonic plate extension or from magmatic activity associated with seafloor volcanic processes. Earthquakes are also generated by crustal cracking that results from hydrothermal cooling of ocean crust. Ocean Networks Canada have operated a cabled observatory at the Endeavour midocean spreading center segment of the Juan de Fuca Ridge off the Pacific Northwest coast since 2011. Initially, the cabled observatory supported a variety of instruments in two hydrothermal vent fields, but just a single seismic station. In 2016, the seismic network expanded to four stations and several more are being added in summer 2018, together with more instruments to monitor the hydrothermal vent fields. This study will create an earthquake catalog for the Endeavour cabled observatory that will update in near real time. Seismicity from two prior experiments with autonomous ocean bottom seismometers will be relocated with three-dimensional seismic velocity models from recent a tomography. The scientific goals are to characterize how the seismicity has evolved as extensional stresses have accumulated since the segment last ruptured volcanically two decades ago, improve the interpretation of two complex earthquake swarms in 2005, constrain the linkages between hydrothermal seismicity and heat uptake above the magma chamber, and provide a public earthquake catalog that can be used by researchers to investigate temporal linkages between seismicity and variations in the hydrothermal fields. In terms of broader impacts, this project will train a postdoctoral researcher and foster an international collaboration with Canadian scientists studying the Endeavour hydrothermal fields.Long term observatories are an important tool for capturing the seafloor spreading events that form the upper oceanic crust and for understanding the evolution of hydrothermal systems and the chemosynthetic biological communities they support. About 1 meter of extensional strain has accumulated since the last non-eruptive diking event on the central Endeavour segment in 1999, and the rates of seismicity and level of tidal triggering are likely to evolve as the current spreading cycle approaches its culmination. The two swarms that occurred in 2005 on the northern Endeavour segment and West Valley propagator were difficult to interpret because of large systematic location errors. Relocating the earthquakes with an accurate three-dimensional model and the double-difference technique will allow earthquakes to be associated with morphological features and facilitate a better understanding of the magmatic and tectonic contributions to these swarms. Existing locations for the earthquakes above the axial magma chamber have absolute errors of several hundred meters and relocating these events with an accurate three-dimensional velocity model will contribute to the understanding of hydrothermal heat uptake by constraining the position of earthquakes relative to the axial magma chamber. It may also resolve changes in the configuration of the hydrothermal system as a result of deepening of the hydrothermal system as heat is mined from the axial magma chamber and/or the injection of fresh magma.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.