Testing the Observation of Annual Seismic Velocity Variations at Axial Volcano

位于东北太平洋Axial火山的海洋观测站倡议电缆阵列为在深海海底环境中进行长时间序列地震测量提供了前所未有的机会，而这一测量以前受到自主海底地震仪部署能力的持续时间的限制。在陆地上，地壳地震速度的年度变化与温度、地下水或积雪/冰层的季节性变化有关。在深海，不应该存在这样的季节性变化，但最近在Axial火山使用电缆海底地震仪网络进行的观测发现，地震速度每年都会发生变化。该项目将通过查看附近其他环境(例如水合物岭)的海底电缆数据来调查深海观测的稳健性。假设要么是噪声源的特性有季节性变化，足以产生观测到的速度差，要么是有某种地球物理过程，如裂缝张开，对每年的强迫做出响应。这些结果可能要么表明，目前陆地观测中关于季节变化的假设需要重新考虑，要么表明地壳可能由于某些外部强迫而变形，例如每年的固体地球潮汐力量。这两种方法都会对现场产生重大的、更广泛的影响，以及如何解释环境噪声结果。该项目还将支持研究生论文的一个章节。环境噪声可以通过地震速度的变化来测量地球物理性质的年度变化。在两个地震仪上记录的连续环境噪声的相关函数与两个地震仪之间介质的格林函数成正比。然后可以利用这一点来成像地球地下的速度结构。Axial的速度年变化的证据表明，无论是噪声源特征的季节性变化，足以产生观测到的速度差，还是存在某种地球物理过程，如裂缝张开，都是对年度强迫的响应。该项目将通过查看其他环境(例如水合物岭)中的海底电缆数据来测试观测的稳健性，并通过限制速度变化的深度范围来调查这两种选择。该奖项反映了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，被认为值得支持。

The Ocean Observatory Initiative cabled array at Axial Volcano in the northeast Pacific provides an unprecedented opportunity to conduct long time-series seismic measurements in the deep seafloor environment, previously limited by the duration of autonomous ocean bottom seismograph deployment capabilities. On land, annual variations in crustal seismic velocities have been tied to seasonal changes in temperature, ground-water, or snow/ice pack. In the deep sea, no such seasonal variations should exist and yet recent observations at Axial Volcano using the cabled ocean bottom seismograph network find annual seismic velocity variations. This project will investigate the robustness of the deep sea observations by looking at seafloor cabled data from other nearby settings (e.g. Hydrate Ridge). It is hypothesized that either there is a seasonal change in the characteristics of the noise source large enough to generate the observed velocity difference, or there is some geophysical process, such as crack opening, that is responding to annual forcings. The results are likely to either demonstrate that current assumptions about seasonal variability in terrestrial observations need to be revisited, or that the crust may be deforming due to some external forcing such as annual solid earth tidal forces. Both have significant broader impacts for the field, and how ambient noise results might be interpreted. The project will also support one chapter of a graduate student's thesis.Ambient noise can be used to measure annual changes in geophysical properties through changes in seismic velocity. The correlation function of continuous ambient noise recorded at two seismometers is proportional to the Green's function of the medium between the two sensors. This can then be utilized to image the velocity structure of Earth's subsurface. Evidence for annual changes in velocity at Axial, which are consistent over 3 years of recording suggest that either there is a seasonal change in the characteristics of the noise source large enough to generate the observed velocity difference, or there is some geophysical process, such as crack opening, that is responding to annual forcings. This project will investigate both these options by looking at seafloor cabled data in other settings (e.g. Hydrate Ridge) to test the robustness of the observation, and by constraining the depth extent of the velocity changes.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.