Testing the Observation of Annual Seismic Velocity Variations at Axial Volcano
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.