| Popis: |
The oceanic crust, the uppermost layer of the rigid oceanic lithosphere, has a well defined general architecture, but the magnitude and wavelength of heterogeneities in crustal structure remain poorly observed in detail. In this thesis, I have characterized the seismic structure of the marine sediments and crust surrounding the Hawaiian islands.I primarily analyzed seafloor compliance data, the deformation of the seafloor in response to infragravity wave loading. These long-period (50+ seconds) measurements are estimated from broadband ocean-bottom seismic data and are sensitive to the elastic structure of oceanic sediments and crust, particularly shear-velocity structure. I produced a map of sediment thickness and shear velocity, which includes high-velocity volcanic sediments and low-velocity pelagic sediments. I then used surface wave dispersion data to constrain the structure of the lower crust and the uppermost mantle beneath the Hawaiian Islands. Initial results indicate that the uppermost mantle may exhibit variations in shear velocity on the order of several percent. These variations could be explained through a combination of thermal anomalies and partial melting. Another focus of this thesis is the analysis of seafloor compliance data as a function of time to investigate melt content in the lower crust before and after the submarine eruption of 2015 at Axial Volcano. I found that Vs dropped dramatically following the eruption and took several years to recover to pre-eruption levels. The changes can be explained by variations in the geometry of melt. The station distribution also allowed me to constrain the lateral extent of the variations to within several hundred meters of the center of the caldera. In the course of these studies, I developed tools and theory to improve OBS data quality. I wrote automated software to determine the orientation of horizontal seismometer components using surface wave arrival angles, and spent considerable time characterizing the long-period response of differential pressure gauges. Finally, I demonstrated the viability of measuring horizontal seafloor compliance, although I was limited in my geologic interpretations due to instrument response issues and data quantity. The techniques and results developed in this thesis should provide insight into the dynamics of the oceanic lithosphere, including the mechanics of intraplate plumes and crustal accretion. These results will become more valuable as OBS instrumentation progresses and expands data coverage into the furthest reaches of the world's oceans. |
