S-Wave Anisotropy under Axial Seamount
Abstract
Axial Seamount is a submarine volcano located at the intersection of the Juan de Fuca Ridge and the Cobb-Eickelberg hotspot. The seamount is characterized by a shallow caldera that is elongated in the NNW-SSE direction. Two eruptive events in 1998 and 2011 motivated the deployment of a real time cabled observatory as part of the Ocean Observatories Initiative that includes a network of seven seismometers that span the southern half of the caldera. Five months after the observatory came on-line, the volcano erupted on April 24, 2015. Well over 100,000 earthquakes have been located in the vicinity of the caldera, delineating an outward dipping ring fault that extends from near the surface to the magma body at 2 km depth and which accommodates inflation and deflation of the volcano.
We are applying shear-wave splitting analysis to measure the spatial and temporal patterns of anisotropy under the main caldera. Our goal is to understand the alignment of cracks and faults and search for a time-dependent component of anisotropy related to the inflation and deflation of the caldera around the 2015 eruption. All the earthquakes have been relocated using three-dimensional P and S wave velocity models and we limit our analysis to records with both P and S wave picks. Using Python and the Obspy package, we have adapted the workflow of Savage et al. (2010). We first estimate the ray incidence and rectilinearity from P wave polarization and if this suggests that the record is suitable for analysis, we use the method of Silver and Chan (1991) to estimate the fast polarization direction and lag time of the S wave. Preliminary results using a subset of 4000 earthquakes with near-vertical rays, reveal a complex anisotropy structure under the caldera. Stations located on the western flank and inside the caldera have fast-directions parallel to the elongated NNW-SSE portion of the ring fault, whereas stations located on the eastern flank have fast-directions orthogonal to the ring fault. Average delay times are similar for most of stations (0.035 s), but station AXAS1 in the south caldera has significantly higher delay times (0.08 s). The spatial patterns of splitting suggest that anisotropy may be influenced both by the ring fault and by the asymmetric pattern of volcano inflation.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43G0209B
- Keywords:
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- 0450 Hydrothermal systems;
- BIOGEOSCIENCESDE: 4825 Geochemistry;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 8416 Mid-oceanic ridge processes;
- VOLCANOLOGYDE: 8424 Hydrothermal systems;
- VOLCANOLOGY