Depth Constraints on Seismic Anisotropy in Iceland from Shear Wave Splitting Measurements
Abstract
Earthquakes associated with volcanic and extensional processes in the Icelandic rift zone provide an excellent source of seismic data with which to investigate the anisotropic properties of crust generated at a mid-ocean ridge. Seismic instruments deployed by the University of Cambridge and the Icelandic Meteorological Office over the last decade have recorded a large catalogue (> 100,000) of seismic events in the central part of the northern volcanic rift zone in Iceland, straddling the A skja an d B árðarbunga central volcanoes and their associated rift segments.
Shear wave splitting is a key indicator of the presence of seismic anisotropy. A shear wave entering an anisotropic medium is polarised into two quasi-S waves which travel with different velocities. Measurements of the orientation and accumulated delay time between these two components allow us to extract information on the underlying anisotropic fabric. We utilise MFAST, an automated shear wave splitting routine, to examine local earthquakes between 2009 and 2016. We observe a shallow anisotropic layer in the upper 4 km with an average delay time of 0.1s (approximately 5% anisotropy). The measured orientation of the fast axis correlates with the strikes of mapped surface features (e.g. surface fissures), which are generated by the rifting process. This is consistent with being caused by stress-induced microcracks, preferentially aligned parallel to the axis of the spreading ridge. The coincidence of the mid-Atlantic ridge with the Iceland plume results in a region of anomalously thickened crust in Iceland. This thick crust hosts pockets of deep seismicity that occur in swarms largely between 14-26 km depth within the otherwise aseismic lower crust. This seismicity allows us to investigate anisotropy at greater depths beneath a mid-ocean ridge than would otherwise be possible elsewhere in the world. A potential second layer of anisotropy, contributing an additional 0.2s of delay time, is consistently observed between 8 km and 15 km depth across the network. At this depth brittle fractures have closed, so we require an alternative mechanism to explain the source of the observed seismic anisotropy.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S41D0559B
- Keywords:
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- 7203 Body waves;
- SEISMOLOGY;
- 7255 Surface waves and free oscillations;
- SEISMOLOGY;
- 7299 General or miscellaneous;
- SEISMOLOGY