2 research outputs found
Discriminating Between Spatial and Temporal Variations in Seismic Anisotropy at Active Volcanoes
This thesis addresses the measurement and interpretation of seismic anisotropy
around active volcanoes via shear wave splitting analysis.
An overpressured magma reservoir will exert a stress on the surrounding country
rock that may or may not be manifest as observable strain. Shear wave splitting
analysis can be a useful indicator of stress in the crust and hence, the pressure induced
by magma movement. Changes in shear wave splitting have already been
observed at Mt. Ruapehu following eruptions in 1995/1996 and are inferred to be
caused by changes in local stress in response to magma pressure. One of the main
problems with the interpretation of temporal changes in shear wave splitting is the
possibility of spatial variations being sampled along differing raypaths and being
interpreted as temporal changes. Using a dense observational network and an automated
shear wave splitting analysis, we examine local earthquakes occurring in
2008 within 100 km of Mt. Ruapehu. We note a strong azimuthal dependence of
the fast direction of anisotropy (phi) and so introduce a spatial averaging technique
and a two-dimensional tomography of recorded delay times (dt), to observe the
spatial variation in more detail. Using this new method of mapping shear wave
splitting parameters, we have created a benchmark of spatial variations in shear
wave anisotropy around Mt. Ruapehu, against which future temporal changes may
be measured. The observed anisotropy is used to define regions in which phi agrees
with stress estimations from focal mechanism inversions, suggesting stress-induced
anisotropy, and those in which phi aligns with structural features such as fault strikes,
suggesting structural anisotropy. Data from past deployments of three-component
seismometers have been analysed in the same way as those recorded during the
2008 experiment and the results compared. We identify a stable region of strong
anisotropy, interpreted to be caused by schistose mineral alignment, and a transient
region of strong anisotropy centred on the volcano during the major magmatic
eruption of 1995.
We also introduce a method of analysing temporal variations in seismic anisotropy
at active volcanoes by using tight clusters of earthquakes and highly correlated
multiplets. At Mt. Ruapehu, changes in shear wave splitting parameters associated
with the 2006 and 2007 phreatic eruptions are detected using a cluster of earthquakes
to the west of the volcano. Similar analyses using another cluster and multiplets
from the stable region of strong anisotropy do not reveal temporal changes, although
examination of the waveform codas of the repeating earthquakes reveals systematic
changes that we interpret as being caused by seismic scatterers associated with the
2006 and 2007 eruptions. These scatterers appear to contaminate the shear wave
coda and so inhibit the detection of any subtle changes in shear wave splitting
parameters.
Finally, we apply some of these methods to data from the 2008 eruption of Okmok
volcano, Alaska. Shear wave splitting analysis at Okmok reveals a change in
anisotropy associated with the 2008 eruption. This change however, is attributed
to a change in dominant hypocentre location. Multiplet analysis at Okmok volcano
reveals a similar scatterer contamination of the shear wave arrival. This spurious
phase is interpreted to be an S to P conversion from interaction with the magma
reservoir