HR: 0800h
AN: T21B-0520    [Abstracts]
TI: Travel Time Residuals and Anisotropic Attenuation in the Central Volcanic Region, NZ
AU: * Greve, S M
EM: grevesonj@student.vuw.ac.nz
AF: Victoria University of Wellington, Box 600, Wellington, 6001 New Zealand
AU: Savage, M K
AF: Victoria University of Wellington, Box 600, Wellington, 6001 New Zealand
AU: Hofmann, S D
AF: Victoria University of Wellington, Box 600, Wellington, 6001 New Zealand
AU: Hofmann, S D
AF: Institute of Geophysics, University of Hamburg, Hamburg, 20146 Germany
AB: New Zealand's Central Volcanic Region is characterized by the subduction of the Pacific Plate underneath the Australian Plate. SKS splitting results show large delay times and trench parallel fast directions. Teleseismic broadband data were acquired in 2001 along a trench perpendicular line (striking northwest to southeast). An unusual attenuation and travel time pattern was observed on an SKS and SKKS record comparing fast and slow components. Going from south-east to northwest across increasing path length in the mantle wedge, the amplitude of the fast component decreases and it appears to arrive earlier with respect to the ak135 propagation model, whereas the amplitude and arrival time of the slow component stays more consistent. To examine the phenomena we determine travel time residuals relative to arrival times estimated from the ak135 propagation model. For earthquake to station distances from 20 to 120 degrees, P waves residual times vary between -3.66 s to 3.24 s. Values for fast and slow S waves range between -5.00 s and 5.00 s. Results are dependent on backazimuth. Waves arriving from west to northwest show a decrease of residual time and therefore an increase of apparent/mean velocity from west to east along the line. This is consistent with what would be expected due to the increasing length of the travelpath through the higher velocity slab. On the other hand there is an increase in travel time residuals for waves arriving from eastern directions, consistent with longer paths through the mantle wedge. The high frequency fast and slow shear waves examined by this technique show similar trends. We attribute the apparent early arrivals of the fast wave in the SKS/SKKS records to waveform modification during attenuation. We are currently examining the spectral characteristics to quantify the anisotropic attenuation. The anisotropic attenuation cannot be caused by fluid filled cracks, as this would cause the amplitude and arrival time of the slow component to decrease. Another hypothesis to be tested is that the attenuation could be due to the loss of energy during phase conversions from aligned heterogeneities.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 5144 Wave attenuation
DE: 7203 Body wave propagation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting