Download Volume II Accomplisments (28 Mb pdf). - IRIS

S-Velocity Mantle Structure at the Subducting Chile Ridge
Simon Lloyd (Northwestern University), Suzan van der Lee (Northwestern University), Raymond M Russo (University
of Florida), Diana Comte (University of Chile), Victor I Mocanu (Bucharest University), Alejandro Gallego (University of
Florida), Ruth Elaine Murdie (Gold Fields Australia, St Ives Gold Mine), John C VanDecar (Carnegie Institution of Washington)
At the triple junction between the Nazca, Antarctica and South American plate an actively spreading ridge is currently being subducted.
The ridge continues to spread as it gets subducted beneath South America. However, no new lithosphere is formed in the process.
As a result slab windows likely exist beneath the overiding plate. These gaps allow asthenospheric mantle to flow through the slab,
effecting mantle chemistry and thermal regime, seismic velocities and anisotropy as well as surface geology (e.g. gaps in arc volcanism).
Slab windows have successfully been imaged using P wave tomography [Russo et al., 2009]. In this study we analyze Rayleigh
waves as they traverse the Chile Ridge Subduction Project (CRSP) array, in order to determine if additional constraints may be
obtained from surface wave analysis.
We easily cross-correlated waveforms from an event at different stations to determine the relative arrival times, which allowed
us to image the wavefront as it passes through the array. Typical for many events recorded at the CRSP array are wavepaths following
the boundary between the Nazca and South American plates. For these events the incoming wavefront is not perpendicular
to the great circle paths connecting the source and the receivers.
In order to get a first order estimate of the Rayleigh wave phase velocities we split the array into four regions defined by the
73°W meridian and 46°S parallel. We then invert relative arrival times for the average phase velocity within the defined regions.
We repeat the inversion for several events, and combine the results. We conclude that
• Imaging Rayleigh wavefronts as they traverse the array region shows that particularly at shorter periods (e.g. 30 s) they are
not perpendicular to the great circle path connecting the earthquake source and the seismometers.
• This needs to be taken into consideration when determining the phase velocities.
• A strong velocity contrast between the E (slow) and the W (fast) is derived from the delay times.
• Possibly a similar distinction can be made between the N (fast) and the S (slow).
References:
Russo, R.M., VanDecar, J.C., Comte, D., Mocanu, V.I., Gallego, A. Murdie, R.E., 2010. Subduction of the Chile Ridge: upper mantle structure
and flow, GSA Today, in press.
Acknowledgements: This work was supported by National Science Foundation grant EAR 0538267.
We deployed 39 broadband
seismic stations from
PASSCAL in southern Chile,
aiming to investitgate the
possible existence and
effects of slab windows.
(left) The average Rayleigh wave phase velocity in region A is distinctly higher than in the other regions. (middle)
Velocites in the west of the array are clearly higher than in the east. (right) The north-south contrast is not as
strong, but the south appears be slightly slower than the north.
Owing to the small station spacing, the
recorded waveforms are very similar at
all stations. (below) For wavepaths following
the boundary between the Nazca
and South American plates, the incoming
wavefront is not perpendicular to the
great circle paths.
II-180 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics