IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2158
Surface Wave Tomography beneath the Gulf of California Rift Zone
Mrs. Xiaomei Zhang
Department of Earth Sciences Utrecht University
Hanneke Paulssen, Sergei Lebedev, Thomas Meier
We present an average 1-D shear velocity model and a set of fundamental mode Rayleigh wave phase
velocity maps including azimuthal anisotropy of the Gulf of California (GofC). The study was based on
applying the two-station-method to earthquake data recorded by the NARS-Baja network. The 1-D
shear velocity model space search result confirms the presence of a low velocity zone in the upper
mantle of the GofC, which was suggested by regional and global models as well. The sensitivity of our
phase velocity period band (10~250 s) reaches into the transition zone (660 km). The Monte Carlo
search suggests low shear velocities to this depth, which indicates that the source of the low velocities
has a deep mantle origin. Therefore the common feature of the entire region is slower shear velocities
than PREM down to the transition zone. From the phase velocity tomography with anisotropy (period
10~100 s, depth range 0~250 km), we observe a change in pattern of the lowest velocity regions as a
function of period (depth). At 30 s the lowest phase velocities are found beneath the plate boundary in
the Gulf (related to upwelling underneath the ridge). At periods larger than 50 s, the lowest velocities
are found beneath the northern GofC (possibly related to a slab gap in the subducted Farallon plate). In
the low period band (≤ 20 s) we distinguish two different directions of azimuthal anisotropy. In the
northern part of the GofC the fast axes move from NNW (10~14 s) to NW (20 s) as the period
increases. In the southern part the fast axes moves from ENE to EW. The fast NW axes are coherent
with Pacific plate motion or the strike slip plate boundary motion. The fast EW axes in the southern part
seem to be associated with gulf extension. In the period band in-between 20 to 40 s, the amplitudes of
the anisotropy become very low; this could be due to upwelling underneath the Gulf. In the period band
above 40 s, we lose resolution of the azimuthal anisotropy in the southern part due to lack of data. The
fast axes in the northern part move from NW (at 50 s) to EW (at 100 s). The NW direction can be
associated with the direction of Pacific Plate motion, and the EW fast axes agree with the fast direction
of SKS-splitting measurements for southern California and northern Baja [Obrebski et al., 2006] and
mantle flow beneath western North America [Silver & Holt, 2002].
Keywords: gulf of california, surface wave, tomography