Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

Rayleigh wave group velocity dispersion measurements from local and regional earthquakes
are used to interpret the lithospheric structure in the Gulf of California region. We compute
group velocity maps for Rayleigh waves from 10 to 150 s using earthquakes recorded by broadband
stations of the Network of Autonomously Recording Seismographs in Baja California
and Mexico mainland, UNM in Mexico, BOR, DPP and GOR in southern California and TUC
in Arizona. The study area is gridded in 120 longitude cells by 180 latitude cells, with an
equal spacing of 10 × 10 km. Assuming that each gridpoint is laterally homogeneous, for each
period the tomographic maps are inverted to produce a 3-D lithospheric shear wave velocity
model for the region.
Near the Gulf of California rift axis, we found three prominent low shear wave velocity
regions, which are associated with mantle upwelling near the Cerro Prieto volcanic field, the
Ballenas Transform Fault and the East Pacific Rise. Upwelling of the mantle at lithospheric
and asthenospheric depths characterizes most of the Gulf. This more detailed finding is new
when compared to previous surface wave studies in the region. A low-velocity zone in northcentral
Baja at ∼28oN which extends east–south–eastwards is interpreted as an asthenospheric
window. In addition, we also identify a well-defined high-velocity zone in the upper mantle
beneath central-western Baja California, which correlates with the previously interpreted location
of the stalled Guadalupe and Magdalena microplates. We interpret locations of the fossil
slab and slab window in light of the distribution of unique post-subduction volcanic rocks in
the Gulf of California and Baja California.We also observe a high-velocity anomaly at 50-km
depth extending down to ∼130 km near the southwestern Baja coastline and beneath Baja,
which may represent another remnant of the Farallon slab.

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