Abstract
The number of broadband three-component seismic stations in southern
California has more than tripled recently. In this study we use the
teleseismic receiver function technique to determine the crustal
thicknesses and VP/VS ratios for these stations and map out the lateral
variation of Moho depth under southern California. It is shown that
a receiver function can provide a very good point measurement of
crustal thickness under a broadband station and is not sensitive
to crustal P velocity. However, the crustal thickness estimated only
from the delay time of the Moho P-to-S converted phase trades off
strongly with the crustal VP/VS ratio. The ambiguity can be reduced
significantly by incorporating the later multiple converted phases,
namely, the PpPs and PpSs +PsPs . We propose a stacking algorithm
which sums the amplitudes of receiver function at the predicted arrival
times of these phases by different crustal thicknesses H and VP/VS
ratios. This transforms the time domain receiver functions directly
into the H-VP/VS domain without need to identify these phases and
to pick their arrival times. The best estimations of crustal thickness
and VP/VS ratio are found when the three phases are stacked coherently.
By stacking receiver functions from different distances and directions,
effects of lateral structural variation are suppressed, and an average
crustal model is obtained. Applying this technique to 84 digital
broadband stations in southern California reveals that the Moho depth
is 29 km on average and varies from 21 to 37 km. Deeper Mohos are
found under the eastern Transverse Range, the Peninsular Range, and
the Sierra Nevada Range. The central Transverse Range, however, does
not have a crustal root. Thin crusts exist in the Inner California
Borderland (21-22 km) and the Salton Trough (22 km). The Moho is
relatively flat at the average depth in the western and central Mojave
Desert and becomes shallower to the east under the Eastern California
Shear Zone (ECSZ). Southern California crust has an average VP/VS
ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain
ranges with Mesozoic basement and lower ratios in the Mojave Block
except for the ECSZ, where the ratio increases.
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