Seismic Anisotropy of the Crust in the Regions of Northern Baja California and Sonora, Mexico, Using Receiver Functions
Abstract
We calculated receiver functions from teleseismic P waves recorded at the NARS-Baja broad band stations to investigate the seismic anisotropy of the crust in the regions of northern Baja California and Sonora. Seismic anisotropy is typically controlled by stress state in the fragile shallow crust and by the rock strain state in the ductile intermediate and deep crust. To emphasize phases produced by the anisotropic structure of the crust we partially remove the source effect performing a frequency domain deconvolution of the radial and transverse components by the vertical one. In order to enhance coherent information and decrease noise, we stack receiver functions obtained from P-wave arrivals with similar back azimuth, since they are expected to yield consistent receiver functions. We gave particular attention to the Ps converted phase arrival, which is the most prominent wave arrival after direct P phase and is generated by the P to S conversion at the Moho due to the sharp velocity drop. When a Ps arrival appears clearly on radial and transverse components, and does not overlap with other phases, we suppose that transverse energy rose from shear wave splitting and we retrieve apparent anisotropy parameters, namely the azimuth of the fast direction and the time delay using a method based on the calculation of the covariance matrix. Using this method we characterized the crustal anisotropy under the NARS-Baja station NE75, located in the Stable Central Peninsula Province. The results obtained at this station suggest that the anisotropy of the crust must be related to the metamorphism induced by the subduction of the Farallon plate. At other stations, the results show high dispersion and back azimuthal dependency, indicating the presence of a more complex structure under the stations analyzed. We also performed forward modeling of the crust structure by matching the receiver functions obtained from observed records with synthetic seismograms calculated using a reflectivity based algorithm. At station NE75, a reasonable fit is achieved by introducing an anisotropic layer in the lower crust. A second anisotropic layer is also required in the mantle lithosphere near the Moho. This feature of the model suggests that metamorphic rocks were probably produced by the release of volatiles from the subducted Farallon plate. At station NE71 in the Transpeninsular Strike Slip Province, the anisotropy appears to lie mainly beneath the Moho. In contrast, stations NE80 and NE81 located in the Mexican Basin and Range Province show a substantial amount of crustal anisotropy, which is sufficient to account for the main features in the receiver functions calculated with the observed seismograms.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2007
- Bibcode:
- 2007AGUSM.S31A..07O
- Keywords:
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- 8100 TECTONOPHYSICS;
- 8104 Continental margins: convergent;
- 8105 Continental margins: divergent (1212;
- 8124);
- 8106 Continental margins: transform