S. C. Semken

Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau using receiver functions

Received 20 October 2004; accepted 2 March 2005; published 28 May 2005. [1] The seismic structure of the crust and upper mantle of the southwestern United States is examined using receiver functions calculated from teleseismic arrivals recorded in the Colorado Plateau–Rio Grande Rift–Great Plains Seismic Transect (LA RISTRA) experiment. We apply receiver function estimation and filtering methods developed by Wilson and Aster (2005) to produce receiver functions with decreased sensitivity to noise and deconvolutional instability. Crustal thickness and Vp/Vs ratios are estimated using both direct and reverberated P-to-S receiver function modes. We apply regularized receiver function migration methods to produce a multiple-suppressed image of the velocity discontinuity structure of the subsurface. Our results show that crustal thickness averages 44.1 ± 2.3 km beneath the Great Plains (GP) and 45.6 ± 1.1 km beneath the Colorado Plateau (CP). Crustal thinning beneath the Rio Grande Rift (RGR) is broadly symmetric about the rift axis, with the thinnest crust (35 km) located directly beneath the rift axis, suggesting a pure shear stretched lithosphere beneath the RGR. We also observe a prominent northwest dipping discontinuity, ranging from 65 to 85 km deep beneath the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are consistent with preserved ancient lithospheric structures such as relict suture zones associated with Proterozoic subduction. We observe an upper mantle discontinuity at 220–300 km depth that may correlate with similar discontinuities observed beneath eastern North America. We also observe relatively flat discontinuities at 410 and 660 km depth, indicating there is not a large-scale thermal anomaly beneath the RGR at these depths.

Lithospheric structure of the Rio Grande rift

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust–mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rifts surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.

Broadband Seismic Background Noise at Temporary Seismic Stations Observed on a Regional Scale in the Southwestern United States

Background noise power spectral density (PSD) estimates for 54 PASS- CAL Colorado Plateau/Rio Grande Rift/Great Plains Seismic Transect (LA RISTRA) stations were computed using data from 1999 to 2000. At long periods (0.01-0.1 Hz), typical vertical noise levels are approximately 12 dB higher than the nearby Global Seismic Network (GSN) borehole station ANMO, but horizontal power spec- tral density (PSD) noise levels are approximately 30 dB higher. Long-period noise levels exhibit essentially no spatial correlation along the LA RISTRA transect, indi- cating that local thermal or atmosphere-driven local slab tilt is the dominant source of noise in this band. Between 0.1 and 0.3 Hz, typical noise levels are dominated by naturally occurring microseismic noise and are essentially identical to those observed at ANMO. At short periods, 0.3-8 Hz, typical noise levels along the network exceed ANMO levels by approximately 15 dB, with the highest levels corresponding to proximity to cultural noise sources. No significant day/night variations were observed in the microseismic band; however, both low- and high-frequency noise levels show an increase of up to 8 dB in median midday versus midnight noise levels. We find that the major shortcomings of these shallow PASSCAL-style temporary vaults rela- tive to a GSN-style borehole installation are increased susceptibility to long-period horizontal (20 sec) noise and to surface noise sources above approximately 2 Hz. Although the high-frequency near-surface noise field is unavoidable in shallow vaults, we suggest that increased understanding and mitigation of local tilt effects in shallow vaults offers the possibility of significantly improving the long-period noise environment.

Rethinking Recruitment: The Comprehensive and Strategic Recruitment of Secondary Science Teachers

The shortage of science teachers has spurred a discussion about their retention and recruitment. While discussion about retaining science teachers has increased dramatically in just the last few years, science teacher educators have not attended to the recruitment of science teachers with the same tenacity. This paper is our effort to initiate this discussion and to focus on secondary science teachers. We begin by suggesting why recruitment is important and explore related research. We then suggest a comprehensive and strategic orientation towards recruitment that serves as a mechanism to examine current practices in the field. In presenting this position paper, we hope that science teacher educators will contemplate their own recruitment practices and begin discussing the recruitment process more openly with one another.