Here is what I have so far of a paper that Anne had me write describing my project.  The things in bold are things I need to check on or edit.  For instance, I'm not sure how to site a map within the text, and I'm not positive that I sited it right in the references.  I'm not sure of all the places that the data came from.  As far as the signal to noise ratio and variance reduction, we looked at the data with various different values for these, but I am unsure which values we ended up with.

I'm also going to be the first to admit it's not perfect.   It's kinda rough and doesn't quite have an end to it, but, Michael wanted a description of our projects, so I thought it would be easiest just to post this.  :)  Also, I refer to a map later on, which I post at the end.  Right now the colors correspond to seconds, but I will eventually change it so that it refers to the actual crustal thickness.

The 37º N latitude Gravity Anomaly of Southern Nevada

Mollie M. Pettit

There is a gravity anomaly at 37º N latitude in Nevada at which the gravity increases drastically from North to South (site map.  How?).  There are many proposed hypotheses concerning the reasoning behind this swift change.  Saltus et al. [1995] have explored various arguments for this gravity anomaly including crustal thickness or density, heat flow, petrologic differences, subduction, seismicity, etc. I  have focused on possibility that a change in crustal properties could be a explanation for a large part of this gravity increase.  

Several different conclusions have been made about the degree to which the crustal thickness can justify the gravity change.  Many studies [Benz et al., 1990; Catchings and Mooney; Holbrook, 1990; 1991; McCarthy et al., 1991; Pakiser, 1989] have come to the conclusion that either crustal thickness or crustal density could possibly explain 100 percent of the anomaly.  Other studies [Jones et al., 1992; Saltus et al., 1995] have suggested that crustal characteristics can account for no more than ½ of the necessary gravity changes.

There are two hypothetical situations within the crust that could create such a gravity change.  First, such a change could result from a huge shift in crustal thickness at  37º N.  Because the average density of the mantle (3.3 g/cm3) is larger than the average density of the crust (2.75 g/cm3), an area where the crust is much thinner will result in a higher density which infers a higher gravity.  Similarly, an area with thicker crust will result in a lower density and lower gravity.  A second situation that could be causing such a change in gravity is if the density of the mantle changes in that location.  I will focusing on testing  the first situation.  

In order to determine the crustal thickness the southern Nevada location, I first examined receiver functions from stations from 1993 to 2008 and between 241.5º  and 247.5º  longitude and 35º  and 40º  latitude available from (where did data come from?).  For this analysis, I used only receiver functions with a signal to noise ratio of (?) and a variance reduction of over (?).  I then visually went through the receiver functions to avoid particularly noisy traces and remove them from the data pool.  

To enhance the impact of the regularities and reduce the impact of the irregularities, the receiver functions were stacked.  The remaining receiver functions, totaling 25114, were then used to plot the back azimuths, slowness, and moho in MATLAB.  

Because we are assuming constant velocities for P and S waves within the mantle and crust, the difference in time between the arrival of the two wave types is a good signifier of the crustal thickness.  An area in which the difference in arrival times is higher will have a thicker crust and an area in which the difference is smaller will have a thinner crust.  By estimating the average arrival time for each station, a map was made of the approximate crustal thicknesses.  This map shows a decrease in crustal thickness from North to South between 36º N and 37º N latitude.

References

Benz, H. M., R. B. Smith, and W. D. Mooney (1990), Crustal structure of the

northwestern Basin and Range Province from the 1986 Program for Array

Seismic Studies of the Continental Lithospher seismic experiment, Journal

of Geophysical Research, 95, 21, 823-21, 842.

Catchings, R. D. and W. D. Mooney (1991), Basin and Range crustal and 

uppermost mantle structure, northwest to central Nevada, Journal

of Geophysical Research, 96, 6247-6267.

Davidson, J. P., W. E. Reed, P. M. Davis (1997), Exploring Earth:  An Introduction 

to Physical Geology, pp. 108-129, Prentice-Hall, inc., Upper Saddle River, 

New Jersey.

Decade of North American Geology Project (1987), Gravity anomaly map of 

North America, The G. S. A. Foundation, United States.

Gilbert, H. J., and A. F. Sheehan (2004), Images of crustal variations in the 

intermountain west, Journal of Geophysical Research, 109, 

doi:10.1029/2003JB002730.

Gilbert, H. J., A. F. Sheehan, K. G. Dueker, and P. Molnar (2003), Receiver 

functions in the western United States, with implications for upper mantle 

structure and dynamics, Journal of Geophysical Research, 108(B5), 2229, 

doi:10.1029/2001JB001194.

Holbrook, W. S. (1990), The crustal structure of the northwestern Basin and Range 

Province, Nevada, from wide-angle seismic data, Journal

of Geophysical Research, 95, 21, 843-21, 870.

Jones, C. H., B. P. Wernicke, G. Lang Farmer, J. D. Walker, D. S. Coleman, L. W. 

McKenna, and F. V. Perry (1992), Variations across and along a major 

continental rift.  An interdisciplinary study of the Basin and Range 

Province, western USA, Tectonophysics, 213, 57-96.

McCarthy, J., S. P. Larkin, G. S. Fuis, R. W. Simpson, and K. A. Howard (1991), 

Deep anatomy of a metamorphic core complex.  Seismic refraction and 

wide-angle reflection profiling in southeastern California and western 

Arizona, Journal of Geophysical Research, 96, 12, 259-12, 291.

Pakiser, L. C. (1989), Geophysics of the intermontane system, in Geophysical 

Framework of the Conterminous United States,  edited by L. Pakiser, and W. 

  D. Mooney, Mem. Beol. Soc. Am., 172, 235-247.

Saltus, R.W. and Thompson, G.A. (1995), Why is it downhill from Tonopah to Las 

Vegas?: A case for mantle plume support of the high northern Basin and 

Range, Tectonics, 14, 1235-1242.

Sheehan, A. F., G. A. Abers, C. H. Jones, and A. L, Lerner-Lam (1995), Crustal 

thickness variations across the Colorado Rocky Mountains from teleseismic 

receiver functions, Journal of Geophysical Research, 100(B10), 20, 391-20, 

404.