Graduate students:
Suzan van der Lee(Now
at ETH, Zuerich, Switzerland)
Triparna Das (Nowat
ARCO, Dallas)
Undergraduates:
Tatiana Petalas('95)
Wendy Deslauriers('97)
Funding:
NSF
Publications:
Das, T. and G. Nolet,Crustal
thickness estimation using high frequency Rayleigh waves, Geophys.Res.
Lett., 22, 539-542, 1995.
Van der Lee, S.and
G. Nolet, Seismic image of the subducted trailing fragments of theFarallon
Plate, Nature, 386, 266-269, 1997.
Van der Lee, S.and
G. Nolet, Upper mantle S velocity structure of North America, J.Geophys.
Res. 102, 22815-22838, 1998.
Das, T. and G. Nolet,Crustal
Thickness Map of the Western Unites States by Partitioned WaveformInversion,
J. Geophys. Res., 103, 30021-30038, 1998.
Project goals:
The aim ofthis
project was to obtain a detailed image of the upper mantle under NorthAmerica,
which may serve to explain a number of important geological questions.
|
Thefirst result (Van der Lee and Nolet, 1997a,b) is a map of the deeper structureas imaged by variations in the velocity of seismic S (shear) waves. Thebig surprise in this map is the image of a pronouncedband of high velocities in the transition zone, coinciding with the expectedlocation of the subducted trailing fragments of the Farallon platen. |
Among the other wellresolved features of the new model NA95 are (1) a high velocity root beneaththe North American craton which extends no deeper than 250 km except nearthe Archean core of the craton where depths of 350 km are reached, (2)a weak band of low velocity along the eastern margin of the North Americancraton, which reaches into the transition zone, (3) a low velocity slabwindow beneath the western United States down to a depth of 300 km, (4)areas of low uppermost-mantle velocity beneath the Cascade volcanoes, theYellowstone hotspot track, the Colorado plateau, the Sierra Madre Occidentaland the grabens bordering the Jalisco block.
In order toimprove on these figures, we do need to obtain a much better understandingof the crustal structure. While the thickness and the seismic velocityof the crust is locally sometimes well known from regional experiments,new methods are needed to investigate the crust everywhere. Such a methodwas developed by Triparna Das (Das and Nolet, 1994) used short period Rayleighwaves to find crustal thickness and average S velocity in the crust. Dasand Nolet (1998) apply this to the western part of the US using modern,digitally recorded seismograms. Wendy Deslauriers investigated the easternpart of the US, where seismicity is low and where we have to us older,photographic seismograms. Her research resulted in a Senior thesis.
