Imaging Crustal Structure in Alaska from Receiver Functions

Alaska is bounded by an active subduction zone in the south where the Pacific plate has been colliding with and subducting under the northwestern edge of the North American Plate. The Denali fault across central Alaska separates complex accreted terranes in the south from older terranes and the stable North American continent in the north. In this thesis, we present a broad-scale image of mean crustal thickness and Vp/Vs ratio across the major tectonic domains of Alaska based on P wave receiver functions.
We have processed seismograms from 174 earthquakes at 110 stations of the USArray Transportable Array and Alaska Regional Network. Individual receiver functions are stacked by station with moveout correction and by the H-k method. Both methods produce similar Moho depth variation of 28 km to 46 km across Alaska. The deepest Moho up to 46 km is located in southern Alaska beneath the Alaska Range and Wrangell - St. Elias Mountains. A thick crust of 38-43 km is also imaged near the Arctic margin in northern Alaska at the Brooks Range. In contrast, the Moho of central Alaska is generally shallow, at 33 km depth on average, except beneath the W. Mackenzie Mountains. Relatively thin crust is also located beneath lowland areas such as the North America craton, the Nanana Basin, and Cook Inlet. Vp/Vs ratios of the crust are estimated at each station from the H-k method. Normal to slightly low Vp/Vs ratios are located to the north of the Denali fault with a range of 1.65-1.75. High Vp/Vs ratios are found in southern Alaska with an average value of 1.85, which can be attributed to broad volcanism in the area. The highest Vp/Vs ratio of 2.1 is in the Wrangell volcanic field, which is caused by metamorphic and mafic intrusive rocks. Relatively high Vp/Vs ratio also exists at other volcanoes in southwest Alaska and the Alaska Range.