This research is to use western U.S. seismic network recordings of local earthquakes and prior seismic refraction studies to better constrain the crustal structure under the Sierra Nevada mountains. Recent studies have revived the controversy over whether the Sierra Nevada has a cold, deep root or whether effects interpreted as a root are caused by an unusually low- velocity, warm, extending mantle that has heated up within the last 10 my. The results will have important implications on our knowledge of the creation and tectonics of the lithosphere and upper mantle of the western U.S.
