Upper mantle P velocities beneath the North America craton

In this work, we determined the detailed mechanisms of three earthquakes occurring in Quebec, Texas and Idaho for use in modeling triplication data. The first event provided pure-path triplication recordings at over 400 USArray stations. Although amplitudes of the direct P waves are small, the depth phase sP is clear and displays shadow-zone characteristics indicative a low velocity layer (LVL) in the upper mantle, where the amplitude of the AB branch decreases rapidly at a distance of 16 degrees. Another feature of the LVL is that the AB branch can be seen at distances larger than 23 degrees. Similar to the Canadian Shield velocity model S25 (LeFevre and Helmberger, 1989), we found a LVL between 160 km and 215 km and obtained excellent fits assuming 1D model. The other two events are located near the craton margins and have been recorded by the MOMA array (Texas event) and CANOE array (Idaho event). These mixed paths are mostly craton with modified 1D models producing good fits. We, also, produced 2D modeling results that use tomographic images for correcting the source structures.