This dissertation details efforts to image the along-strike continuity of major seismic discontinuities in subduction zones using dense (<20 km spacing) arrays of seismometers. The seismic discontinuities were located using receiver-side P-to-S and P-to-P scattered phases from teleseismic (30°<epicentral distance<90°, Mw>~5) earthquake P waves. The time series of scattered phases are referred to as “receiver functions” (RFs) though they are slightly different from traditional RFs. Chapter 1 investigates the location and orientation of the subducting Juan de Fuca (JdF) Moho beneath Mount St. Helens (MSH) using RFs from a dense seismometer array that was deployed in a circular area around the volcano. Two-dimensional (2D) migration and three-dimensional (3D) common-conversion point (CCP) stacking imaging methods were used to reveal a continuous JdF slab at 68±2 km beneath MSH, which is the shallowest directly imaged subducting slab beneath an arc volcano in the world. Between 2016 and 2018, we deployed, serviced, and recovered 36 broadband seismometers in a dense array across the road system of southcentral Alaska, which provided the key data for Chapters 2 and 3. RFs from that array and two other dense seismometer deployments across southcentral Alaska were generated using an improved method that accounts for variations in surface geology. 2D migration of the RFs revealed the Yakutat terrane, an oceanic plateau, subducting down to at least 110-km depth on the eastern half of southcentral Alaska. 3D CCP stacking revealed a continuous LVZ atop the Yakutat terrane for over 450 km along strike, at depths <35 km. Chapter 3 describes the use of RF data from Chapter 2 and data from an older dense seismometer array across central Washington, USA, in estimating the thickness and the ratio of P wave velocity to S wave velocity (Vp/Vs) of the LVZ atop the subducting Yakutat terrane in Alaska and the JdF plate in Cascadia. Results show consistent estimates of Vp/Vs that are lower than those of previous studies. In addition, synthetic seismograms were also generated and used to show that previous measurements of Vp/Vs may have been biased to higher values.