In this dissertation, I use seismic imaging and waveform modeling methods to investigate melt migration processes and the structure of the magma plumbing system beneath the East Pacific Rise (EPR) and Endeavour segment of the Juan de Fuca Ridge, respectively. This work begins by studying shallow mantle reflections beneath the EPR. I find the amplitude versus offset and waveform characteristics of the reflections to be consistent with a sub-horizontal dunite channels located up to 20 km off-axis. The depth of the dunite channels correlate with patterns of mantle melt delivery and the predicted base of the thermal lithosphere, suggesting the channels are thermally controlled and may have formed in situ via dissolution by focused flow at the base of the lithosphere. This interpretation is consistent with field observations in ophiolites and numerical modeling of melt-focusing channels. The three-dimensional velocity structure of the Endeavour segment is then investigated to identify how patterns of mantle melt delivery influence the segment-scale distribution of crustal melt and crustal accretion. The results from this study indicate that the mantle magmatic system is skewed relative to the ridge-tracking crustal magmatic system and that this skew exerts primary control on magmatic, tectonic, and hydrothermal activity at the Endeavour segment. In regions where mantle melt delivery is axis-centered, mantle-derived melts are efficiently transported from the mantle to the crust, resulting in frequent crustal melt replenishment, associated seismogenic cracking, and enhanced crustal melt content that drives vigorous hydrothermal activity. Conversely, sites of off-axis melt delivery are characterized by less efficient vertical melt transport, resulting in infrequent crustal melt injection and hence, reduced crustal melt content and hydrothermal activity. Next, I focus on how along-axis variations in magma replenishment modulate crustal permeability and the intensity of hydrothermal circulation. Using full-waveform inversion, I show that sites of localized magma replenishment to the axial magma lens, along with induced seismogenic cracking, coincide with enhanced permeability. I conclude that the frequency of magma injection governs hydrothermal circulation patterns and heat flux at mid-ocean ridges. This dissertation includes previously published and unpublished coauthored material.

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