Mapping The Mantle Transition Zone Beneath Eastern North America An Automated Receiver Function Approach

Abstract

Despite being ever-present beneath our feet, the interior of the Earth remains an enigmatic world of its own. In the last two centuries, the science of seismology has begun to peel back the mystery of this world within a world, producing maps of the crust, mantle, outer core, and inner core. The boundaries between these layers are marked by distinct jumps or discontinuities, often in chemical composition, and nearly always in seismic velocity. In this thesis, we investigate a particular set of these discontinuities in the mid-mantle, known as the mantle transition zone.This thesis begins with a review of the historical developments in the discovery of the mantle transition zone, and then introduces the method we used to study it—teleseismic receiver functions. After this introduction, we present a technical discussion of a key element of seismometry, the instrument response. We frame this discussion in the context of a, MLg 3.1 earthquake that occurred in Marlboro, New Jersey in 2020. We then continue developing our methods with a receiver function analysis of a seismometer deployed in Bermuda. After discussing the geologic history of the island, we introduce the iterative time-domain deconvolution method for calculating receiver functions. With this method, we produce images of the mantle transition zone, and we additionally devise an automated quality control criterion for subsequent receiver function analyses. We find that the transition zone beneath Bermuda is thickened, and discuss potential interpretations in the context of various mineral systems, such as olivine and garnet. Following this, we apply our methods to a problem of a much larger scale—imaging the mantle transition zone beneath the entirety of eastern North America. Using common conversion point stacks, we produce high-resolution images of the mantle transition zone. We find three noteworthy features in our stacks: two thinned, and one thickened region. We suggest that the thinned regions are associated with the northern and central Appalachian anomalies. The thickened region is coincident with the supposed location of the Laramide slab. We additionally present an analysis on the effects of different three-dimensional velocity models on the depth correction of receiver functions.