CHANGES IN THE SHAPE OF THE EARTH HAVE BEEN OBSERVED FOR SEVERAL DECADES BY ADVANCED SPACE-GEODETIC TECHNIQUES INCLUDING THE GLOBAL POSITIONING SYSTEM (GPS). THE NASA-SUPPORTED MISSIONS HAVE PRODUCED LONG AND ACCURATE DATASETS WHICH PROVIDE AN UNPRECEDENTED OPPORTUNITY TO INVESTIGATE AND DIFFERENTIATE BETWEEN GEOPHYSICAL CONTRIBUTIONS TO SURFACE DEFORMATION INCLUDING TECTONIC MOTIONS AND THE REDISTRIBUTION OF SURFACE FLUIDS. THE SURFACE OF THE EARTH IS DISPLACED IN ELASTIC RESPONSE TO CHANGES IN SURFACE MASS LOADS. SPATIAL GEODETIC OBSERVATIONAL TECHNIQUES SUCH AS GPS ACCURATELY MEASURE THE CRUSTAL LOADING DEFORMATION AT MILLIMETER-LEVEL ACCURACY. LOADING DEFORMATION DEPENDS ON BOTH SURFACE-LOAD MASS CHANGES AND THE ELASTIC PROPERTIES OF EARTH S INTERIOR. IF SURFACE-LOAD CHANGE CAN BE PRECISELY DETERMINED THE RESIDUAL BETWEEN OBSERVED AND MODELED LOADING DEFORMATION MAY BE USED TO INVESTIGATE THE ELASTIC STRUCTURE OF THE EARTH. HERE WE PROPOSE TO USE OBSERVATIONS OF MASS-LOADING RESPONSE IN THE GREAT LAKES REGION OF NORTH AMERICA TO PROBE THE ELASTIC STRUCTURE OF THE REGIONAL CONTINENTAL SHIELD. A BETTER UNDERSTANDING OF THE SUBSURFACE STRUCTURE OF THE GREAT LAKES REGION IN THE NORTH AMERICAN PLATE IS IMPORTANT FOR ALL SOLID EARTH SCIENCES. PREVIOUS SEISMIC TOMOGRAPHY STUDIES INDICATE THAT CENTRAL AND NORTHERN PARTS OF NORTH AMERICA ARE UNDERLAIN BY A RELATIVELY COLD CRATONIC KEEL EXTENDING TO DEPTHS OF MORE THAN 200 KM WITHIN THE MANTLE. A CONSPICUOUS FEATURE OF SOME RECENT MODELS IS A LOW-VELOCITY CHANNEL AT ~200 KM DEPTH EXTENDING FROM THE EAST COAST OF NORTH AMERICA INTO THE LOWER GREAT LAKES REGION. THE DEVIATIONS OF THE REGIONAL EARTH STRUCTURE IN THE GREAT LAKES REGION FROM STANDARD GLOBALLY AVERAGED EARTH STRUCTURE WILL AFFECT THE EARTH S LOADING RESPONSE TO VARIATIONS IN SURFACE MASS. CORRECTLY CHARACTERIZING THE REGIONAL EARTH STRUCTURE WILL IMPROVE OUR UNDERSTANDING OF THE FORMATION AND EVOLUTION OF THE NORTH AMERICAN PLATE. THE PROPOSED PROJECT WILL SYSTEMATICALLY ANALYZE GPS-OBSERVED LOADING DEFORMATION AROUND THE GREAT LAKES REGION AND DEVELOP AN INVERSION STRATEGY TO BETTER CONSTRAIN THE EARTH STRUCTURE IN A CENTRAL REGION OF THE NORTH AMERICAN PLATE. CHANGES IN LAKE VOLUME ARE KNOWN RELATIVELY ACCURATELY BY WATER-LEVEL MONITORING STATIONS SO WE WILL FOCUS ON LAKE-MASS CHANGES FOR THE TOMOGRAPHIC INVESTIGATIONS. ADDITIONAL LOADING SIGNALS INCLUDING FROM THE ATMOSPHERE OCEAN AND REGIONAL HYDROLOGY WILL BE MODELED AND REMOVED PRIOR TO INVERSION FOR STRUCTURE. THE PROPOSED PROJECT CLOSELY MATCHES ESI S 2018 SOLICITATION WITH A FOCUS ON INNOVATIVE SOLID-EARTH SCIENCE: GEODETIC TOMOGRAPHY. THE PROPOSAL AIMS TO ADVANCE OUR UNDERSTANDING OF SOLID-EARTH PROCESSES AND PROPERTIES FROM CRUST TO CORE WHICH IS FUNDAMENTAL TO THE ESI PROGRAM AND DIRECTLY ADDRESSES KEY SCIENTIFIC CHALLENGES HIGHLIGHTED BY THE ESI PROGRAM: HOW DOES THE SOLID EARTH RESPOND TO CLIMATE-DRIVEN EXCHANGE OF WATER AMONG EARTH SYSTEMS AND WHAT ARE THE IMPLICATIONS FOR SEA-LEVEL CHANGE AND HOW DO TECTONIC PROCESSES AND CLIMATE VARIABILITY INTERACT TO SHAPE EARTH S SURFACE AND CREATE NATURE HAZARDS.