Understanding　offshore　site　amplification　(AF)　characteristics　is　crucial　for　marine　engineering.　Thus　far,　the　spectral　ratio　(SR)　has　been　widely　used　to　analyze　offshore　AF　characteristics,　despite　its　reliability　not　yet　being　fully　investigated.　In　this　study,　we　used　826　ground-motion　data　recorded　at　6　offshore　and　19　onshore　stations　in　the　Sagami　Bay　region　to　reveal　the　source,　path,　and　AF　characteristics　using　the　generalized　inversion　technique　(GIT).　The　average　stress　drops　for　the　shallow　crustal,　subduction　interface,　and　subduction　slab　earthquakes　were　3.55,　6.59,　and　7.07　MPa,　respectively.　The　offshore　sites　were　categorized　into　flat　and　steep　stations　based　on　the　local　slope　gradient　(LSG),　and　systematic　AF　characteristics　were　observed.　We　found　that　the　topography　had　a　negligible　influence　on　the　flat　free-field　station　KNG205.　AFs　obtained　using　the　GIT　and　SR　were　analyzed　and　compared,　revealing　two　main　differences:　(1)　SR-obtained　AF　values　were　significantly　larger　than　GIT-obtained　AF　values　above　0.5　Hz　and　5　Hz　at　the　flat　and　steep　sites,　respectively;　(2)　resonant　frequencies　estimated　using　the　SR　drifted　to　lower　and　higher　frequencies　for　flat　and　steep　sites,　respectively.　The　ineffectiveness　of　the　SR　for　flat　and　steep　stations　was　attributed　to　the　effects　of　topography　and　seawater　pressure　on　low-　and　high-frequency　vertical　ground-motions,　respectively.　These　results　suggest　that　the　SR　is　only　reliable　for　estimating　AF　curves　at　marine　free-field　sites　and　low-frequency　AFs　at　steep　sites.　The　LSG　may　play　an　important　role　in　offshore　ground-motion　studies.　©　2024　Elsevier　Ltd