In　densely　populated　urban　areas,　the　presence　of　tunnels　may　modify　the　input　motion　for　the　seismic　design　of　the　aboveground　However,　neither　building　codes　nor　seismic　zonation　guidelines　have　yet　considered　this　matter.　In　this　paper,　the　effects　of　an　underground　tunnel　on　the　ground　are　numerically　investigated,　in　terms　of　the　peak　ground　acceleration　and　acceleration　response　spectrum.　Compared　with　existing　related　studies,　this　paper　goes　further,　considering　obliquely　incident　seismic　waves.　The　results　indicate　that　the　underground　tunnel　influences　both　the　magnitude　and　distribution　of　the　ground　accelerations.　The　amplification　of　the　peak　ground　accelerations　due　to　the　presence　of　the　underground　tunnel　can　be　significant,　and　the　amplification　factors　are　functions　of　the　incident　angle　of　seismic　waves.