Theroleplayedbythediffractionfieldontheproblemofseismicsiteeffectsisstudied.ForthatpurposewesolveandanalyzesimplescatteringproblemsunderPandSVin-planewaveassumptions,usingtwowellknowndirectboundary-element-basednumericalmethods.Afterestablishingthedifferencebetweenscatteredanddiffractedmotions,andintroducingtheconceptofartificiousandphysicallybasedincomingfields,weobtaintheamplitudeoftheFourierspectraforthediffractedpartoftheresponse:thisisachievedafterestablishingtheconnectionbetweenthespatialdistributionofthetransferfunctionoverthestudiedsimpletopographiesandthediffractedfield.Fromthenumericalsimulationsitisobservedthatthisdiffractedpartoftheresponseisresponsiblefortheamplificationofthesurfacegroundmotionsduetothegeometriceffect.Furthermore,itisalsofoundthatthediffractionfieldsetsinafingerprintofthetopographiceffectinthetotalgroundmotions.Theseconclusionsarefurthersupportedbyobservationsinthetime-domainintermsofsnapshotsofthepropagationpatternsoverthecompletecomputationalmodel.Inthissensethegeometricsingularitiesareclearlyidentifiedassourcesofdiffractionandfortheconsideredrangeofdimensionlessfrequenciesitisevidentthatlargeramplificationsareobtainedforthegeometriescontainingalargernumberofdiffractionsourcesthusresultinginastrongertopographiceffect.Theneedforclosed-formsolutionsofcanonicalproblemstoconstructarobustanalysismethodbasedonthediffractionfieldisidentified.
