ThenorthernmarginoftheSouthChinaSea,asatypicalextensionalcontinentalmargin,hasrelativelystrongintraplateseismicity.ComparedwiththeactivezonesofNanaoIsland,Yangjiang,andHeyuan,seismicityinthePearlRiverEstuaryisrelativelylow.However,aML4.0earthquakein2006occurrednearDanganIsland(DI)offshoreHongKong,andthissitewasadjacenttothesourceofthehistoricalM5.8earthquakein1874.TorevealtheseismogenicmechanismofintraplateearthquakesinDI,wesystematicallyanalyzedthestructuralcharacteristicsinthesourceareaofthe2006DIearthquakeusingintegrated24-channelseismicprofiles,onshore–offshorewide-angleseismictomography,andnaturalearthquakeparameters.WeascertainedthelocationsofNW-andNE-trendingfaultsintheDIseaandfoundthattheNE-trendingDIfaultmainlydippedsoutheastatahighangleandcutthroughthecrustwithanobviouslow-velocityanomaly.TheNW-trendingfaultdippedsouthwestwithasimilarhighangle.The2006DIearthquakewasadjacenttotheintersectionoftheNE-andNW-trendingfaults,whichsuggestedthattheintersectionofthetwofaultswithdifferentstrikescouldprovideafavorableconditionforthegenerationandtriggeringofintraplateearthquakes.Crustalvelocitymodelshowedthatthehigh-velocityanomalywasimagedinthewestofDI,butadistinctentitywithlow-velocityanomalyintheuppercrustandhigh-velocityanomalyinthelowercrustwasfoundinthesouthofDI.Boththe1874and2006DIearthquakesoccurredalongtheedgeofthedistinctentity.Twoverticalcross-sectionsnearlyperpendiculartothestrikesoftheintersectingfaultsrevealedgoodspatialcorrelationsbetweenthe2006DIearthquakeandthelowtohighspeedtransitioninthedistinctentity.Thisresultindicatedthatthetransitionalzonemightbeaweaklystructuralbodythatcanstorestrainenergyandreleaseitasabrittlefailure,resultinginanearthquake-pronearea.
