- 期刊
- OpenAccess
Active Tectonics of Taiwan Orogeny From Focal Mechanisms of Small-to-Moderate-Sized Earthquakes
臺灣的地震地體構造及潛在的地震震源區之辨識
並列摘要
A bottom-simulating reflector(BSR)has been identified in the offshoreTaiwan accretionary prism using both migrated 6-channel and 120-channelreflection profiles.The BSR is marked by a reversed polarity reflectorthat increases in sub-bottom depth with increasing water depth,suggestingthat the BSR marks the base of the methane hydrate stability field.TheBSR is located in offscraped sediments derived from the Taiwan orogenand the Chinese continental margin,which may contain high amounts oforganic carbon,thereby providing a source for the methane.Gas seepagefound on land north of this region also supports the presence of methane inthese strata.The BSR is typically located in the crests of anticlines andmud volcanoes,implying the migration and entrapment of methane gas.Recent uplift of the seafloor in anticlines may reduce pore pressure,therebydecomposing the hydrate phase into free gas,which may enhance the acousticimpedance contrast across the BSR.BSRs are conspicuously absent beneathsubmarine canyons in the region.A”flat spot”was found under theBSR within an anticline,suggesting that the hydrate-filled pore-space mayresult in lower sediment permeability,therefore enhancing the entrapmentof free gas.We document the areal extent and sub-bottom depth of theBSR over a 77000 km^(2)-wide region around southern Taiwan.In particularat least 30% of the seafloor,to as much as 60%,along the trenchward slopeof the accretionary prism is underlain by the BSR.The BSR is located atshallower sub-bottom depths on the trenchward(western)side of the accretionaryprism than on the arcward(eastern)side,even though waterdepths are similar,implying enhanced fluid flow from depth,resulting in ahigher geothermal gradient as sediments are accreted and dewatered alongthe Manila trench.