Site Effect Analysis in the Taipei Basin: Results from TSMIP Network Data

A new 500-m gridded digital bathymetric data set has been producedby compiling available shipboard bathymetric data supplemented by globalbathymetric data sets in the area between 18°N and 27°N,and from117°E to 125°E.Combined with topographic data from GTOPO30,a globalland data set in 30 arc-second grid spacing,this new digital elevation model(DEM)reveals the regional as well as local morphology of Taiwan and itsoffshore area.Spatial resolution of 1km is achieved in the area off easternand southern Taiwan where swath bathymetric data are available.In otherareas where ship tracks are sparse,a spatial resolution of 4 arc-minute isretained.This DEM provides the best topographic information at presenton a regional scale,which helps to reveal many of the morphotectonic featuresrelated to the active tectonic processes of subduction and arc-continentcollision in this region.Using 2-D shaded topographic maps and 3-D physiographic diagramsgenerated from the DEM,the major morphologic features in each tectonicprovince of the region are presented.The Taiwan Strait is characterized bylow relief sea floor with two NE-SW trending depressions and a shallowbank in the center of the strait.Submarine canyons mark the continentalslope.In the area off southern Taiwan,N-S trending ridges and troughs arethe major morphological features,however,several NE-SW trending lineamentshave been identified in the Luzon forearc region.Off eastern Taiwan,submarine canyons and topographic features related to sedimentaryprocesses along the eastern flank of the Luzon Arc are revealed in detail.Aprominent N-S trending linear ridge,the Gagua Ridge,located along 123°Eon the West Philippine Basin floor is entering the Ryukyu Trench and hasproduced a big re-entrant at the frontal portion of the Yaeyama Ridge.E-W to NW-SE trending linear shear zones are observed over the YaeyamaRidge.These linear faults are the results of westward migration of the frontalportion of the accretionary wedge due to oblique convergence.A series offour forearc basins have been identified.Different depths of the forearcbasins reflect lateral variation of the forearc region from oblique subductionto collision.Along the northern wall of the Southern Okinawa Trough,faulted slope and subsided shelf blocks suggest that this region is underpost-collisional extension,and the active extension of the Southern OkinawaTrough is advancing westward toward Taiwan.