Velocity Structure Of Crust And Uppermost Mantle In Cross-Straits Of Southeast China And Adjacent Region

The cross-straits of Southeast China are located in the southeast Eurasian Plate,which exsits strong geological activities between Eurasian Plate and Pacific Plate.It is important to detect the velocity structure of crust and uppermost mantle to understand the tectonic evolution,earthquake generation processes,earthquake prediction and risk assessment.Based on new method and data,multi-dimesions velocity structure of crust and uppermost mantle were constructed for the study of network earthquake determination,earthquake generation,and tectonic evolution.The main achievements can be summarized as follows:Data from explosions provide a unique advantage to study velocity models as the source locations are precisely known.In this study,we hand-picked first Pg and Pn arrivals and secondary Pg arrivals from the explosion seismograms recorded at provincial stations.Using a joint inversion method,we inverted for a new 1-D P-wave velocity model,Fujian explosion model(FJEM).The FJEM model shows significant improvement in fitting the travel-time data over previous models.Similar velocity models inverted from subsets of explosion data suggest a relative stable and simple velocity structure in Fujian province.Relocations of the explosions suggest that the traditional South China model has small horizontal location errors(average 0.52±0.45 km),but errors in depth are significant(average 4.7±1.2 km),while the FJEM model has similar horizontal location capability but much smaller errors in depth(average 1.3±1.1 km).Synthetic tests using real earthquake bulletins show similar results for the South China model:(1)for earthquakes in the interior of Fujian with good azimuthal coverage,the horizontal mislocation is small;(2)the horizontal mislocations for earthquakes along the coast and in Taiwan Strait with poor azimuthal coverage can be very large;and(3)mislocations in depth can be large regardless station distribution,which have strong trade-off with origin time errors.Essentially,the horizontal location is largely affected by station azimuthal coverage and much less by the reference model.But the improved FJEM model not only is closer to the real Earth(fit data better)but also can reduce depth mislocation.We propose that South China model be replaced and the FJEM model be used in routine earthquake locations in Fujian and perhaps neighboring regions as well.The uppermost mantle structure and Moho depth are important for understanding the crust evolution.We combined bulletin and handpicked data to conduct Pn tomography of the region.The results show distinct features that are correlated with the complex geology at surface,suggesting a lithosphere-scale tectonics of the region.Low Pn velocities 'are found along a belt of the western Pacific transpressional system from the Okinawa Trough and eastern East China Sea,across central and eastern Taiwan orogeny,to the island arcs of the Luzon Strait and the entire Philippine Islands,as well as under the Palawan Island and part of the continental margin north of the Pearl River Basin.High velocities are found under Ryukyu subduction zone,part of the Philippine subduction zone,part of the Eurasian subduction beneath the southwestern Taiwan,and the continent-ocean boundary between the south China and the SCS basin.The Taiwan Strait,the Mainland SE coast,and the main SCS basin sea are relatively uniform with average Pn values.Crustal thicknesses show large variations in the study region but also coherency with tectonic elements.The crust thickness under SE Mainland coast(Fujian,Guangdong,and Guangxi)is quite uniform.The Pn pattern in Taiwan shows linear trends of surface geology and suggests strongly lithosphere-scale deformation of the young Taiwan orogenic belt marked by the deformation boundary under the Western Foothill and the Western Coastal Plain at depth,and the crustal thickness shows a complex pattern from the transpressional collision.Because of crustal strong heterogeneity under Taiwan Island,it will have an influence on the crustal thickness from station delay.We calculated the average crustal velocity using the latest 3-D velocity model and corrected for the station delay.The result from crustal thickness is similar to previous study from receive functions.It is concluded that the station delays in Taiwan are influenced significantly by velocity heterogeneities.Thus,our new method is effective to reduce error of crustal thickness for traditional Pn tomography.The inversion of uppermost mantle velocity and Moho depth are strongly influenced by crustal velocity heterogeneity.3D P-wave velocities were inverted by multi-phases joint inversion method in Fujian area.The fault zones in Fujian province have various velocity patterns.Our results show that shallow crust is characterized by high velocity that represents mountains,while the mid-lower crust shows low velocities.The anomaly velocities are correlated closely with tectonic faults in Fujian province.A lower velocity zone are found along Zhenghe-Dapu fault zone as mentioned by previous study,however our result shows those lower velocity exists at depth of 20?30 km in middle-lower crust.Compared with previous study,this lower velocity zone is larger and deeper at range and depth,respectively.Low velocity anomalies from the vertical profiles show that both Binhai fault zone and Zhenghe-Dapu fault zone are deep fractures that cut through the bottom of the crust.The crustal thickness of 28-35 km from joint inversion is similar to the result from receive function.Thin crustal thicknesses are found in offshore of Fujian province,thick crustal thicknesses are found in mainland.We also found that crustal thickness is thin along the Yongan-Jinjiang fault.The left side of Zhenghe-Dapu fault zone under mainland mainly shows high Pn velocity,while the Fuzhou basin and onshore show low Pn velocity.