| A China Seismological Reference Model(CSRM)provides important constraints on determining the temperature,pressure,and composition of bedrocks in the crust and mantle of the continental China,advancing our understandings of regional geodynamics and tectonic evolution history.In this study,we construct a CSRM from top to bottom using constraints of multiple seismic observations with three projects:(ⅰ)we develop a new method to combine seismic ambient noise and regional earthquake surface wave for better resolving the Earth’s subsurface structure,improving the surface wave data coverage in areas with sparse seismic station coverage;(ⅱ)we construct a highresolution shallow three-dimensional(3-D)seismic model in the top 10 km of the upper crust in the continental China,using constraints of the short-period Rayleigh wave ellipticity,P-wave polarization and receiver function;(ⅲ)we construct a highresolution 3-D seismic model in the top 100 km of the crust and uppermost mantle in the continental China,using constraints of the tele-seismic Raleigh wave ellipticity,receiver function and Raleigh wave phase/group velocity dispersion.(ⅰ)Both the regional earthquake surface wave and seismic ambient noise provide important constraints on the Earth’s structure,and yet no study both minized tomographic errors from seismic sources and combined them for the best imaging of subsurface seismic structure.In this study,we address this issue by developing a new method to simultaneously determine surface wave phase velocity and earthquake centroid parameters in three steps:1)preliminary phase velocity inversion based on seismic ambient noise,2)preliminary earthquake relocation based on earthquake surface wave data,and 3)simultaneous inversion for phase velocity and earthquake centroid parameters with constraints of inter-station phase velocity measurements based on seismic ambient noise and event-station phase velocity measurements based on earthquake surface wave data.Application of the method in the North South Seismic Belt region in China results in high-resolution Rayleigh wave phase velocity maps and accurate earthquake centroid parameters.The additional earthquake data notably improve resolution of the inverted phase velocity model in west Yunnan and central Tibetan blocks,the regions with sparse seismic station coverage.The inverted phase velocity model exhibits high-velocity anomalies in cratonic regions and the Emeishan Large Igneous Province,and low-velocity anomalies in the interior and surrounding regions of the Tibetan Plateau.Relocation places earthquakes in shallow depths with geotherm above the crustal rock’s brittle-ductile transition temperature of～400℃,revealing thermal control on thickness of the seismogenic zone.With earthquake centroid parameters constrained,earthquake data are expected to provide further constraints on the deep seismic structure that is beyond the sampling limit of seismic ambient noise.(ⅱ)We construct a high-resolution shallow 3-D seismic model in the top 10 km of the upper crust in the continental China,with constraints of P polarization,Rayleigh wave ellipticity and receiver function obtained from records of 4193 seismic stations.Our 3-D seismic model has a spatial resolution of 0.6°-1.2° in the north-south seismic belt and the trans-north China orogen,and 1°-2° in the rest of the continental China(except the Tarim basin and the southwest Tibet).The seismic model exhibits low velocity anomalies of deposits in major sedimentary basins and high velocity anomalies of crustal bedrocks in young orogenic belts and old tectonic blocks.The inferred sediment thickness maps display thick deposits in major sedimentary basins,some compacted sediments in the intermontane basins in young orogenic belts and little sediments in old tectonic blocks.We also discuss compaction effects of the sediments and implications of tectonic history and geological evolution of the major basins in the continental China based on the inferred seismic models.(ⅲ)We construct a high-resolution 3-D seismic model(i.e.,CSRM)in the top 100 km of the crust and uppermost mantle,using constraints of the receiver function,teleseismic Rayleigh wave ellipticity and phase/group velocity dispersion obtained from records of 4193 seismic stations,and prior constraints of the shallow seismic model[1]obtained in the first step,a crustal Vp/Vs model[2],and a Pn-wave velocity model[3].Spatial resolution of the CSRM is about 0.6° in the north-south seismic belt zone and trans-north China orogen,and about 1.2° in the rest of the continental China.In the upper crust,the seismic velocity(Vs and Vp)model exhibits low-velocity anomalies in major sedimentary basins and high-velocity anomalies of heterogeneities in the crust.In the middle and lower crust,the seismic velocity model reveals several prominent low-velocity anomalies of crustal bedrocks in the central Tibetan Plateau and its peripheral region,and high-velocity anomalies in the cratonic regions.At the uppermost mantle,heterogeneities in the northern Qiangtang block,eastern Songpan-Ganzi block,Tengchong and Datong volcanic regions are characterized by low seismic velocity and high mantle Vp/Vs,while the cratonic roots in the mantle characterized by high seismic velocity and low mantle Vp/Vs.The crust beneath the continental China manifests a conspicuous eastward thining trend,and major basins in the eastern China have a thinned crust in comparison with their surroundings.Analysis based on the CSRM reveals that the intrusion rocks in the continental China are mainly granite at the depth shallower than 6 km,and transfer into metamorphism rocks of amphibite and gneiss at the depth deeper than 6 km.The CSRM obtained in this study provides constraints on the temperature and composition of bedrocks in the crust and uppermost mantle,and can be used as a reference seismic structure in future studies of the seismic hazard assessment,earthquake source and high-resolution seismic tomography of the deeper mantle. |
