| The precise information of the location,morphology,and velocity perturbation of the slab in the deep mantle is critical for deciphering the subduction history and the mantle dynamic processes.Generally,the deep mantle slab structure is mainly depicted from global tomography.However,most tomographic images could have underestimated amplitude and smearing morphology due to the smoothing and damping applied in the inversion process,resulting in bias in tectonic interpretation and mantle dynamics simulation.Therefore,a more precise deep slab structure is needed which will help us to explore the deep mantle comprehensively and gain better insight into their corresponding dynamic processes.The seismic waveforms provide extra constrains on deep mantle anomalies.In particularly deep mantle anomalies with sharp boundaries can generate multipathing effects with rays passing through them,and produce distorted waveforms.Meanwhile,the waveform information observed on the seismic array can be used to evaluate the multipathing effects and constrain the shape and physical characteristics of the anomalies.In this thesis,we apply the multipathing detection method to the waveform data recorded by the China National Seismograph Network,and find that the multipathing patterns vary significantly for the earthquakes from different directions.Especially,strong multipathing effects are observed from the southeast events.Comparing with tomography models,we find high-velocity anomalies located in the lower mantle of the East Asia margin,upper mantle below north Taiwan and the top of the lower mantle of southern Philippines.Combined with waveform modeling,we construct the deep mantle slab structures in these three representative regions.The obtained slab parameters,such as thickness,dipping angle,and lateral extension,provide basic mantle slab images for plate reconstruction in the western Pacific region.The multipathing patterns observed for events from the Solomon Island region vary among different events,which provide strong constraints on the depth of the lower mantle slab.Through detailed waveform modeling,we confirm the existence of a lower mantle slab dipping southeastward at a depth of 1000 km below the East Asia margin,with a S-wave velocity perturbation of 2.5%,a length of 1000 km,and a thickness of 200 km.Its lateral length along the northeast southwest direction is～1100 km.The spatial correlation between the imaged lower mantle slab and the plate reconstruction models suggests that this deep slab may be a fragment of the Izanagi Plate subducted beneath the proto-South China Sea during the late Cretaceous or the intra-oceanic subduction within the Izanagi Plate since the late Cretaceous.The earthquakes from New Guinea recorded by stations in the southeast coast of China and Taiwan region show strong waveform distortions.Although the stations in both southern and northern Taiwan show advanced arrivals,waveform distortions mainly occur in northern Taiwan.In order to better use the multipathing patterns to invert the deep mantle slab structure,we perform inversion tests by incorporating multipathing parameters in the objective function,which can achieve more stable result when there are significant gaps in the data and the signal-to-noise ratios is low.Combined with inversion and waveform modeling,we confirm that the waveform data recorded at the northern part of Taiwan can be fitted with a SE-dipping high velocity anomaly located at a depth of 500～650 km,with a length of 400 km,a thickness of 125～150 km,and an S-wave velocity perturbation of 4%～6%.Moreover,threedimensional simulations suggest that this high velocity anomaly in the north extends approximately 460 km in the southwest-northeast direction.We speculate that this highvelocity anomaly may be associated with the Eurasian plate subducting beneath the Philippine Sea Plate,while the different multipathing patterens between the southern and northern Taiwan suggest that a slab tear occurred beneath the northern Taiwan.The earthquakes recoded by stations in the Taiwan region from the Banda Sea exhibit strong out-of-plane multipathing.Tomography models also suggest a highvelocity anomaly at the top of the lower mantle beneath the southern Philippines.The waveforms recorded by the station in Taiwan show complicated azimuthal multipathing patterns,which are mainly caused by rays passing through the western boundary of the high-velocity anomaly.Moreover,the waveform simulations based on threedimensional model suggest that the high-velocity anomaly is located beneath the southern Philippines at a depth of 800～900 km,with a thickness of 100 km,and S-wave velocity perturbation of 4%to 8%.Comparing the detected lower mantle slab with plate reconstruction models,it appears that the high-velocity anomaly may have originated from the subducted Philippine Sea Plate during the Miocene. |
