We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31 degrees N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

S U M M A R Y3-D V P and V S models for the crust and upper mantle beneath the Taiwan area have been determined using selected high-resolution earthquake data from an island-wide seismic network and two local seismic arrays. Lateral structural variations in the upper crust, as also evident from surface geology, are responsible for the observed large traveltime residuals or station corrections. Prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination (JHD) station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion. A finite-difference method, that is efficient and accurate for a highly heterogeneous velocity structure, is applied to calculate P-and S-wave traveltimes from the source to receiving stations. All earthquakes in the Taiwan Central Weather Bureau's catalogue are then relocated using the resultant 3-D V P and V S models. The depth of the Moho varies significantly, especially along the east-west direction. In the western Coastal Plain and Western Foothills the depth of the Moho is around 35 km, which deepens gradually eastward, reaches a maximum depth of ∼55 km beneath the eastern Central Mountain Range, shallows up rapidly beneath the Longitudinal Valley and Coastal Range, and merges with the thin Philippine Sea Plate offshore of eastern Taiwan. In central Taiwan, the Central Mountain Range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km. Therefore, the uplifted and thickened Central Mountain Range serves as a backstop for the converging Eurasian and Philippine Sea plates. The crust beneath the Central Mountain Range is characterized by a brittle, high-velocity and seismically active upper crust (<15 km) and a ductile, low-velocity and aseismic mid-to-lower crust (below 15 km), most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east, from the surrounding hotter upper mantle beneath the thickened continental crust, and from shear heating during active collision. The collision zone in eastern Taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low V P and high V P /V S ratio near the Taitung region in southeastern Taiwan. It transforms into an active westward steeply dipping seismic zone along a transition zone between the high V P and V S oceanic crust and the low V P and V S continental crust near Hualien region in central eastern Taiwan. There is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion. However, a few basins are either bounded on one side by an active fault or are characterized by blind faults beneath. The geometry of the subduction zone in northeastern Taiwan can be clearly imaged from the relocated earthquake locations. Behind the subduction, a region of low V P and high V P /V S ratio at depths of 5 to 10 km can be identified beneath the Tatun-Ch...

[1] The repetition of slip during rupture process of earthquake is a debate issue which had never been confirmed clearly in the past big events due to the lack of dense nearfield observations and limited resolution in time of source model. The 2011 M9.0 Tohoku-Oki earthquake generated a wealth seismic records which provided us an unprecedented opportunity to study the rupture evolution of giant earthquake at a high spatio-temporal resolution. Here we use teleseismic, local strong motion and near-field coseismic geodetic data to investigate the source rupture process of this event based on the parallel inversion technique. The results reveal a broad slip zone with remarkable large scale repeating slip during the earthquake. The inverted source model shows several time periods of energy release with three main peaks. These energy bursts and temporal rupture snapshots suggest repetition of a large scale slip on the biggest asperity. This rupture behavior resulted in >50 m slips on the slip zone and prolonged the entire rupture process for a long duration of ∼160 seconds. The proposed source model is in a good agreement with the aftershock distribution and can interpret the characteristics of local strong motions. Further investigations of repeating slip during this event are crucial which will deeply transform earthquake science from dynamic point of view. Citation: Lee,

We combine light detection and ranging (LiDAR) digital terrain model (DTM) data and an improved mesh implementation to investigate the effects of highresolution surface topography on seismic ground motion based upon the spectralelement method. In general, topography increases the amplitude of shaking at mountain tops and ridges, whereas valleys usually have reduced ground motion, as has been observed in both records from past earthquakes and numerical simulations. However, the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations, especially the seismic response of the true ground surface. Here, we use LiDAR DTM data, which provide two-meter resolution at the free surface, and a spectral-element method to simulate three-dimensional (3D) seismic-wave propagation in the Yangminshan region in Taiwan, incorporating the effects of realistic topography. A smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces. Numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content. The amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat-topped hills experience lower levels of ground shaking. Interaction between small-scale topographic features and high-frequency surface waves can produce unusually strong shaking. We demonstrate that topographic variations can change peak ground acceleration (PGA) values by 50% in mountainous areas, and the relative change in PGA between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response. This suggests that high-resolution, realistic topographic features should be taken into account in seismic hazard analysis, especially for densely populated mountainous areas.

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