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.
To understand the complex damage pattern produced by the interaction between a 3-D sedimentary basin and 3-D spherical wavefronts, scattering of seismic waves by 3-D models due to a local point source is investigated by using the Pseudo-Spectrum method.The 3-D wavefields at different time steps are evaluated n111 nerically for a corner diffraction and a sediment-filled basin model.3-D wavefronts of both models are investigated from snapshots over free surface and vertical cross-sections.Numerical results show that model corners generate strong out-of-plane scattering energy which causes strong seismic energy focusing and defocusing in some specific locations.For an incident wave from a point source below the basin, the sediment-filled basin traps wave energy which propagates inward and focuses near the basin center.This energ)r extends to the basin b• ottom with decreasing amplitude.Besides, part of the incident energy is blocked by this basin, resulting in low amplitudes at the surrounding rock sites.This blocking effect is not predicted by the plane wave incidence.Comparisons are made with the results from 2-D models, and they show that the 3-D wavefronts from a point source or from in-plane scattering can be approximated by 2-D models; however, wavefronts from out-of-plane scattering cannot be reproduced.
The paper presents an estimation of the Earth’s crustal motion from the continuous GPS data at 6 stations (MTEV, MLAY, DBIV, TGIV, SMAV and SLAV) in the Northwestern and at PHUT (Hanoi) station using GAMIT/GLOBK software. The absolute displacements of the Earth’s crust at 7 stations in the IGS14 frame are respectively: 34.10±0.71 mm/yr (DBIV), 34.31±0.65 mm/yr (PHUT), 34.51±0.75 mm/yr (SMAV), 34.55±0.80 mm/yr (MLAY), 34.80±0.72 mm/yr (TGIV), 34.93±0.99 mm/yr (SLAV) and 35.59±0.73 mm/yr (MTEV), in the southeastward with the azimuth range 104-108o. The Son La fault is a right-lateral slip fault with a shear amplitude of ~1.5 mm/yr. The Lai Chau-Dien Bien fault is a left-lateral slip fault with a shear amplitude of ~1.9 mm/yr. Although the absolute velocities at the DBIV, SMAV, SLAV, TGIV and MLAY stations are evaluated with the error <1 mm/yr, the relative displacement on the Ma River fault is of ~0.5 mm/yr, and it seems that we still do not have a reliable assessment of the slip rate on the Ma River right-lateral slip fault.
Abstract The M w 8.4 Illapel earthquake occurred on 16 September was the largest global event in 2015. This earthquake was not unexpected because the hypocenter was located in a seismic gap of the Peru‐Chile subduction zone. However, the source model derived from 3‐D spectral‐element inversion of teleseismic waves reveals a distinct two‐stage rupture process with completely different slip characteristics as a composite megathrust event. The two stages were temporally separated. Rupture in the first stage, with a moment magnitude of M w 8.32, built up energetically from the deeper locked zone and propagated in the updip direction toward the trench. Subsequently, the rupture of the second stage, with a magnitude of M w 8.08, mainly occurred in the shallow subduction zone with atypical repeating slip behavior. The unique spatial‐temporal rupture evolution presented in this source model is key to further in‐depth studies of earthquake physics and source dynamics in subduction systems.
Abstract In observational seismology, the long-period background noise level in horizontal seismometer components is always higher than the vertical component at the same location. It is believed that the horizontal long-period noise is mostly attributed to ground tilt, yet the authentic tilt recordings of background noise in the seismic frequency band (0.01–20 Hz) have not been recorded successfully. We demonstrate the use of a newly developed optical-based rotation sensor, blueSeis-3A, to correct for the tilt effect in a horizontal seismometer. A station with a horizontal long-period (1 < T < 100 s) background noise level higher than the global new high-noise model was chosen, where a seismometer, a tiltmeter, and a rotation sensor were collocated. We first confirmed that the horizontal recordings from a tiltmeter and a seismometer were visually consistent, although it was unclear if tilt contamination existed in both recordings. It became evident until the ground tilt recorded by a rotation sensor was found to be waveform-matched with them, indicating that both sensors were contaminated by tilt. The spectrum of tilt records showed that ground tilts were mostly at period bands >1 s. By subtracting tilt from seismometer records, the horizontal background noise levels could be improved, approximately to the resolution level of the rotation sensors. Our results demonstrate that collocating rotation sensors with seismometers is a feasible way to improve the quality of the long-period background noise level in seismometer horizontal components when rotation sensors are sufficiently sensitive.
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.
A method for the finite element computation of the s)•nthetic seismo gran1s of SH wave propagation in a two-dimensional (2-D) medium is pro posed.The finite element method has the ad,rantage of specifying the arbi trar)' density and velocity field in the medium.To complete the waveform modeling for an earthquake, the transformation of line source snapshots and seismograms of the 2-D finite element modeling to the point source responses is critically necessary.The accuracy of the finite element model ing is verified b)1 comparing it with the generalized ray method and \\1ith a finite difference method which computes the 2-D wave field from a differ ent approach.The aim of this study is to demonstrate the successfulness of seismogram synthesis by the finite element method and to propose the potential applications of this method in the Taiwan area.
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