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The 5 February 2016, Meinong, Taiwan, earthquake brought extensive damage to nearby cities with significant pulse-like velocity ground motions. In addition to the spatial slip distribution determination using filtered strong-motion data, we show that, with the advantage of the densely distributed seismic network as a seismic array, we can project the earthquake sources (asperities) directly using nearly unfiltered data, which is crucial to the understanding of the generation of the pulse-like velocity ground motions. We recognize that the moderate but damaging ML 6.6 Meinong earthquake was a composite of an Mw 5.5 foreshock and an Mw 6.18 mainshock with a 1.8-5.0 s time delay. The foreshock occurred at the hypocenter reported by the official agency, followed by the mainshock with a centroid located at 12.3 km to the north-northwest of the hypocenter and at a depth of 15 km. This foreshock-mainshock composition is not distinguishable in the finite-fault inversion because it filtered the seismic data to low frequencies. Our results show that the pulse-like velocity ground motions are mainly attributed to the source of mainshock with its directivity and site effects, resulting in the disastrous damages in the city of Tainan. Although finite-fault inversion using filtered seismic data for spatial slip distribution on the fault has been a classic procedure in understanding earthquake rupture processes, using a dense seismic network as a seismic array for unfiltered records helps us delineate the earthquake sources directly and provide more delicate information for future understanding of earthquake source complexity. [ABSTRACT FROM AUTHOR]

Seismic data collected during explosion experiments performed as part of the TAiwan Tiitegrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high-resolution images of the structure of the crust and upper man- tle beneath Taiwan. The most significant feature observed at near-source stations located on the western coastal plain in Taiwan is high-energy later arrivals. These high-amplitude multiples almost completely mask the lower-amplitude signals (seismic refraction and wide-angle reflection) from the deep crust. The later arrivals are identified as free-surface- reflected multiples. The nature and generation of these high-energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer-velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1 D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P-wave velocity of 1.76 km/s, and an average velocity gradient of about 0.72/s from the surface to 3.0-km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multi- ple diving waves with the aim of enhancing the deep-surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [ABSTRACT FROM AUTHOR]

We combine light detection and ranging (LiDAR) digital terrain model (DTM) data and an improved mesh implementation to investigate the effects of high-resolution surface topography on seismic ground motion based upon the spectral-element 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 fiat-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. [ABSTRACT FROM AUTHOR]

Topography influences ground motion and, in general, increases the amplitude of shaking at mountain tops and ridges, whereas valleys have reduced ground motions, as is observed from data recorded during and after real earthquakes and from numerical simulations. However, recent publications have focused mainly on the implications for ground motion in the mountainous regions themselves, whereas the impact on surrounding low-lying areas has received less attention. Here, we develop a new spectral-element mesh implementation to accommodate realistic topography as well as the complex shape of the Taipei sedimentary basin, which is located close to the Central Mountain Range in northern Taiwan. Spectral-element numerical simulations indicate that high-resolution topography can change peak ground velocity (PGV) values in mountainous areas by ±50% compared to a half-space response. We further demonstrate that large-scale topography can affect the propagation of seismic waves in nearby areas. For example, if a shallow earthquake occurs in the I-Lan region of Taiwan, the Central Mountain Range will significantly scatter the surface waves and will in turn reduce the amplitude of ground motion in the Taipei basin. However, as the hypocenter moves deeper, topography scatters body waves, which subsequently propagate as surface waves into the basin. These waves continue to interact with the basin and the surrounding mountains, finally resulting in complex amplification patterns in Taipei City, with an overall PGV increase of more than 50%. For realistic subduction zone earthquake scenarios off the northeast coast of Taiwan, the effects of topography on ground motion in both the mountains and the Taipei basin vary and depend on the rupture process. The complex interactions that can occur between mountains and surrounding areas, especially sedimentary basins, illustrate the fact that topography should be taken into account when assessing seismic hazard. [ABSTRACT FROM AUTHOR]

We perform full elastic wave-field simulations within the Taipei basin by using a three-dimensional (3D) discontinuous finite-difference method. The 3D Taipei basin model is determined from a seismic reflection study. Two major subsurfaces, the Songshan formation (surface soil layer) and the basin basement, are constituted in the model. A parallel-based composite grid technique, a containing scalene grid and a discontinuous grid, is developed in this study to deal with the possible numerical problem of thin depth and low velocity of the Songshan formation. Taking advantage of the composite grid, the resolution of the subsurface structure can be reached to 20 m, and a higher frequency (up to 3 Hz) of the synthetic waveform can be achieved. In our strong ground motion simulations, we assume a constant velocity in each subsurface. Three different types of models are considered in the study: the Songshan formation with a basement structure model, a basin basement model, and a layered half-space model. Results indicate that only the model with both the Songshan formation and the basement structure can produce the apparent basin amplification effects. First, the surface wave generated after the primary S wave is trapped at the shallow part of the basin. Then, when the wave propagates through the deepest part of the basin, most of the energy is reflected from the boundary and focused back into the basin. In addition, part of the seismic wavefront turns and follows the shallow basin edge resulting in further amplification. Our study indicates that the complex Taipei basin geometry and fairly low velocity of the Songshan formation dominate the amplification and wave propagation behavior that result in extraordinary strong shaking patterns in the Taipei metropolitan region. [ABSTRACT FROM AUTHOR]

We use the spectral-element method to simulate strong ground motion throughout the Taipei metropolitan area. Mesh generation for the Taipei basin poses two main challenges: (1) the basin is surrounded by steep mountains, and (2) the city is located on top of a shallow, low-wave-speed sedimentary basin. To accommodate the steep and rapidly varying topography, we introduce a thin high-resolution mesh layer near the surface. The mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave-speed contrasts. Variations in Moho thickness beneath Northern Taiwan are also incorporated in the mesh. Spectralelement simulations show that ground motion in the Taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains. The amplification of ground motion is mainly controlled by basin depth and shallow shear-wave speeds, although surface topography also serves to amplify and prolong seismic shaking. [ABSTRACT FROM AUTHOR]