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The strong Hualien earthquake (Mw 6.1) occurred along the suture zone of the Eurasian Plate and the Philippine Sea Plate, which struck the Hualien city in eastern Taiwan on April 18, 2019. The focal mechanism of this earthquake shows that it is caused by a rupture within a thrust. In the present study, the rupture plane responsible for this earthquake has been modeled using the modified semi-empirical technique (MSET). The whole rupture plane is assumed to be composed of strong motion generation areas (SMGAs) along which the slip occurs with large velocities. The spatiotemporal distribution of aftershocks of this earthquake within identified rupture plane suggests that there are two SMGAs within the rupture plane. The source displacement spectra (SDS) obtained from the observed records have been used to compute the source parameters of these two SMGAs. The MSET efficiently simulates strong ground motion (SGM) at the rock site. The shallow subsurface shear wave velocity profile at various stations has been used as an input to SHAKE91 algorithm for converting records at the surface to that at the rock site. The simulated records are compared with the observed records based on root-mean-square error (RMSE) in peak ground acceleration (PGA) of horizontal components. Various parameters of the rupture plane have been selected using an iterative forward modeling scheme. The accelerograms have been simulated for all the stations that lie within an epicentral distance ranging from 5 to 100 km using the final rupture plane parameters. The comparison of observed and synthetic records validates the effectiveness of the simulation technique and suggests that the Hualien earthquake consists of two SMGAs responsible for high-frequency SGM. [ABSTRACT FROM AUTHOR]

Surface wave tomography usually adds a smooth term into the objective function to make the edge smoothing. This paper will make an improvement on the inversion method under Gaussian probability distribution condition during the iterative process of near-surface inversion, which is based on ray tracing, using the sensitivity parameters in G matrix as spatial smoothing constraints. We try to select self-adapted parameters, following the laws of physics. These parameters include damping factor, model deviation, smoothing factor, and sensitivity parameters. Improved inverse algorithm will be used in numerical simulation and real data, from ambient noise of Taipei Basin of Taiwan, to achieve the near-surface shear wave velocity structure. Compared with the earlier research results in this region, we find the same points and differences. By the process above, we verify the reasonability of the selection of the self-adaption parameters, validity of the improved inverse algorithm and the reliability of the code. The improved algorithm provides a fast, self-adapted, less empirical and universal tool and method to study the near-surface shear wave velocity structure. [ABSTRACT FROM AUTHOR]

The Chung-Li very-high-frequency (VHF at 52 MHz) atmospheric radar and a dual-polarized microwave radiometer were operated collaboratively to investigate strongly convective precipitation while the typhoon Trami just passed through the Taiwan in Aug, 2013. First, respective Doppler velocities of clear-air and precipitation echoes were identified automatically by the VHF radar as clearly as possible. Two approaches were designed for this purpose: contour-based and peak-finding processes. The two approaches initially determined some major spectral centers or peaks, which were usually redundant, and then proper sifting and clustering were performed for the redundant spectral centers or peaks to yield several mean locations of Doppler velocities for profiling. The outcomes of the two approaches were consistent in general. With the estimated Doppler velocities, a tracing process was developed for Doppler profiling, in which Doppler velocity shear was one of the significant criteria in the tracing process. The radar echoes collected by the VHF radar during rainy and strongly convective atmosphere have been investigated to validate the two approaches and the tracing process. About 98% of the tracings could identify the Doppler profiles of clear air and precipitation, even the atmosphere was disturbed severely. The radar spectral parameters, Doppler profiles, and the information from a dual-polarized microwave radiometer as well as the simulation of weather model, were examined jointly. It signified that strong updraft and turbulent atmosphere could bring the liquid water to the height above the melting layer, and then the Bergeron effect and coalescence process on formation of ice crystal and graupel above the height of the melting layer occurred accordingly. [ABSTRACT FROM AUTHOR]

The high-frequency decay parameter kappa (κ) was computed by fitting Fourier amplitude spectra from the seismic network of the Taiwan Strong Motion Instrumentation Program (TSMIP). The κ of shear horizontal waves (SH waves) was calculated for individual recordings, and the relationship between κ values and the epicentral distance (Repi) of each station was derived for each station. The κ value at Repi = 0 (denoted as κ0) can be used as a site parameter. There are totally 26,277 seismograms that have been recorded at 679 TSMIP stations over the period of 1993 through 2014 with local magnitudes of 4.0–7.1 and focal depths less than 30 km. The estimation of κ0 for Taiwan ranges from 0.0208 to 0.147 s, and the spatial distribution of κ0 was closely related to geology and velocity. The site-specific κ0 values from 425 stations were correlated with the averaged shear wave velocity of the top 30 m of strata (VS30), and the relationship could be described as κ0 = (0.125 ± 0.005) − (0.011 ± 0.001) × ln(VS30), and an acceptable linear correlation (R2 = 0.57) was performed. The results may be used in the future application of ground motion prediction equations (GPMEs) and serve as simulation parameters. Also, the relationship between κ0 and the depth to engineering rock (Z1.0) is not well correlated, whereas an acceptable correlation exists between κ0 and other site proxies (i.e., elevation and resonant frequency). [ABSTRACT FROM AUTHOR]

Combining cruise observations, satellite altimeter and Argos drifter data in the northeastern South China Sea in the spring of 2017, this paper reports a Luzon Cold Eddy (LCE), which originated near the northwestern coast of Luzon Island. Then it migrated northwestward and was sampled by the cruise transect to the southwest of Taiwan Island. In the core of the LCE, the isotherm, isohaline and isopycnal all uplifted by approximately 100 m within the depth range of 150–300 m. The corresponding geostrophic currents were consistent with satellite altimeter results. In addition, a double-index was applied to study the dynamical process for the LCE evolution. The index reflects well the intraseasonal variability of eddies and the Kuroshio pathway in the spring of 2017. During the second half of April, the enhancement of the LCE was regulated by the velocity shear resulting from a large anticyclonic eddy east of the Luzon Strait. Taken together, the results of this study demonstrate a better understanding of the circulation and mesoscale patterns and a potential implication for water mass renewal in the northeastern South China Sea. [ABSTRACT FROM AUTHOR]

Seismic risk assessment of lifeline networks or building portfolios requires the quantification of spatial correlation of ground motion intensity over a region during an earthquake event. In this paper, the spatial correlation of vertical spectral accelerations (SAs) at single and multiple periods is studied. In the first part, the spatial correlations of vertical SAs at eight periods in the range of 0.0–3.0 s are investigated. The geostatistical analysis is performed to compute the spatial correlations of SAs using records compiled from ten earthquake events occurred in California, Japan, Taiwan, and Mexico. The results show that the spatial correlations of the vertical SAs at short periods are dependent on the regional site conditions indicated with the spatial correlation of shear wave velocity in the top of 30 m (Vs30). However, the effect of the correlation of Vs30 on the correlation of the vertical SAs decreases as the spectral period increases. A simple predictive model is proposed to quantify the correlation ranges of the vertical SAs considering regional site conditions. The proposed model is compared with the models presented for horizontal SAs. The comparison shows that the correlation ranges of horizontal SAs are generally larger than those of the vertical ones. As an illustrative example, seismic performance of a hypothetical bridge network is studied with different ranges of spatial correlations. The results indicate that ignoring spatial correlation may significantly affect the exceedance probability for spatially distributed engineering demand parameters of bridges in the network. In the second part, the multivariate spatial correlation is investigated using linear model of coregionalization (LMC) approach. Based on the correlation estimates obtained from geostatistical analysis, a predictive model is proposed to simulate vertical SAs at multiple periods and different sites. [ABSTRACT FROM AUTHOR]

The Fujian-Taiwan Strait-Taiwan area has undergone multiple periods of alternate compressional and extensional tectonic events, forming a complex geological structure. The underground structure and deformation characteristics, reflected by three-dimensional velocity structure and anisotropy, are the basis for analyzing the influences of different geological processes. In this study, utilizing 2 years of continuous waveform data from more than 100 permanent stations in Fujian and Taiwan, we extracted Rayleigh and Love wave dispersion data based on the cross-correlation of vertical and tangential components. A direct inversion method was employed to obtain the 3-D shear wave velocity and radial anisotropy structure of the crust. Vertical and horizontal polarized shear wave velocity structures were also inverted separately, and then used to calculate another radially anisotropic velocity model for comparison. The results show that the middle and lower crust of Fujian and the Taiwan Strait are dominated by positive radial anisotropy attributed to the near-horizontal alignment of minerals caused by extension. Although in an extension environment, most of the shallow crust exhibits weak anisotropy due to the vertical cutting of fractures and fissures, except for the strait area covered by horizontal sedimentary layers leading to strong positive radial anisotropy. In addition to the influence of faults and other shallow structures, the stress distribution caused by metamorphism also plays an important role; thus, the upper crust of the marine metamorphic belt shows negative radial anisotropy. Different from the strong positive anisotropy in other parts of the lower crust, relatively weak anisotropy appears beneath the Penghu area, which is related to two-stage Cenozoic extension. • A direct inversion method was adopted to obtain the crustal radial anisotropy beneath Fujian and the Taiwan Strait. • Most of the study region is dominated by positive radial anisotropy due to Mesozoic to Cenozoic extension. • The marine metamorphic belt shows negative anisotropy and may be affected by Mesozoic metamorphism. [ABSTRACT FROM AUTHOR]

The convergence between the Eurasian Plate and the Philippine Sea Plate results in rapid uplifting of the Taiwan Orogen. To study the mountain building in Taiwan, we present three transects of shear wave velocity across the Taiwan mountain belt with joint inversion of teleseismic Rayleigh wave H/V ratios and phase velocities derived from ambient noise tomography. The Particle Swarm Optimization algorithm is introduced into the joint inversion and is proved to be an efficient tool in solving the geophysical inverse problems. In determining the H/V ratios, cross-correlation techniques are used to reduce the non-great circle effects of the Rayleigh wave packets and to correct sensor misorientations. The cross-correlation techniques improve the accuracy of the H/V measurements, especially at the short periods. The observed up-to-middle crustal thickening in the central segment of the Central Range suggests lithospheric collision tectonism, whereas the lower crustal thickening in southern segment is proposed to be related to subduction of the Eurasian continental crust. In the north, however, no crust thickening is observed. The variations of the crust architecture along the axis of the Taiwan Orogen favor a close coupling relationship between mountain building and slab interactions in the mantle. Beneath the Central Range, the twist of the fate of the Eurasian continental crust from consumption in the south to collision in the central segment is proposed to be resulted from the slab breakoff beneath central Taiwan. • High-resolution shear wave velocity transects across the Taiwan Island from joint inversion. • Deflection of Rayleigh wave packet from the great-circle path. • Coupling of the mountain building with the subsurface slab interactions in Taiwan. [ABSTRACT FROM AUTHOR]

Taiwan lies in the circum-Pacific earthquake zone. The seabed soil of offshore wind farms in Taiwan is mainly composed of loose silty sand and soft, low-plasticity clay. The seismic demand for offshore wind turbines has been given by the local code. Ground-motion analysis is required to consider the site effects of the soil liquefaction potential evaluation and the foundation design of offshore wind turbines. However, the depth of the engineering bedrock for ground motion analysis is not presented in the local code. In this study, we develop a three-dimensional ground model of an offshore wind farm in the Changhua area, through use of collected in situ borehole and PS (P wave (compression) and S (shear) wave velocities) logging test data. The engineering bedrock is the sediment at the depth where the average shear wave velocity of soil within 30 m, Vsd30, is larger than 360 m/s. In this ground model, the shear wave velocity of each type of soil is quantified using the seismic empirical formulation developed in this study. The results indicate that the engineering bedrock lies at least 49.5–83 m beneath the seabed at the Changhua offshore wind farm. Based on these findings, it is recommended that drilling more than 100 m below the seabed be done to obtain shear wave velocity data for a ground response analysis of the seismic force assessment of offshore wind farm foundation designs. [ABSTRACT FROM AUTHOR]

The shear wave velocity profile and dynamic soil properties are known to be affected by aleatory uncertainty. This paper aims to investigate the effect of a statistical variation in the initial stiffness profile, stiffness degradation and damping curves on ground response predictions by conducting stochastic analysis. The Large Scale Seismic Test site in Lotung, Taiwan, is back-analysed with a fully-coupled finite element procedure using an advanced kinematic hardening soil model. Two ground motions recorded at the site, one strong and one weak, are applied at bedrock level. The results reveal that the site response prediction is sensitive to the seismic intensity of the input motion. When the level of induced shear strain is higher, i.e. in the case of the stronger motion, the spatial variability of the stiffness degradation and damping curves has a pronounced effect on the predicted site response. In contrast, when the weaker motion is considered the prediction is particularly sensitive to the statistical variation in the initial stiffness profile. This is mainly due to the stiffness degradation at very small strains shown by the laboratory data on LSST soils, which is captured in the paper by assuming an appropriate elastic domain in the constitutive model calibration. [ABSTRACT FROM AUTHOR]

The shear wave velocity (V s) of sediments plays a key role in seismic wave amplification and is required in site response analysis. Such information is usually lacking during field exploration, but standard penetration test blow count (N) is typically available. Therefore, several studies have established empirical correlations between N and V s for engineering use. However, these empirical correlations significantly vary in terms of model form and are only applicable to specific soil types, such as sand or clay. A unified empirical correlation for a wide range of soil types, which contains several soil properties (e.g., liquid limit and plasticity index (PI)) in addition to the V s and N of strata, is developed in this study using the Engineering Geological Database for the Taiwan Strong Motion Instrumentation Program. Influences of confining stress, fines content (FC), PI, and soil types on small-strain properties (i.e., V s) and large-strain measurements (i.e., N) are first evaluated through the developed correlations with these parameters. The unified correlation between V s and N that is dependent on confining stress, FC, and PI is then proposed through the conditional prediction approach. The model successfully applies to different regions in Taiwan that includes various types of soil deposits and, thus, is potentially used for the other regions. • A unified correlation between SPT–N and Vs for a wide range of soil types is developed. • The model considers the different behaviors of small strain (Vs) and large strain (SPT-N). • The model is developed based on the conditional prediction approach. • The model contains confining stress, fines content, and plasticity index in addition to the Vs and N. • The model successfully applies to different regions in Taiwan and is potentially used for the other countries. [ABSTRACT FROM AUTHOR]

Recent advances in understanding the scalar transfer across the wind-driven air-water interface, including both experimental observations and numerical simulations, reveal that the transport processes are dominated by the along-wind circulatory motions within the aqueous boundary layer. Such circulatory motions are attributed to coherent elongated eddies arising within the turbulent shear layer and Langmuir cells formed from the interaction between wave drift and shear layer. These streamwise vortices manifest themselves by inducing distinct elongated streaks on the water surface. The population density of these streaks, which can be quantified by the distribution of transverse spacing between streaks, thus characterizes the rate of interfacial scalar transfer. The prevalent notion of transverse spacings between streaks on non-breaking wind-driven wavy surfaces is that, similar to the streaks within a wall boundary layer, the statistics of non-dimensional streak spacing are essentially invariant with Reynolds number exhibiting constant mean value and remarkably similar probability distributions conforming to lognormal behavior. In this study, such statistics are reappraised by analyzing infrared images of non-breaking wind waves at various wind speeds. The experiments were conducted in the wind-wave facility Aeolotron at the University of Heidelberg. Thermal footprints induced by gravity-capillary waves and Langmuir cells are first extracted from the infrared images, and the streaky signatures solely attributed to elongated coherent vortices of the turbulent shear layer are therefore derived. Image segmentation technique is applied to identify the elongated streaks. The streak transverse spacings are then measured, and their ensemble statistics are calculated. The results reveal that the probability distributions of streak spacing all exhibit lognormal probability distribution for various non-breaking wind-wave conditions. The mean streak spacing decreases as the wind friction velocity increases. The non-dimensional mean streak spacing in viscous length scale, however, increases with the wind friction velocity; this contradicts to the previous conclusions that non-dimensional spacings remain to be approximately constant. The oscillatory motions of surface waves on the turbulent shear layer might be the cause of such a variation.This work was supported by the Taiwan Ministry of Science and Technology (MOST 107-2611-M-002 -014 -MY3). [ABSTRACT FROM AUTHOR]

In order to avoid the casualties caused by damaged buildings during strong earthquakes, it's important and essential to understand the seismic design specifications of buildings. The motion of a building depends on the interferometry of building and ground motion, the coupling between building and ground, and the mechanical properties of the building. We applied deconvolution to the motion recorded to separate the building response. Deconvolution interferometry is a powerful and convenient tool which allow us to extract structural parameters (e.g., shear wave velocity, resonance frequency and attenuation parameter) from seismic records. By applying deconvolution to earthquake records, it can provide a complete response of the building and understanding the motion pattern of the structure under strong motion. Therefore, we can check medium's state by tracking resonance frequency, shear wave velocity and the attenuation parameter Q to avoid damage of building in the next mega-earthquake. We deployed a seismograph array in the library of National Chung Cheng University, Taiwan, which recorded earthquakes and few months of ambient noise record. The shear wave velocity from the seismic record was calculated to be 225.26±24.29 m/s. We also used seismic record to calculate the Q value of each floor and found that the Q value for the lower floors (Q=16) was lesser on average than the higher floors (Q=23). We believe that this reflects partial energy of the wave dissipated to the surface during transfer, making the Q value of the lower floors is lesser. On the other hand, we estimated the similar shear wave velocity 289.9±7.6 m/s from ambient noise record for the entire building as well. In advance, we divided the structure into low- and high-floors to calculate the shear wave velocity from the upward and downward propagation. We found that the shear wave velocities of the low and high floors are significantly different but exhibit a stable variation. For the monitoring phase, calculating the velocity variation from ambient noise depends on stacking signal to gain better S/N ratio and more convergent result. Therefore, for ambient noise calculation, we tried different stacking time-lengths and calculated the related shear wave velocities which the results are similar to the analysis in earthquake. From the above results, we concluded that the physical parameters of the building are stable and implied that the structure is in a healthy condition. Finally, we hope these results would be helpful to build a long-term monitoring of building's healthy status and the assessment of seismic hazard. [ABSTRACT FROM AUTHOR]

Typhoon and earthquake are mainly natural disasters in Taiwan. Accordingly, the dynamic properties of submarine sediments are important issues while there are referring to offshore site investigation, marine structure construction, and resources exploration. The gas hydrate is a potential energy resource and distributed over the continental margin offshore of the south-west (SW) of Taiwan. Because of the requirement of exploration works, lots of marine areas were selected to study their engineering properties of submarine sediments. The geophysical seismic method and geotechnical laboratory tests were performed to establish physical properties and dynamic soil parameters of these selected sites. Data obtained from P-waves and S-waves generated by P–S conversion on reflection from airgun shots recorded along one line of ocean bottom seismometers (OBS) were used to construct 2-D shear wave velocity (Vs) sections. The cores sampled at test sites utilized gravity sampling method and conducted soil laboratory tests. The cored specimens carried out physical property test and the bender element test to obtain physical properties, Vs, and maximum shear modulus (Gmax). Correlations of porosity, Vs, and Gmax were constructed to predict the in situ properties. Results show the prediction equation get approximate values in comparison with the Vs and Gmax measured by field geophysical seismic method. This paper introduces the field and laboratory tests on obtaining dynamic properties, presents all of the test results, and correlate the relationship of dynamic soil properties between field and lab test outputs. [ABSTRACT FROM AUTHOR]