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The 2018 February 6 M w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s−1) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9 V s speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake. [ABSTRACT FROM AUTHOR]

This study used the results from 45 microtremor array measurements to construct a shallow shear wave velocity structure in the western plain of Taiwan. We constructed a complete 3D velocity model based on shallow and tomography models for our numerical simulation. There are three major subsurfaces, engineering bedrock (VS = 600 m s-1), Pliocene formation and Miocene formation, constituted in the shallow model. The constant velocity is given in each subsurface. We employed a 3D-FD (finite-differences) method to simulate seismic wave propagation in the western plain. The aim of this study was to perform a quantitative comparison of site amplifications and durations obtained from empirical data and numerical modelling in order to obtain the shallow substructure soil response. Modelling clearly revealed that the shallow substructure plays an important role in strong ground motion prediction using 3D simulation. The results show significant improvements in effective shaking duration and the peak ground velocity (PGV) distribution in terms of the accuracy achieved by our developed model. We recommend a high-resolution shallow substructure as an essential component in future seismic hazard analyses. [ABSTRACT FROM AUTHOR]

The 1909 Taipei earthquake (M 7.3) occurred beneath the Taipei metropolitan area (TMA) causing substantial damage according to the historical literature. According to the hypocenter relocation and tectonic implications provided in a previous study, we simulated ground motions within the TMA using a hybrid simulation method involving the spectral-element method (SEM) and the empirical Green's function method (EGFM). We used the SEM for simulating low-frequency components and the EGFM for simulating high-frequency components. These high and low frequency components were subsequently combined. For the EGFM we used the records from a recent ML 4.9 earthquake (11 October 2013, depth = 143.8 km) in the Taipei area as the empirical Green's function. According to the historical literature, the observed PGA (peak ground acceleration) values are 59.2 and 67.0 gal at ancient stations TAP and KEE, with periods of 1.21 and 1.34 s, respectively. By comparing the simulated PGA values at modern stations TAPB and WFSB to the historical documented ones for 12 different models, our result suggests that the 1909 Taipei earthquake was an event with a magnitude of about Mw 7.3 and stress drop of approximately 30 bars, or a smaller equivalent magnitude between Mw 6.8 - 7.3 but with much higher average stress drop of more than 100 bars. For a deep event beneath TMA a larger vertical P-wave motion and longer period shaking wave, as addressed in the historical literature, might be expected with prolonged shaking as found in the simulation. A seismic hazard assessment is necessary for metropolitan Taipei to better understand the long period shaking from deep subduction zone intra plate events. [ABSTRACT FROM AUTHOR]

The 2013 ML 6.4 Ruisui earthquake struck a seismic gap around the northern part of the Longitudinal Valley where rare earthquakes have occurred for at least two decades. Seismic data with different frequency bands connote a diverse scale of source characteristics. The slip model derived by a previous study using long-period seismic data provided us with the preliminary earthquake source properties in low-frequency energy radiation. However, the high-frequency part of the seismic energy needs to be considered for the strong motions in the comprehensive frequency band. As a case study, we carried out broadband strong motion simulations through the hybrid scheme, which simulated waveforms combining the low-frequency motions (frequency-wavenumber integration method) and high-frequency motions (empirical Green's function method) to reveal the strong ground motion and source characteristics of the 2013 Ruisui earthquake. The results show that even though the local structure is complicated this hybrid simulation can effectively rebuild the overall broadband strong ground motion characteristics for the 2013 Ruisui earthquake. Additional study issues are required to better understand the nature of localized high peak ground acceleration and the related seismic hazards for future earthquakes. [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]

We present earthquake shaking scenarios by a physics-based ground-motion simulation scheme for seismic risk assessments. In fitting requirements of the application of loss estimations, scenarios can refer to various kinds of damage and losses to constructions (buildings, bridges, etc.), casualties, economic losses, social losses and so on. We quantitatively assess the ground motion effects by the physics-based full-waveform ground-motion simulation which is a powerful tool for earthquake-induced ground-motion prediction. The entire time history of the ground motion induced by an earthquake depends on a number of factors, including source-rupture process, subsurface velocity structure, topographic relief, and local site condition. Then, the further risk assessment can also be revealed by a combination with exposure data. As an example, we carried out simulations to model rupture scenario of the Shanchiao fault and the potential large subduction earthquake in northern Taiwan, with accounting for effects of (1) source slip distribution, (2) seismic wave propagation, (3) surface topography and (4) site amplification. Output ground-motion parameters, including peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), 0.3-s-period spectral acceleration (SA0.3), and 1.0-s-period spectral acceleration (SA1.0), can be adopted to further implement to loss estimation. Our results referred different shaking-level potentials of earthquake scenarios can benefit to risk management, especially for the further application of strategy making of emergency response and disaster prevention and preparations. [ABSTRACT FROM AUTHOR]