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Earthquakes often occur in regions with complex material structure, such as sedimentary basins or mantle wedges. However, the majority of co-seismic modelling studies assume a simplified, often homogeneous elastic structure in order to expedite the process of model construction and speed up calculations. These co-seismic forward models are used to produce Green's functions for finite-fault inversions, so any assumptions made in the forward model may introduce bias into estimated slip models. In this study, we use a synthetic model of a sedimentary basin to investigate the impact of 3-D elastic structure on forward models of co-seismic surface deformation. We find that 3-D elastic structure can cause changes in the shape of surface deformation patterns. The magnitude of this effect appears to be primarily controlled by the magnitude of contrast in material properties, rather than the sharpness of contrast, the fault orientation, the location of the fault, or the slip orientation. As examples of real-world cases, we explore the impact of 3-D elastic structure with a model of the Taipei basin in Taiwan and a simulated earthquake on the Sanchaio fault, and with a 3-D geologic model of the San Francisco Bay Area and a slip model of the 1984 Morgan Hill earthquake on the Calaveras fault. Once again, we find that the presence of the basin leads to differences in the shape and amplitude of the surface deformation pattern, but we observe that the primary differences are in the magnitude of surface deformation and can be accounted for with a layered elastic structure. Our results imply that the use of homogeneous Green's functions may lead to bias in inferred slip models in regions with sedimentary basins, so, at a minimum, a layered velocity structure should be used. [ABSTRACT FROM AUTHOR]

Ocean tide loading (OTL) displacements are sensitive to the shallow structure of the solid Earth; hence, the high-resolution spatial pattern of OTL displacement can provide knowledge to constrain the shallow Earth structure, especially in coastal areas. In this study, we investigate the sensitivity of the modeled M2 OTL displacement over Taiwan Island to perturbations of three physical quantities, namely, the density, bulk modulus, and shear modulus in the upper mantle and crust. Then, we compare the sensitivity of the modeled M2 OTL displacement to Earth models with the sensitivity to ocean tide models using root mean square (RMS) differences. We compute the displacement Green's function and OTL displacement relative to the center of mass of the solid Earth (CE) reference frame, analyze the sensitivity to the three physical quantities in the CRUST1.0 model and the Preliminary Reference Earth Model (PREM), and present their spatial patterns. We find that displacement Green's functions and OTL displacements are more sensitive to the two elastic moduli than the density in the upper mantle and crust. Moreover, their distinctive sensitivity patterns suggest that the three physical quantities might be constrained independently. The specific relationships between the perturbed structural depths and the distance ranges of peak sensitivities from the observation points to the coastline revealed by the shear modulus can mitigate the nonuniqueness problem in inversion. In particular, the horizontal tidal components observed by the Global Positioning System (GPS) can yield better results in inversions than the vertical component owing to the smaller OTL model errors and the higher structural sensitivity (except for the shear modulus in the asthenosphere). [ABSTRACT FROM AUTHOR]

The present article focusses on determining the response of reduced micropolar half-space subjected to specific loading conditions. Analytical expressions for translational and rotational displacements are derived for the half-space subjected to impulses using the method of potentials. The derived analytical expressions are numerically evaluated to determine the response of reduced micropolar half-space subjected to buried impulses acting in both horizontal and vertical directions. The derived Green's functions are further used to obtain the surface translations and rotations for the half-space subjected to earthquake force. Parametric analysis is carried to understand the effect of additional material parameters of the reduced micropolar medium on the surface ground motions. Finally, ground motions are simulated for 2012 Wutai, Taiwan, earthquake by modelling the medium as a homogeneous reduced micropolar half-space consisting of rock material properties. [ABSTRACT FROM AUTHOR]

We report on the near-surface (< 500 m) seismic structure (Vp, Vs, and Vs anisotropy) of Taiwan using the empirical Green's functions of body waves between vertical station pairs at 60 borehole sites. The near-surface anisotropy in metamorphic rocks and Miocene or older sedimentary strata are mostly categorized as orogeny-parallel anisotropy (OPA), and sites exhibiting stress-aligned anisotropy (SAA) are located in areas with late Quaternary sedimentary deposits or in alluvial coastal plains. The results show that there is a significant variation of the anisotropy strength even within the same geological unit. Since all the major geological units of Taiwan are well sampled by borehole arrays, and drilling data for 52 sites are available, this provides a unique opportunity to investigate the geological implications of strong variations in near-surface anisotropy. The investigation indicates that (1) near-surface anisotropy caused by structural fabric is generally stronger than that resulting from the alignment of stress-induced open cracks, (2) SAA strength in sedimentary rocks, igneous rocks, and gravel sediments is generally higher than that in fine-grained sediments, and (3) the sites with the strongest OPA (> 30%) are located in mountain ranges and the mean OPA strength of these sites is significantly higher than that of foothills, suggesting that foliated metamorphic rocks lead to stronger OPA than do dipping sedimentary strata. The results also suggest that the fluid-saturation of micro-cracks is not a necessary condition for the occurrence of the SAA mechanism in the near-surface depth range. • Structure-related anisotropy is generally stronger than stress-induced anisotropy. • Consolidated rocks and gravels show higher anisotropy than fine-grained sediments. • Foliated metamorphic rocks show stronger OPA than dipping sedimentary strata. • Fluid-saturation is not necessary for SAA occurrence in near-surface depth range. [ABSTRACT FROM AUTHOR]

In Taiwan, the Nantou area is a seismically active region where several moderate events have occurred, causing some disasters during the past century. Here, we applied the strong ground motion simulation with the empirical Green's function method to investigate the source characteristics for the eight moderate blind-fault events that struck the Nantou area in 1999 and 2013. The results show that for these Nantou events, a high stress drop and focal depth dependence were noted, which might be related to the immature buried fault in this area. From the viewpoint of seismic hazard prevention and preparation, future earthquake scenarios that include high stress drop should be applied to more analyses, especially the moderate-to-large events originating from the immature blind faulting. [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]

We investigated broadband evidence of the rupture directivity effect and estimated stress drop for the 2013 Nantou blind-thrust earthquakes (the 0327 and 0602 crustal events) from three individual analyses using empirical Green's functions: deconvolution source time function analysis, source spectral ratio analysis, and broadband waveform modeling of a strong-motion generation area. Although these two adjacent events have similar focal mechanisms, the 0327 event shows only up-dip directivity to the west, with an estimated stress drop of 9.0 MPa, and the 0602 event displays significant rupture directivity to the southwest, with an estimated stress drop of 14.2 MPa. The obvious variance in the rupture behavior and stress drop of these two nearby events, which might be related to immature blind faulting, suggests that this region requires additional study of the stress heterogeneity to better assess the seismic hazard. [ABSTRACT FROM AUTHOR]

SGOTL, a computer package coded in FORTRAN, has been developed to model the gravity effect due to ocean tide loading (OTL), especially for a coastal station with large ocean tides. SGOTL uses a regional and a global tide model to account separately for near (inner) and far (outer) zone contributions, and optimizes an inner-zone region and grid interval for numerical convolution. Height dependent Green's functions for Newtonian and elastic effects are developed. The coastline is defined by the full-resolution GMT shoreline, and optionally a digital elevation model (DEM). A case study using gravity observations at the Hsinchu superconducting gravity station and some offshore islands around the Taiwan Strait suggests that SGOTL outperforms some selected global OTL programs and achieves an accuracy of 0.1 µGal for 8 leading tidal constituents. [ABSTRACT FROM AUTHOR]

Using the ambient noise cross-correlation technique, we derived empirical Green's functions (EGFs) of short-period Rayleigh waves from the continuous vertical component of data recorded by 34 seismic stations in northern Taiwan. We measured the group velocity of the EGFs in the frequency range of 0.2-0.5 Hz. The derived EGFs and group velocity measurements were carefully examined, and only quality results were used for the tomographic inversion. We implemented a wavelet-based multiscale inversion technique to construct the group velocity map for 0.35-Hz Rayleigh waves in northern Taiwan. The resulting model shows excellent correlation to the surface topography and geological units. To compare with models derived from traditional body-wave tomography, we calculated the group velocity maps and their fit to the noise-derived EGFs for two recent body-wave models. The comparison indicates that body-wave velocities, particularly Vs, at shallow depth in the west coast plain are overestimated in two recent body-wave-derived models, and/or a much higher Vp/Vs ratio may exist for this area. Our results suggest that the noise-derived short-period EGFs may provide important constraints on shallow Earth structure, in complement to that provided by body waves. [ABSTRACT FROM AUTHOR]

Improving seismic hazard mitigation of the densely populated metropolitan area of and around the capital of Taiwan requires detailed knowledge of the 3D crustal structure of Taipei basin. The high levels of ambient noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic exploration or earthquake tomography methods. We investigate the shallow crust in the metropolitan region using surface wave array tomography with time domain empirical Green's function (TDEGF) inferred from correlation of ambient seismic noise. Analysis of the TDEGF amplitudes suggests that the dominant sources of ambient seismic noise are the coastlines and shallow continental shelf of the Taiwan Strait, northwest of the study region. Our study demonstrates that ambient seismic noise tomography is feasible at periods of 0.5-3 s, which is much shorter than the 10-30 s used in most other studies, and which opens new opportunities for high resolution studies of near-surface heterogeneity. The lateral variation in Rayleigh wave phase velocity correlates well with surface geology and suggests that faults play an important role in the regional tectonic setting. High phase velocities mark the Tatun volcanic area, the Kuanyin Mountain dominated by Quaternary igneous rock, and the Miocene Western Foothills south of the Taipei fault. Low phase velocities characterize regions are along western and southeastern edges of the Taipei basin and the Pleistocene Linkou tableland. Main faults in the region are either marked by low phase velocities or define transitions between regions of high- and low-velocity anomalies. [ABSTRACT FROM AUTHOR]

The "max gain" which is the maximum absolute value of the reciprocal Green's Function of a city vulnerable to tsunami damage is used to evaluate the efficiency of the location for tsunami detecting instruments. The max gain is the correlation between the water elevation variations at the observatory and the vulnerable coastal city. After vulnerable cities are determined, the max gain diagram can be used to determine the most favorable location for observation. To verify the applicability of the methodology of a tsunami monitoring system, the Marine Cable Hosted Observatory (MACHO) project in Taiwan is analyzed in detail. Four vulnerable cities, Tou-cheng, Chi-long, Su-ao, and Hua-lien, are chosen first. It is found that from the hydrodynamic point of view the Ryukyu Island Arc is more important than the Yaeyama Ridge. The max gain distribution is found to be useful for optimizing the tsunami monitoring system in that some modification on the node locations is predicted to significantly improve the efficiency of the tsunami sensors without changing the route or the cost. [ABSTRACT FROM AUTHOR]

Abstract  This study presents predicts ocean tidal loading (OTL) effects using a Green’s function approach and validates a novel tidal model for Taiwan. Numerical integration of OTL is performed using the Gauss quadrature method and a local tidal model for the inner zone and a global model for the outer zone. Observed time series of GPS-derived vertical displacements and gravity variations (3–7 days) at five co-located GPS-gravimeter stations along the South East China and Taiwan coasts were utilized to assess the accuracy of the proposed models and two other models. The OTL-induced gravity variations are 3–16 μgal and the vertical site displacements are 13–27 mm. Generally, an OTL model using a mixed global and local tidal model generates better agreement with the observations than an OTL model using a global tidal model only. However, containing a local model inside a global model does not always produce a good agreement with the observations. The relatively large discrepancies between modeled and observed OTL values at some stations indicate that there is a need for an improved local tidal model in the study area. [ABSTRACT FROM AUTHOR]

Using 2017 data, 54 volatile organic compounds (VOCs) in Taiwan's atmospheric environment from nine photochemical assessment monitoring stations (PAMSs) were collected and their characteristics and sources were analyzed. High VOC concentration spots were exposed in urban and industrial areas (Wanhua, Tucheng, Chungming, Tainan, Qiaotou, and Xiaogang). The major categories of VOC were alkanes and aromatics. Among them, benzene, toluene, ethylbenzene, and m,p-xylene (BTEX) predominated and contributed to ozone formation potential (OFP) and secondary organic aerosol formation (SOAF). T/B and X/E ratios showed that VOCs in urban and industrial areas originated from fresh, mixed sources of local emissions. Positive matrix factorization (PMF) analysis indicated that industrial emissions (solvent usage, industry, and petrochemical plant) were the primary contributors, followed secondarily by traffic emissions (vehicular emission and vehicular fuel evaporation), and aged air-mass. Analysis by potential source contribution function (PSCF) was conducted to confirm the effects of aged air-mass sources. The estimated VOC sources OFP and SOAF indicated that industrial emissions were the greatest contributors to OFP and SOAF. Results from this study imply that VOC control measures should prioritize control of industrial emissions. Image 1 • BTEX was the major VOC species, especially toluene, which was the dominant potential contributor to ozone formation potential (OFP) and secondary organic aerosol formation (SOAF). • Positive matrix factorization (PMF) and potential source contribution function (PSCF) are used for tracking regional aged air-mass transport pathways which confirm the results of PMF approach. • The analysis of backward trajectories by PSCF showed that the contribution of aged air-masses affects VOC distribution mainly in the northeast to southwest Taiwan. [ABSTRACT FROM AUTHOR]

Seismic attenuation parameters in Taiwan have been obtained by using a modified "MultipleLapse Time Window Analysis" approach in order to take into account the scatteringanisotropy. MLTWA method exploits the spatio-temporal dependence of the ratio betweencoherent and incoherent seismic intensity to determine the scattering and absorptionproperties of a heterogeneous medium. In addition to the mean free path l (or the scatteringquality factor Qsc), the seismic wave propagation is controlled by a parameter g(independent of l) which determines the angular redistribution of energy upon scattering(scattering anisotropy). Preferentially forward (resp. backward) scattering correspondsto g &#x003E; 0 (resp. g &#x003C; 0), with g = 0 for isotropic scattering. We use Monte-Carlosimulations to compute Green functions and sensitivity kernels for Qsc, Qi (absorptionquality factor) and g in media with an heterogeneous crust overlying a transparentmantle. We analyse seismic records of local earthquakes with magnitude greater than 3.5which occurred between 1994 and 2016. For each stations, we collect events withina radius of 100 km and a maximum depth of 40 km. About 350000 waveformswith a S/N ratio greater than 4 in the coda are selected. Seismic energy in three15 s time windows from the S-wave arrival are measured in 3 frequency bands(1-2, 2-4,4-8 Hz). To account for site amplification and source amplitude, wenormalize observed energy by the average energy in a late coda window at 65 s fromthe origin time of the earthquakes. For every station, we analyse the normalizedenergy in the three time widows as a function of the hypocentral distance. In order todetermine the attenuation parameters Qsc−1, Qi−1 and g with related errors, weimplement a Levenberg-Marquardt algorithm. To guide the inversion, we use thesensitivity kernels for the attenuation parameters computed with our Monte-Carloapproach. Overall, Taiwan is more attenuating than most orogens with a mean effective scatteringloss (Qsc∗)−1 = Qsc−1(1 − g) about 0.012 and a mean intrinsic absorption Qi−1 about0.009 at 1.5 Hz. Our maps reveals strong lateral variations of attenuation with scale lengthfrom 10 to 100 km. Scattering loss (Qsc∗)−1 varies over more than one order of magnitudeacross Taiwan (between 0.0022 and 0.16 at 1.5 Hz). By contrast, absorption fluctuations areabout 30%. The more attenuating zones are the Coastal Range and the Costal Plain wherescattering dominates over absorption at low frequency, and inversely at high frequency. Theseregions are also characterized by strong backscattering (g &#x003C; −0.85) at 1.5 Hz and ratherhigh V P∕V S ratio. Scattering becomes much more isotropic at high frequency.We speculate that the observed strong backscattering at low frequency is relatedto strong impedance fluctuations in the crust induced by the presence of fluids. [ABSTRACT FROM AUTHOR]

Coda wave interferometry is exploited to investigate the temporal changes of crustal properties preceding and following the Oct. 31, 2013 Mw 6.2 Ruisui Earthquake which struck the previously seismically quiet area in the northern segment of the Longitudinal Valley collision suture between the Eurasian and Philippine Sea Plate in eastern Taiwan. The coda is retrieved from the daily-stacked empirical Green's functions (EGFs) calculated by the correlations of continuous ambient noise recorded at three-component seismograph stations and volumetric strainmeters between 2013 and 2014. By comparing the time-lapse traveltime and waveform changes between the pre- and post-event coda arrivals at a wide frequency range of 0.05-8 Hz, we observe explicitly not only coseismic velocity reduction but also preceding decorrelation of waveforms in both the interferometric seismic and strain field. Considering the sensitivity of the observed coseismic velocity changes at frequencies of 0.1-0.9 Hz s to velocity perturbations at depth is dominated by surface wave sensitivity, their correlation with the changes of the volumetric strain at depths of 3-5 km induced by the earthquake slip indicates the static stress perturbations on the fault zone are the predominant cause. In addition, the quasi-periodic variation with a dominant cycle of about 4 months that appear only in the strain derived EGFs is correlated well with the fluctuation of the groundwater levels. During the yearly summer monsoon and typhoon seasons between July and October when the wind speed and the strength of noise source in Taiwan are relatively strong, the retrieved coda amplitudes are highly attenuated which lead to the abnormally large and random fluctuations in the estimated velocity perturbations and significant drop in waveform correlation. Such phenomena may be associated with the disturbance of subsurface structures by the increase in seasonal precipitation or hydrological extreme events such as floods and consequently rapid rise of groundwater levels. The use of the borehole seismometers and multiple-component EGFs can prevent this obstruction and obtain more stable and accurate estimates in the earthquake-induced perturbations. [ABSTRACT FROM AUTHOR]