Taipei Basin Research Articles

Growth Normal Faulting at the Western Edge of the Metropolitan Taipei Basin since the Last Glacial Maximum, Northern Taiwan

Growth strata analysis is an useful tool in understanding kinematics and the evolution of active faults as well as the close relationship between sedimentation and tectonics. Here we present the Shanchiao Fault as a case study which is an active normal fault responsible for the formation of the 700-m-thick late Quaternary deposits in Taipei Basin at the northern tip of the Taiwan mountain belt. We compiled a sedimentary record, particularly the depositional facies and their dated ages, at three boreholes (SCF-1, SCF-2 and WK-1, from west to east) along the Wuku Profile that traverses the Shanchiao Fault at its central segment. By incorporating the global sea level change curve, we find that thickness changes of sediments and changes of depositional environments in the Wuku area are in a good agreement with a rapid sea level rise since the Last Glacial Maximum (LGM) of about 23 ka. Combining depositional facies changes and their ages with their thickness, we are able to introduce a simple back-stripping method to reconstruct the evolution of growing strata across the Shanchiao Fault since the LGM. We then estimate the vertical tectonic slip rate since 23 ka, which exhibits 2.2 mm yr-1 between SCF-2 and WK-1 and 1.1 mm yr-1 between SCF-1 and SCF-2. We also obtain the Holocene tectonic subsidence rate of 2.3 mm yr-1 at WK-1 and 0.9 mm yr-1 at SCF-2 since 8.4 ka. We thus conclude that the fault zone consists of a high-angle main fault to the east between SCF-2 and WK-1 and a western lower-angle branch fault between SCF-1 and SCF-2, resembling a tulip structure developed under sinistral transtensional tectonism. We find that a short period of 600-yr time span in 9 8.4 ka shows important tectonic subsidence of 7.4 and 3.3 m for the main and branch fault, respectively, consistent with possible earthquake events proposed by previous studies during that time. A correlation between geomorphology and subsurface geology in the Shanchiao Fault zone shows that an array of subtle geomorphic scarps corresponds to the branch fault, however, that the surface trace of the main fault seems to be completely erased by the surface force of erosion and sedimentation. We recommend that analysis in the light of growth-faulting scheme should be conducted in a more systematic way for earthquake geology study in other portions of the Shanchiao Fault within the Taipei Basin, as well as other areas worldwide currently under extensional deformation.

Monitoring of Surface Deformation in Northern Taiwan Using DInSAR and PSInSAR Techniques

We investigated the surface deformation of the northern Taiwan area, including the Taipei basin and its surrounding mountainous areas of the last fifteen years using the ERS-1, ERS-2 and ENVISAT SAR images. Although the Taipei basin now is well developed and amenable to research gathering using the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique, the mountainous areas surrounding the basin are densely covered with various vegetation throughout different seasons inducing high noise ratio in interferograms. Therefore the DInSAR technique is ineffective for observation of surface deformations of these areas. As a result, we developed the Persistent Scatterer (PS) InSAR technique to extract the phase signal of the chosen PS points for this study. Our analysis result shows that the atmospheric disturbance and DEM residual can be successfully reduced and the precise information of surface deformation can be effectively obtained by the PSInSAR technique not only in the basin but also in the mountainous areas. Integrating the DInSAR and PSInSAR results, we observed conspicuous deformation events in northern Taiwan including: (1) the slight uplift in the Western Foothills, the Tatun volcanoes, the Linkou Tableland and the Taoyuan area; (2) the subsidence at the border of the Taipei basin; and (3) relative slight uplift rebound in the center of Taipei basin. The displacements along the Shanchiao, Chinshan, and Kanchiao Faults are large enough to be observed; the Taipei, Hsinchuang, and Nankang Faults are too small and cannot be discerned. Further comparison between the DInSAR, PSInSAR, and their corresponding leveling data shows a very coincidental pattern and measurably improves the authenticity of radar interferometry.

Investigation for Strong Ground Shaking across the Taipei Basin during the MW 7.0 Eastern Taiwan Offshore Earthquake of 31 March 2002

According to reconstructed ground motion snapshots of the northern Taiwan area during the Mw 7.0 eastern Taiwan offshore earthquake of 31 March 2002, the composite effects indicated complicated wave propagation behavior in the ground motion of the Taipei basin. A major low frequency pulse arose after the S-wave with the duration of about 20 seconds was observed in northern Taiwan and dominated the radial direction. Observed waveforms of a low frequency pulse show amplification during the seismic wave across the Taipei basin from its eastern edge to western portion. This effect has been considered to be generated by an unusual source radiation, deep Moho reflection or basin bottom surface. In this study, recorded ground motions from a dense seismic network were analyzed using a frequency-wavenumber spectrum analysis for seismic wave propagation properties. We investigated temporal and spatial variations in strong shaking in different frequency bands. Results show that a simple pulse incident seismic wave strongly interacts with inside soft sediments and the surrounding topography of the Taipei basin which in turn extends its shaking duration. Evidence showed that seismic waves have been reflected back from its western boundary of basin with a dominant frequency near one Hz. Findings in this study have been rarely reported and may provide useful information to further constrain a three-dimensional numerical simulation for the basin response and velocity structure, and to predict ground motions of further large earthquakes.

A Blind Normal Fault beneath the Taipei Basin in Northern Taiwan

The Taipei basin, historically low in seismicity, is located in northern Taiwan. A dense broadband seismic array was deployed in the basin in June 2004 to monitor seismic activity. During the period of operation, three felt earthquakes occurred near the eastern part of the Taipei basin, about 3 km to the south of Taipei 101 then the tallest building in the world. Relocated earthquakes show a southeast-dipping distribution of hypocenters beneath the Taipei basin. The seismicity pattern and focal mechanisms of the three felt events suggest the existence of a blind normal fault whose surface projection is along the river channel in the middle of the basin.

Time of a Near Holocene Volcanic Eruption in the Tatun Volcano Group, Northern Taiwan: Evidence from AMS Radiocarbon Dating of Charcoal Ash from Sediments of the Sungshan Formation in Taipei Basin

Time of a Near Holocene Volcanic Eruption in the Tatun Volcano Group, Northern Taiwan: Evidence from AMS Radiocarbon Dating of Charcoal Ash from Sediments of the Sungshan Formation in Taipei Basin

Simulating the Taipei basin response by numerical modeling of wave propagation

Taipei, the capital of Taiwan, suffered from destructive earthquakes four times during the 20th century (ML = 7.3 on April 15, 1909; ML = 6.8 on November 15, 1986; the Chi-Chi ML = 7.3 earthquake on September 21, 1999; and ML = 6.8 on March 31, 2002). Analysis of recorded data shows a strong dependence of spectral amplification in the Taipei Basin on earthquake depth and azimuth. At low frequencies ( f <3 Hz) significant largeramplificationsareobservedforshallowearthquakesascomparedtointermediatedepth events. The former ones also display strong azimuthal dependence. As structures with large response periods such as bridges and tall buildings are sensitive to these low frequencies the understanding of the associated wave effects within the basin and their role for site effect amplification is critical. The tool we employ is 3D finite-difference modeling of wave propa- gation of incident wave fronts. The available detailed model of the basin allows studying the wave effects. Modeling clearly reveals that basin edge effects as observed in data are related to surface wave generation at the basin edges with a high degree of azimuthal dependency. The reproduced site amplification effects are in qualitative agreement with the observations from strong motion data.

Reliability-Based Code Calibration for Axial Ultimate Bearing Capacities of Single Bored Piles in Taipei Basin

ABSTRACTIn Taipei, many pile proof load tests were not conducted to failures but only to a multiple (e.g., 2) of the design load. This leads a difficulty of incomplete information: For these test results, the ultimate bearing capacities of the test piles are unknown. This paper addresses the issue of calibrating resistance factors of piles for the axial ultimate bearing capacities based on the incomplete information from these tests. A simplified probabilistic method is proposed to resolve this issue. A local pile test database of Taipei is presented, and the analysis results show that the inclusion of the incomplete pile load test data helps in calibrating the resistance factors. Moreover, it is found that the calibrated resistance factors for the axial ultimate bearing capacities are consistent to the safety factors that are adopted in the current Taiwan design code. This paper also addresses another important issue for the pile design in Taipei: the end bearing capacity in the sandstone layer is very uncertain. A maximum likelihood method is taken to identify the best estimate for this end bearing capacity. The conclusion of this paper may be useful for reliability-based designs for the axial ultimate bearing capacities of single bored piles in the Taipei region.

Development of seismic force requirements for buildings in Taiwan

This paper describes static and dynamic procedures to calculate seismic demand specified by the current seismic design code for buildings in Taiwan, which was issued in 2005. For design levels with a return period of 475 years, the design spectral response acceleration can be developed for general sites, near-fault sites and Taipei Basin. In addition, in order to prevent building collapse during extremely large earthquakes and yielding of structural components and elements during frequent small earthquakes, the required seismic demands at the maximum considered earthquake level (MCE, 2%/50 years) and operational level are also included in the new seismic design code. For dynamic analysis procedures, both the response spectrum method and time history method are specified in the new seismic design code. Finally, procedures to generate spectrum compatible ground motions for time history analysis are illustrated in this paper.

Case studies on bermed excavation in Taipei silty soil

Bermed excavation (deep excavation supported by an excavation wall and a berm) can be an economical method for basement construction in urban areas if ground movement around the excavation is not the primary concern and the excavation site is large and (or) irregular in shape. This paper reviews three bermed excavation projects carried out in the Taipei Basin. One project was successful, one was successful after near failure, and one was a failure. Ironically, extensive ground improvement work had been done for the near-failure and failure cases; but no ground improvement was done for the successful case. It is of interest to find the similarities and differences among these bermed excavation projects. The causes of instability that result in unsuccessful bermed excavation projects and the applications of limit equilibrium analysis to the stability of bermed excavation are also discussed here. These three cases show that the existence of grout columns does not help to effectively increase the stability of the berm if the original safety factor against sliding of a berm slope is less than 1.0 and the improvement ratio of grout columns is low (say &lt;20%).

Performance and analysis of pumping tests in a gravel formation

Two single-well and group-well pumping tests in the gravel formation of the Taipei basin were conducted to investigate the hydraulic parameters of the gravel formation and to understand the characteristics of the drawdown at both the construction and remote sites. As the base of the gravel formation at the construction site was unknown, the wells were assumed to be partial penetration wells. A simple method was developed to derive the hydraulic parameters from the pumping test results, taking into account the site-specific influencing factors. The parameters obtained were verified by the group-well pumping test results.

High frequency site amplification evaluated from Borehole data in the Taipei Basin

Based on the quarter-wavelength approximation, the frequency-dependent site amplifications, A(f ), at 18 free-field strong-motion stations in and near the Taipei Basin are evaluated from well-logging data. The V30, which is the average of the S-wave velocities in the topmost 30 m, is a significant factor in classifying the sites. Results show that the site amplifications at all sites in study are larger than 1 and functions of frequency. Compared with the Haskell method, the quarter-wavelength approximation is almost an average and a good representation of overall amplifications. It is noted that the site amplifications evaluated in this study can apply only to frequencies greater than about 1.1 Hz for class C sites and 3.1 Hz for class D.

Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan

Abstract A total of 336 three-component strong-motion recordings from the M w 6.35 Niu Dou earthquake of 25 June 1995 at a focal depth of 39.9 km in northern Taiwan are used to study the effects on strong ground motion due to Moho reflection of S waves. The residuals of both horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) recorded from the earthquake are analyzed. The results confirm that many Class E soft soil stations in the Taipei Basin and the Ilan Plain had the expected large amplification of about 1.7 and 1.5 times, respectively, the predicted median PGA values. Surprisingly, a large group of Class C or D dense and stiff soil sites in Taoyuan (TCU007), Lungtan (TCU013), Guanshi (TCU021), Hsinchu (TCU095), and Miaoli (TCU047) areas in northwestern Taiwan had unusually large amplification of about 3.4–8.1 and 1.7–3.3 times the predicted median PGA and PGV values, respectively. They are interpreted in terms of focusing and interference between SmS waves reflected from the horizontal and inclined portions of an east-dipping Moho discontinuity in this area. This interpretation is supported by the close agreement between the expected amplitudes and arrival times of the largest shear waves with the observed data. Our results suggest that when a damaging earthquake occurs near an inclined Moho boundary, the reflected SmS waves can result in significantly amplified ground motions at distances beginning about 50 km. The exact distance range will depend on the thickness of the crust and the dip angle of the Moho boundary.

Effects of Topography on Seismic-Wave Propagation: An Example from Northern Taiwan

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.

Applying hourly measurements of meteorological data and aerosol soluble ions in Taipei Basin, Taiwan

This study applies observational data composed of hourly weather and aerosol to discuss the aerosol characteristics within different weather systems. Based on cluster analysis, spring weather in Taipei Basin 2004 was characterised as five weather systems: humid/low south wind speed, dry cold/high east wind speed, humid cold/east wind, dry cold/northeaster, and northwestern convection wind. Under the humid/low south wind speed and northwestern convection wind systems, there are predominant influences of local vehicle/motor pollutant emissions and secondary pollution on the air quality. Asian dust storms were usually occurred under the weather of dry cold/high east wind speed. The regional air quality is more acceptable during the period of prevalent humid cold/east wind and dry cold/northeaster.

Analysis of Taipei Basin Response for Earthquakes of Various Depths and Locations Using Empirical Data

The response of Taipei basin upon earthquake excitation was studied using records of recent earthquakes. The strong-motion database includes records obtained at 32 stations of the Taipei TSMIP network from 83 deep and 142 shallow earthquakes (M＞4.0) that occurred in 1992-2004. The characteristics of frequency-dependent site response were obtained as spectral ratios between the actual earthquake records (horizontal components) and those modelled for a hypothetical Very Hard Rock (VHR) condition. The models for VHR spectra of Taiwan earthquakes had been recently proposed by Sokolov et al. (2005b, 2006). Analysis of site response characteristics and comparison with simple 1D models of the soil column resulted in the following conclusions: (1) The spectral ratios throughout the basin obtained from deep earthquakes (depth ＞35 km) exhibit good agreement with the theoretical ratios calculated using the 1D models constructed using available geological and geotechnical data. (2) The spectral ratios obtained from shallow earthquakes show influence of: (a) surface waves generated when travelling from distant sources to the basin and (b) relatively low-frequency (＜1-2 Hz) waves generated within the basin. (3) Some shallow earthquakes produce extremely high amplification at frequencies 0.3-1 Hz within the basin that may be dangerous for high-rise buildings and highway bridges. (4) The obtained results may be used in probabilistic seismic microzonation of the basin when many possible earthquakes located at various distances are considered. 2D and 3D simulation is necessary to model the seismic influence from particularly large earthquakes.

Seismic analysis of the world’s tallest building

Taipei 101 (officially known as the Taipei Financial Center) with 101 stories and 508 m height, located in Taipei where earthquakes and strong typhoons are common occurrences, is currently the tallest building in the world. The great height of the building, the special geographic and environmental conditions, not surprisingly, presented one of the greatest challenges for structural engineers. In particular, its dynamic performance under earthquake or wind actions requires intensive research. The structure of the building is a mega-frame system composed of concrete filled steel tube (CFT) columns, steel brace core and belt trusses which are combined to resist vertical and lateral loads. In this study, a shaking table test was conducted to determine the constitutive relationships and finite element types for the CFT columns and steel members for establishing the finite element (FE) model of the tall building. Then, the seismic responses of the super-tall building were numerically investigated. An earthquake spectrum generated for Taipei Basin was adopted to calculate the lateral displacements and distributions of interior column forces. Furthermore, time-history analyses of elastic and inelastic seismic response were carried out using scaled accelerograms representing earthquake events with return periods of 50-year, 100-year, and 950-year, respectively. The computational results indicate that the super-tall building with the mega-frame system possesses substantial reserve strength, and the high-rise structure would satisfy the design requirements under severe seismic events. The output of this study is expected to be of considerable interest and practical use to professionals and researchers involved in the design of super-tall buildings.

Calibration of Reliability-Based Resistance Factors for Flush Drilled Soil Anchors in Taipei Basin

The goal of this research is to calibrate the reliability-based resistance factor of flush drilled soil anchors for their ultimate pullout capacities based on in situ anchor pullout test data in the alluvial soil underlying the Taipei Basin. Efforts are taken to quantify the uncertainties with a full probabilistic analysis approach. The resistance factor is calibrated based on the in situ test results of 46 anchors with a rigorous theoretical approach which constructs the relationship between the resistance factor and failure probability. With this relationship, the reliability corresponding to the code regulation can be verified. From the results of the analysis, it is found that the borehole enlargement due to the flush drilling is quite significant: the actual diameter of the fixed anchor end may be much larger than the nominal diameter of the drilling casing. Consequently, the safety factor of 3.0, recommended by most anchor codes, is found to be too conservative. The results should be valuable for re...

Simulations of Strong Ground Motion and 3D Amplification Effect in the Taipei Basin by Using a Composite Grid Finite-Difference Method

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 prob- lem 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 addi- tion, 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.

The effects of synoptical weather pattern and complex terrain on the formation of aerosol events in the Greater Taipei area

The aerosol in the Taipei basin is difficult to transport outward under specific weather patterns owing to complex terrain blocking. In this study, seven weather patterns are identified from synoptic weather maps for aerosol events, which occurred from March 2002 to February 2005. Among the identified weather patterns, High Pressure Peripheral Circulation (HPPC), Warm area Ahead of a cold Front (WAF), TYPhoon (TYP), Pacific High Pressure system stretching westerly (PHP), Weak High Pressure system (WHP), and Weak Southern Wind (WSW) are related to terrain blocking. The remaining pattern is High Pressure system Pushing (HPP). The classification of the pollution origin of the air masses shows that 15% of event days were contributed by long-range transport (LRT), 20% by local pollution (LP), and 65% by LRT/LP mix. Terrain blocking causes aerosol accumulation from high atmospheric stability and weak winds occurring under HPPC, TYP, and PHP weather patterns when the Taipei basin is situated on the lee side of the Snow Mountains Chain (SMC). Terrain blocking also occurs when the Taipei basin is situated on the upwind of SMC and Mt. Da-Twen under WAF and WSW patterns. To study the variation of aerosol properties under the mixed influence of terrain and pollution origin, we conducted a field observation simultaneously at the urban, suburban, and background sites in the Greater Taipei area from April 14 to 23, 2004. Terrain blocking plays an important role in aerosol accumulation in the stagnant environment when the Taipei basin is on the lee side of SMC. On the other hand, the PM 2.5 sulfate level is stable with a fraction of 30% in PM 2.5 during the observation period at the urban (25%−33%) and background (25%−41%) sites. It indicates that background PM 2.5 sulfate is high on the West Pacific in winter.

Three-Dimensional Simulations of Seismic-Wave Propagation in the Taipei Basin with Realistic Topography Based upon the Spectral-Element Method

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. Spectral-element 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.