Browsing by Author "Wu, Yih-Min"

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A Comprehensive Analysis of Attenuation Characteristics Using Strong Ground Motion Records for the Central Seismic Gap Himalayan Region, India
(1970-01-01)
Mittal, Himanshu
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Sharma, Babita
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Chao, Wei-an
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Wu, Yih-Min
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Lin, Ting-Li
This study aims to estimate attenuation characteristics of the central Himalayan region of India concerning various strong-motion parameters such as Kappa value (kappa) and site effects. We have tried to elaborate on the regional structural heterogeneities and their implications towards the seismic hazard assessment of the study region. A total of 81 earthquakes recorded at 50 stations situated in the central Himalayan region of India are used for the purpose. The particular focus is kept on Kappa value, which shows variability from 0.03 s to 0.095 s, inferring the higher values obtained in plains with deep sediment accumulations proving high-frequency energy dissipation and stiff-soil/rocky sites exhibit comparatively limited attenuation accordingly. To substantiate these results various attenuation parameters such as coda wave quality factor (Q(c)), intrinsic attenuation parameter (Q(i)), and scattering attenuation parameter (Q(s)), have been estimated for two regions in the central seismic gap Himalayan region of India employing the single backscattering model and Wennerberg formulation. The estimated values of Q(c), Q(i), and Q(s) are found to be highly dependent on frequency in the frequency range 1.5-24 Hz for both the regions. The average frequency-dependent relationships (Q = Q(0)f(eta)) estimated for both regions are Q(c) = 158f(1.18)and Q(c) = 194f(1.2), respectively. The low value of Q(0) shows that the region is highly heterogeneous while the higher value of. indicates higher seismicity in the area. It is also found that intrinsic attenuation is predominant over the scattering attenuation, envisaging the behavior of the wave attenuation through the absorption within the granitic layer at shallow depths. At lower frequencies, Q(c) values are found close toQ(s) values, which is in agreement with the theoretical measurements suggesting the presence of complex crustal heterogeneities beneath the region affecting the propagation of seismic waves experiencing considerable decay of energy through scattering. To confirm the aggregate attenuation on the stations, the site characteristics are also determined for examining the behavior of the amplification as the ground motion is comprised of the combined effect of the source, path, and site. The sites are amplified at a predominant frequency (f(peak)) in between 1.5 to 10 Hz for the central Himalayan region. The different attenuation and amplification parameters like kappa, Q, and site effects can be utilized for detailed seismic hazard analysis (based on ground motion prediction equations) of the area as this region is of great importance from a socio-economic point of view.
A first near real-time seismology-based landquake monitoring system
(2017-03-02)
Chao, Wei-An
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Wu, Yih-Min
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Zhao, Li
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Chen, Hongey
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Chen, Yue-Gau
Hazards from gravity-driven instabilities on hillslope (termed 'landquake' in this study) are an important problem facing us today. Rapid detection of landquake events is crucial for hazard mitigation and emergency response. Based on the real-time broadband data in Taiwan, we have developed a near real-time landquake monitoring system, which is a fully automatic process based on waveform inversion that yields source information (e. g., location and mechanism) and identifies the landquake source by examining waveform fitness for different types of source mechanisms. This system has been successfully tested offline using seismic records during the passage of the 2009 Typhoon Morakot in Taiwan and has been in online operation during the typhoon season in 2015. In practice, certain levels of station coverage (station gap < 180 degrees), signal-to-noise ratio (SNR >= 5.0), and a threshold of event size (volume > 10(6) m(3) and area > 0.20 km(2)) are required to ensure good performance (fitness > 0.6 for successful source identification) of the system, which can be readily implemented in other places in the world with real-time seismic networks and high landquake activities.
A First-Layered Crustal Velocity Model for the Western Solomon Islands: Inversion of the Measured Group Velocity of Surface Waves Using Ambient Noise
(2018-11-01)
Ku, Chin-Shang
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Kuo, Yu-Ting
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Chao, Wei-An
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You, Shuei-Huei
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Huang, Bor-Shouh
Two earthquakes, M-w 8.1 in 2007 and M-w 7.1 in 2010, hit the western province of the Solomon Islands and caused extensive damage, which motivated us to establish a temporary seismic network around the rupture zones of these earthquakes. With the available continuous seismic data recorded from eight seismic stations, we cross correlate the vertical component of ambient-noise records and calculate Rayleigh-wave group velocity dispersion curves for interstation pairs. A genetic algorithm is adopted to fit the averaged dispersion curve and invert a 1D crustal velocity model, which constitutes two layers (upper and lower crust) and a half-space (uppermost mantle). The resulting thickness values for the upper and lower crust are 6.9 and 13.5 km, respectively. The shear-wave velocities (V-S) of the upper crust, lower crust, and uppermost mantle are 2.62, 3.54, and 4: 10 km/s with V-P/V-S ratios of 1.745, 1.749, and 1.766, respectively. The differences between the predicted and observed travel times show that our 1D model (WSOLOCrust) has average 0.85- and 0.16-s improvements in travel-time residuals compared with the global iasp91 and local CRUST 1.0 models, respectively. This layered crustal velocity model for the western Solomon Islands can be further used as a referenced velocity model to locate earthquake and tremor sources as well as to perform 3D seismic tomography in this region. Electronic Supplement: Figures showing the misfit of inversion process and the comparison between observed and synthetics and the location of experiments in previous studies and tables listing information about the seismic network, parameters of the genetic algorithm (GA), information of earthquakes used in this study, and results obtained from different 1D models.
Building Effects on the P-Alert-Based Real-Time Shaking Map Determination
(2018-11-01)
Wang, Kai-Shyr
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Chao, Wei-An
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Mittal, Himanshu
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Wu, Yih-Min
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土木工程學系
Recently, the P-wave-alert-device (P-alert) network, which is a dense array of microelectromechanical system (MEMS) accelerometers that was developed and installed by National Taiwan University for the purposes of earthquake early warnings, has recorded a large number of strong-motion records for moderate-to-large earthquakes throughout Taiwan. However, many of these stations are mounted on the vertical walls of buildings in ways such that further studies of the sensor-structure interactions on recorded acceleration data are required before the data is used in the production of high-quality shake maps. In this study, we collect the free-field accelerograms recorded by the Taiwan Strong-Motion Instrumentation Program (TSMIP) network that were operated by the Central Weather Bureau (CWB), where MEMS accelerometers were in the vicinity. Then, we compare the peak ground acceleration (PGA) ratio (R-value) between P-alert and TSMIP stations. Finally, we demonstrate how to use the R-value correction on the P-alert data, in order to rapidly produce high-resolution shake maps for relief work to be done soon after major earthquakes. At present, the shake maps produced by the P-alert network are posted automatically in real time on Facebook and are provided to the National Science and Technology Center for Disaster Reduction (NCDR) in order to allow for their relief work. These timely products provide improved information for disaster risk reduction, emergency preparedness, and emergency response.
How Well Can We Extract the Permanent Displacement from Low-Cost MEMS Accelerometers?
(2017-11-01)
Jan, Jyh Cherng
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Chao, Wei-An
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Wu, Yih-Min
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Chen, Chien-Chih
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Lin, Cheng-Horng
Following the recent establishment of a high-density seismic network equipped with low-cost micro-electro-mechanical system (MEMS) P-wave-alert-device (P-Alert) by the earthquake early warning (EEW) research group at the National Taiwan University, a large quantity of strong-motion records from moderate-magnitude earthquakes (M-L > 6) around Taiwan has been accumulated. Using a data preprocessing scheme to recover the dynamic average embedded within the P-Alert data, we adopted an automatic baseline correction approach for the P-Alert accelerograms to determine the coseismic deformation (Cd). Comparisons between the Cd values determined using global positioning system (GPS) data, strong-motion records from the P-Alert network, and data from the Taiwan Strong Motion Instrumentation Program (TSMIP) demonstrates that the near-real-time determination of Cd values (>2 cm), which provide crucial information for seismic hazard mitigation, is possible using records from low-cost MEMS accelerometers.
Relationship Between Earthquake b-Values and Crustal Stresses in a Young Orogenic Belt
(2018-02-28)
Wu, Yih-Min
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Chen, Sean Kuanhsiang
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Huang, Ting-Chung
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Huang, Hsin-Hua
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Chao, Wei-An
It has been reported that earthquake b-values decrease linearly with the differential stresses in the continental crust and subduction zones. Here we report a regression-derived relation between earthquake b-values and crustal stresses using the Anderson fault parameter (A) in a young orogenic belt of Taiwan. This regression relation is well established by using a large and complete earthquake catalog for Taiwan. The data set consists of b-values and A values derived from relocated earthquakes and focal mechanisms, respectively. Our results show that b-values decrease linearly with the A values at crustal depths with a high correlation coefficient of -0.9. Thus, b-values could be used as stress indicators for orogenic belts. However, the state of stress is relatively well correlated with the surface geological setting with respect to earthquake b-values in Taiwan. Temporal variations in the b-value could constitute one of the main reasons for the spatial heterogeneity of b-values. We therefore suggest that b-values could be highly sensitive to temporal stress variations.
The Large Greenland Landslide of 2017: Was a Tsunami Warning Possible?
(2018-07-01)
Chao, Wei-An
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Wu, Tso-Ren
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Ma, Kuo-Fong
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Kuo, Yu-Ting
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Wu, Yih-Min
Tsunamis generated by mass movements such as landslides, underwater slumps, and rock avalanches can lead to serious inundation of nearby populated areas. The lack of timely estimations of the moving mass volume, however, makes providing operational early warnings for landslides particularly challenging. In June 2017, a large landslide in Greenland generated tsunami waves of about 1 m high that impacted the small town of Nuugaatsiaq. We show how the seismic analysis of real-time seismic records from the Greenland Ice Sheet Monitoring Network (GLISN) can provide estimates of essential physical properties of the landslide such as collapse mass and sliding velocity shortly after origin time. The estimation of the landslide source parameters can be utilized for tsunami-wave simulations. We demonstrate how the real-time integration of seismic waveform inversion with forward tsunami-wave simulation could have enabled a timely operational warning (about 10 min) before the arrival of the impending tsunami waves at the village of Nuugaatsiaq.