Browsing by Author "Chao, Wei-An"

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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
;
Chen, Hongey
;
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
;
You, Shuei-Huei
;
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
;
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.
Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath
(2018-06-14)
Schoepa, Anne
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Chao, Wei-An
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Lipovsky, Bradley P.
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Hovius, Niels
;
White, Robert S.
Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min The bulk sliding mass was 7-16 x 10(10) kg, equivalent to a collapsed volume of 35-80 x 10(6) m(3). The sliding mass was displaced downslope by 1260 +/- 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term "afterslides" for this subsequent, declining slope activity after a large landslide.
Evaluating critical rainfall conditions for large-scale landslides by detecting event times from seismic records
(2018-11-06)
Kuo, Hsien-Li
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Lin, Guan-Wei
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Chen, Chi-Wen
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Saito, Hitoshi
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Lin, Ching-Weei
One purpose of landslide research is to establish early warning thresholds for rainfall-induced landslides. Insufficient observations of past events have inhibited the analysis of critical rainfall conditions triggering landslides. This difficulty may be resolved by extracting the timing of landslide occurrences through analysis of seismic signals. In this study, seismic records of the Broadband Array in Taiwan for Seismology were examined to identify ground motion triggered by large landslides that occurred in the years 2005 to 2014. A total of 62 landslide-induced seismic signals were identified. The seismic signals were analyzed to determine the timing of landslide occurrences, and the rainfall conditions at those times - including rainfall intensity (I), duration (D), and effective rainfall (Rt) - were assessed. Three common rainfall threshold models (I -D , I-Rt, and Rt-D) were compared, and the crucial factors of a forecast warning model were found to be duration and effective rainfall. In addition, rainfall information related to the 62 landslides was analyzed to establish a critical height of water model, (I - 1.5) . D = 430.2. The critical height of water model was applied to data from Typhoon Soudelor of 2015, and the model issued a large landslide warning for southern Taiwan.
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
;
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.
Massive collapse of two glaciers in western Tibet in 2016 after surge-like instability
(2018-02-01)
Kaab, Andreas
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Leinss, Silvan
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Gilbert, Adrien
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Buhler, Yves
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Gascoin, Simon
Surges and glacier avalanches are expressions of glacier instability, and among the most dramatic phenomena in the mountain cryosphere. Until now, the catastrophic collapse of a glacier, combining the large volume of surges and mobility of ice avalanches, has been reported only for the 2002 130 x 10(6) m(3) detachment of Kolka Glacier (Caucasus Mountains), which has been considered a globally singular event. Here, we report on the similar detachment of the entire lower parts of two adjacent glaciers in western Tibet in July and September 2016, leading to an unprecedented pair of giant low-angle ice avalanches with volumes of 68 +/- 2 x 10(6) m(3) and 83 +/- 2 x 10(6) m(3). On the basis of satellite remote sensing, numerical modelling and field investigations, we find that the twin collapses were caused by climate-and weather-driven external forcing, acting on specific polythermal and soft-bed glacier properties. These factors converged to produce surge-like enhancement of driving stresses and massively reduced basal friction connected to subglacial water and fine-grained bed lithology, to eventually exceed collapse thresholds in resisting forces of the tongues frozen to their bed. Our findings show that large catastrophic instabilities of low-angle glaciers can happen under rare circumstances without historical precedent.
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
;
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.