Browsing by Author "Hwang, C."

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Co-seismic Gravity Change of M-w 7.9 Wenchuan Earthquake and Pre-Seismic Gravity Anomaly Detection by Superconducting Gravimeter at Hsinchu, Taiwan, from April to June 2008
(2012)
Lan, S. C.
;
Yu, T. T.
;
Hwang, C.
;
Kao, R.
;
土木工程學系
Earthquakes are caused by crustal movement that releases a huge amount of energy and leads to deformation. The gravity near the epicenter is changed by the deformation of rock materials. Precursors may be detected during the period of seismogenic activity. On 12 May, 2008, a M-w 7.9 earthquake struck near Wenchuan in China. A co-seismic gravity offset of 10: 5 mu Gal and a pre-seismic gravity anomaly about 2 days before the main shock was found in gravity records of the superconducting gravimeter (SG) at Hsinchu, Taiwan. SG records from April to June 2008 were used in this study. During this period of time, 8 gravity anomaly events caused by earthquakes were found. Sonograms of time-frequency analysis show that the perturbations occurred in 1-2 major bands (0.1-0.15 Hz and 0.2-0.3 Hz). The results of canonical correlation analysis show a significant positive correlation between the duration of gravity perturbation before the main shock and the magnitude of the earthquake. This study suggest that SG is a promising instrument for the analysis of earthquake events.
Detailed lithospheric structure of an arc-continent collision beneath Taiwan revealed by joint inversion of seismological and gravity data
(2019-07-01)
Gautier, S.
;
Tiberi, C.
;
Lopez, M.
;
Foix, O.
;
Lallemand, S.
The largest compilation of seismological and gravimetry data are combined in a sequential inversion to shed light on unresolved crustal structures in Northern Taiwan and Southernmost Ryukyu area. Our density and seismic velocity models reveal a NW-SE trending Transfer Zone (TZ) between 23.25 degrees N and 24 degrees N. This primary TZ delimits major changes in the Eurasian Plate (EP) slab geometry and dynamics, as well as first order crustal structure variations. North of this TZ, the EP slab is breaking off from north to south as a result of (1) the extreme EP bending favouring plastic deformation, (2) the stretching of the plate caused by the downward pull of the oceanic part of the slab versus the buoyancy of continental part of the plate still deforming in the Taiwan orogen and (3) the indentation of the EP slab by the western slab edge of the Philippine Sea Plate (PSP). The change of EP slab dip angle is clearly observed in the density profiles. The subvertical northernmost tip of EP slab allows asthenospheric upwelling through slab tear and subsequent invasion of the region beneath the Longitudinal Valley. This process is migrating southward up to the TZ, which marks the transition to a different rheological behaviour. The combination of deep arc-continent collision, clockwise asthenospheric toroidal flow and westward propagation of the southern Okinawa rift results in a 100 degrees clockwise rotation of the northernmost tip of the Central Range. The eastward extrusion of this crustal block (approximate to 90km in diameter) overlays and deflects the PSP slab. The indentation of the EP by the PSP (Luzon Arc) thus results in complex lithospheric accommodation processes and structures.
Erosion effects assessed by repeated gravity measurements in southern Taiwan
(2013-01-01)
Mouyen, M.
;
Masson, F.
;
Hwang, C.
;
Cheng, C. -C.
;
Le Moigne, N.
We analyse temporal variations of gravity measured in southern Taiwan since November 2006 at 10 sites using absolute gravimeters and, since November 2008, at 70 sites using a relative gravimeter. We describe and apply methods to interpret the gravity changes in terms of local hydrological processes and vertical ground motions. The effect of land water is computed from local rainfall data and a model of rain accumulation and discharge in the ground. The effect of the vertical motions of the ground is estimated using time-series of permanent Global Positioning System (GPS) stations and the theoretical gravity to height ratio of -2 mu Gal cm(-1). Unexpectedly, Morakot typhoon (2009 August), the strongest typhoon in Taiwan in 50 yr, was responsible for the highest gravity changes. Morakot triggered numerous large landslides and debris flow deposits. Their thicknesses are estimated from the gravity changes (up to similar to 280 mu Gal) along with field observations and satellite images.
Expected temporal absolute gravity change across the Taiwanese Orogen, a modeling approach
(2009-12-01)
Mouyen, M.
;
Masson, F.
;
Hwang, C.
;
Cheng, C. -C.
;
Cattin, R.
The island of Taiwan is located on the convergent boundary between the Philippine Sea plate and the Chinese continental margin. It offers very active mountain building and collapsing processes well illustrated by the rugged topography, rapid uplift and denudation, young tectonic landforms, active faulting and numerous earthquakes. In this paper, using simple models, we have estimated vertical movements and associated absolute gravity variations which can be expected along a profile crossing the southern part of the island and probably suffering the highest rates of rising. The two different tectonic styles proposed for the island, thin-skinned and thick-skinned, were taken into account. Horizontal and vertical movements were modeled by an elastic deformation code. Gravity variations due to these deformations are then modeled at a second step. They are dominated by plate and free-air effects, i.e. elevation of the topography, with several mu Gal yr(-1). By comparison, gravity changes generated by mass transfers are weak: maximum 0.1 mu Gal yr(-1) with the thin-skinned tectonic and 0.3 mu Gal yr(-1) with the thick-skinned tectonic. Though elastic rheology has limitations, this modeling offers interesting results on what gravity signal can be expected from the AGTO project (Absolute Gravity in the Taiwanese Orogen), which proposes to study the dynamic of these mountain ranges using absolute gravimetry (AG) and also including relative gravimetry (RG) and GPS measurements. (C) 2009 Elsevier Ltd. All rights reserved.
GPS Height Variations Affected by Ocean Tidal Loading Along the Coast of Taiwan
(2016-05-15)
Chung, Y. D.
;
Yeh, Ta-Kang
;
Xu, G.
;
Chen, C. S.
;
Hwang, C.
The Global Positioning System (GPS) uses static relative positioning to mitigate common errors and has the ability to remove the effects of ocean tidal loading (OTL). OTL effects cannot be completely removed by increasing the baseline length during GPS data processing and must be corrected with OTL models, especially in Taiwan, which is surrounded by the ocean. In this paper, we collected GPS data from ten stations that are located along the coast of Taiwan. The periodic effect of OTL is discussed through a wavelet analysis and a harmonic analysis. We also calculated the amplitudes and phases of the semi-diurnal and diurnal tides to understand the effect of OTL on the GPS heights along the coast of Taiwan. The GPS data were divided into short-term (1 month) and long-term (2 years) categories and were calculated using the Bernese 5.0 software. The tidal data and the GPS heights were transferred into a wavelet spectrum for analysis. We found that in the tidal data, the periodic M2 signal was stronger than the K1 signal, which was not the case for the GPS height data. This result indicated that the common errors were not completely removed from the GPS heights and that the corresponding period was approximately 1 day, which might result in a stronger K1 signal. Moreover, five OTL models were chosen and compared in this case. Although the GPS heights with different OTL corrections were very similar, NAO. 99b was the best fitting model for the ocean around Taiwan. Finally, we analyzed the tidal data and the M2 amplitudes and phases of the GPS heights using a harmonic analysis and compared the results with the NAO. 99b model. The comparison indicated that the best agreement occurred in southwestern Taiwan and that the poorest agreement occurred in southeastern Taiwan with a maximum error of 0.3 cm. The phase results showed that the OTL model overlapped with the tidal data in the open ocean around central Taiwan, and both the GPS heights and the heights from the OTL model were 1.5 to 2 h earlier than the tidal data.
Investigating possible gravity change rates expected from long-term deep crustal processes in Taiwan
(2014-07-01)
Mouyen, M.
;
Simoes, M.
;
Mouthereau, F.
;
Masson, F.
;
Hwang, C.
We propose to test if gravimetry can prove useful in discriminating different models of long-term deep crustal processes in the case of the Taiwan mountain belt. We discuss two existing tectonic models that differ in the deep processes proposed to sustain the long-term growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal gravity changes caused only by long-term rock mass transfers at depth for each of them. We show that the underplating model implies a rate of gravity change of -6 x 10(-2) mu Gal yr(-1), a value that increases to 2 x 10(-2) mu Gal yr(-1) if crustal subduction is neglected. If the accretion of the whole Eurasian crust occurs, a rate of 7 x 10(-2) mu Gal yr(-1) is obtained. The two models tested differ both in signal amplitude and spatial distribution. The yearly gravity changes expected by long-term deep crustal mass processes in Taiwan are two orders of magnitude below the present-day uncertainty of land-based gravity measurements. Assuming that these annually averaged long-term gravity changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable rates of gravity change. However, as gravity is sensitive to any mass redistribution, effects of short-term processes such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any long-term deep signal. This study indicates that temporal gravity is not appropriate for deciphering the long-term deep crustal processes involved in the Taiwan mountain belt.
Investigating possible gravity change rates expected from long-term deep crustal processes in Taiwan
(2014-07-01)
Mouyen, M.
;
Simoes, M.
;
Mouthereau, F.
;
Masson, F.
;
Hwang, C.
We propose to test if gravimetry can prove useful in discriminating different models of long-term deep crustal processes in the case of the Taiwan mountain belt. We discuss two existing tectonic models that differ in the deep processes proposed to sustain the long-term growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal gravity changes caused only by long-term rock mass transfers at depth for each of them. We show that the underplating model implies a rate of gravity change of -6 x 10(-2) mu Gal yr(-1), a value that increases to 2 x 10(-2) mu Gal yr(-1) if crustal subduction is neglected. If the accretion of the whole Eurasian crust occurs, a rate of 7 x 10(-2) mu Gal yr(-1) is obtained. The two models tested differ both in signal amplitude and spatial distribution. The yearly gravity changes expected by long-term deep crustal mass processes in Taiwan are two orders of magnitude below the present-day uncertainty of land-based gravity measurements. Assuming that these annually averaged long-term gravity changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable rates of gravity change. However, as gravity is sensitive to any mass redistribution, effects of short-term processes such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any long-term deep signal. This study indicates that temporal gravity is not appropriate for deciphering the long-term deep crustal processes involved in the Taiwan mountain belt.
Lithospheric structure of Taiwan from gravity modelling and sequential inversion of seismological and gravity data
(2012-11-20)
Masson, F.
;
Mouyen, M.
;
Hwang, C.
;
Wu, Y. -M.
;
Ponton, F.
Using a Bouguer anomaly map and a dense seismic data set, we have performed two studies in order to improve our knowledge of the deep structure of Taiwan. First, we model the Bouguer anomaly along a profile crossing the island using simple forward modelling. The modelling is 2D, with the hypothesis of cylindrical symmetry. Second we present a joint analysis of gravity anomaly and seismic arrival time data recorded in Taiwan. An initial velocity model has been obtained by local earthquake tomography (LET) of the seismological data. The LET velocity model was used to construct an initial 3D gravity model, using a linear velocity-density relationship (Birch's law). The synthetic Bouguer anomaly calculated for this model has the same shape and wavelength as the observed anomaly. However some characteristics of the anomaly map are not retrieved. To derive a crustal velocity/density model which accounts for both types of observations, we performed a sequential inversion of seismological and gravity data. The variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple LET. Moreover, the sequential model explained about 80% of the observed gravity anomaly. New 3D model of Taiwan lithosphere is presented. (C) 2012 Elsevier B.V. All rights reserved.
Multiple sensors applied to monitorland subsidence in Central Taiwan
(2015)
Hung, W-C
;
Wang, C.
;
Hwang, C.
;
Chen, Y. -A.
;
Chiu, H. -C.
During 1992-2013, pumping of groundwater caused large-scale aquifer-system compaction and land subsidence in the Choshui River Alluvial Fan (CRAF) in Taiwan. The subsidence has already endangered the operation of Taiwan High Speed Rail (THSR). In this paper, we introduce the multiple sensors monitoring system to study the extent of subsidence in CRAF and its mechanism, including GPS (Global Positioning System), PSI (Persistent Scatterer Interferometry), leveling and multi-layer compaction monitoring well. These sensors complement each other in spatial and temporal resolutions.