Browsing by Author "Kao, Ricky"

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Absolute gravity change in Taiwan: Present result of geodynamic process investigation
(2017-12-01)
Kao, Ricky
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Hwang, Cheinway
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Kim, Jeong Woo
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Ching, Kuo-En
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Masson, Frederic
Gravity values at 24 sites over 2004-2016 measured with absolute gravimeters are used to study geodynamic processes in Taiwan. We model rain-induced gravity effects and other temporal effects of non-geodynamic origins to obtain residual gravity, which cannot be fully explained by GPS-derived vertical displacements. We explain the gravity changes associated with deposited debris, earthquake, volcanism and Moho deepening. Gravity changes of 53.37 and 23.38 mu Gal near Sinwulyu and Laonong Rivers are caused by typhoon Morakot, leading to estimated volumes of 6.0 x 10(5) and 3.6 x 10(5)m(3) in deposited debris. The observed co-seismic gravity change near the epicenter of the M 6.9 Pingtung earthquake (26 December 2006) is 3.12 +/- 0.99 mu Gal, consistent with a dislocation-based gravity change at the mu Gal level, thereby supplying a gravity constraint on the modeled fault parameters. The AG record at the Tatun Volcano Group is the longest, but large temporal gravity effects here has led to a current gravity signal-to-noise ratio of less than one, which cannot convince a sinking magma chamber, but supply an error bound for gravity detections of long-term or transient magma movements. The gravity values at Ludao and Lanyu decline steadily at the rates of -2.20 and -0.50 mu Gal yr(-1), consistent with the expected magma states of the two extinct volcanoes. The gravity rates at an uplifting site in central Taiwan and three subsiding sites in eastern Taiwan are negative, and are potentially caused by Moho deepening at a rate of -3.34 cm yr(-1) and a combined Moho deepening and plate subduction at the rates of -0.18, -2.03, and -1.34 cm yr(-1).
An Analysis of Mechanical Constraints when Using Superconducting Gravimeters for Far-Field Pre-Seismic Anomaly Detection
(2011-06-01)
Lan, Shyh-Chin
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Yu, Teng-To
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Hwang, Cheinway
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Kao, Ricky
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土木工程學系
Pre-seismic gravity anomalies from records obtained at a 1 Hz sampling rate from superconducting gravimeters (SG) around East Asia are analyzed. A comparison of gravity anomalies to the source parameters of associated earthquakes shows that the detection of pre-seismic gravity anomalies is constrained by several mechanical conditions of the seismic fault plane. The constraints of the far-field pre-seismic gravity amplitude perturbation were examined and the critical spatial relationship between the SG station and the epicenter precursory signal for detection was determined. The results show that: (1) the pre-seismic amplitude perturbation of gravity is inversely proportional to distance; (2) the transfer path from the epicenter to the SG station that crosses a tectonic boundary has a relatively low pre-seismic gravity anomaly amplitude; (3) the pre-seismic gravity perturbation amplitude is also affected by the attitude between the location of an SG station and the strike of the ruptured fault plane. The removal of typhoon effects and the selection of SG stations within a certain intersection angle to the strike of the fault plane are essential for obtaining reliable pre-seismic gravity anomaly results.
Determination of gravity at MunGyung (Mungyeong) superconducting gravity observatory, Korea
(2009-06-01)
Kim, Ki-Dong
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Kim, Jeong Woo
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Neumeyer, Juergen
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Kao, Ricky
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Hwang, Cheinway
Absolute gravity measurements were made to calibrate Korea's first superconducting gravimeter (SG) at MunGyung (MG, Mungyeong) Observatory. A calibration coefficient (CC) of the MG SG was determined by a parallel registration with an FG5 absolute gravimeter. A total of 8,541 drops were measured over a period of 37 hours between October 8(th) and 10(th), 2007. We first determined the absolute gravity value to be 979,859,179.3 +/- 88.481 mu Gal (nu Gal=10(-8) m center dot s(-2)) after atmospheric pressure, Earth tide and ocean loading corrections. In a linear regression analysis between the FG5 recordings and the raw SG data, a CC of 64.548 +/- 0.224 mu Gal center dot volt(-1) was determined, having previously removed invalid drops and outliers from the data sets. Together with the absolute measurements, a vertical gravity gradient of 2.72 mu Gal center dot cm(-1) was calculated using a Graviton-EG spring gravimeter to take the absolute gravity value down to the SG observatory platform level. The validity of the CC was additionally tested by a comparison between the recorded SG data and the theoretical tides (HW95 and Wahr-Dehant models) as reference. Gravity variations induced by atmospheric pressure and ocean loading were added to the theoretical Earth tides. The CC based on the theoretical tide was determined to be 64.560 mu Gal center dot volt(-1). The difference between the two coefficients is 0.012 mu Gal center dot volt(-1), which lies within the standard error of the determined coefficient, 0.224 mu Gal center dot volt(-1). Therefore, a value of 64.548 mu Gal center dot volt(-1), determined by the parallel registration with the absolute gravimeter, was accepted as the CC of the SG (GWR Instrument Inc. #045) installed at MG Observatory. During the gravity measurements, the other gravity values and heights such as normal gravity and the gravity gradient, orthometric and dynamic heights were also calculated.
Multi-Decadal Monitoring of Lake Level Changes in the Qinghai-Tibet Plateau by the TOPEX/Poseidon-Family Altimeters: Climate Implication
(2016-06-01)
Hwang, Cheinway
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Cheng, Yung-Sheng
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Han, Jiancheng
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Kao, Ricky
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Huang, Chi-Yun
Lake levels in the Qinghai-Tibet Plateau (QTP) provide valuable records for climate change studies. We use two decades of measurements (January 1993-December 2014) from the TOPEX/Poseidon (T/P)-family satellite altimeters (T/P, Jason-1 and -2) to detect lake level variations at 23 lakes along their repeat ground tracks every 10 days. We employ an optimal processing technique to obtain quality measurements, including outlier detection, averaging and filtering. The lake level accuracies are improved by subwaveform retracking. Jason-1 delivers few measurements after waveform retracking and a cluster classification at most lakes. From January 1993 to December 2014, most lake levels in eastern Tibet rose, while those in western Tibet declined. In Qinghai, lake levels dropped before 2005 and then rose afterwards, coinciding with the measure in 2005 that protects the Qinghai ecosystem (e.g., grassland conservation). The overall pattern of lake level change in the QTP is largely affected by monsoons and lake locations. Most lake levels show clear annual and inter-annual oscillations. Certain lakes show alternating level highs and lows in the same seasons and varying amplitudes of annual oscillations due to lake level changes. We detect a sudden rise of lake level by 7 m caused by floods, varying lake level trends associated with the 199798 El Nino and other factors, and persistently rising and declining lake levels associated with the long-term precipitation trends in the QTP. The T/P-family satellites will continue to monitor lake levels here as long as the sea level monitoring program lasts, collecting a long-term climate record at highlands echoing sea level change.
Precise determination of ocean tide loading gravity effect for absolute gravity stations in coastal area of China: Effects of land-sea boundary and station coordinate
(2013-08-01)
Zhou, Jiangcun
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Hwang, Cheinway
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Sun, Heping
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Xu, Jianqiao
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Zhang, Weimin
Gravity effect due to ocean tide loading (OTL) is an important signal and correction in various gravimetric applications. In this paper, we assessed the OTL gravity effects at four absolute gravity (AG) stations in coastal China from several perspectives. The integrated Green's function of the Newtonian part was derived analytically and that of the elastic part was computed based on the PREM earth model. Ocean tide (OT) records near the four AG stations were used to enhance the accuracy of the global ocean tide model of NA099b and the regional model of NA099jb for the innermost field (<0.02 degrees). The high-resolution digital elevation model (DEM) from the Shuttle Radar Topography Mission (SRTM) was used to define the land-sea boundary in the optimized near field (<1.0 degrees). Results show that the high-resolution land-sea boundary is indispensible for OTL gravity modeling when the shoreline near a coastal station is complex. The SRTM-based OTL model outperforms the GMT-based model shoreline in terms of the agreement between modeled and observed gravity residuals at the four stations. The final gravity residuals, corrected for OIL, at the four stations are significantly smaller than those without OTL corrections. We give examples of accuracy requirements in coordinates at Qingdao for different station heights. At a station height of 80 m and to ensure a 0.1 mu gal accuracy in OTL modeling, the required accuracies in the horizontal and vertical coordinate are 2.5 and 1.3 m, respectively. For a new coastal station and an expected OTL accuracy, one should inspect the variation of OTL due to coordinate variation to find a best strategy to determine the required accuracy of coordinate. (C) 2013 Elsevier Ltd. All rights reserved.
Results from parallel observations of superconducting and absolute gravimeters and GPS at the Hsinchu station of Global Geodynamics Project, Taiwan
(2009-07-21)
Hwang, Cheinway
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Kao, Ricky
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Cheng, Ching-Chung
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Huang, Jiu-Fu
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Lee, Chiung-Wu
The Hsinchu (HS) superconducting gravimeter (SG, serial T48) station is a newly established site in the Global Geodynamics Project (GGP). Simultaneous observations of T48, three FG5 absolute gravimeters, and GPS at four stations are studied. GPS shows few mm a(-1) of horizontal and vertical motions around HS. The calibration factor and drifting rate of T48 are -75.96 +/- 0.07 mu Gal V(-1) and 0.2 +/- 0.7 mu Gal a(-1) (1 mu Gal = 10(-8) m s(-2)). Both the SG and absolute gravity records contain trends of about 2-3 mu Gal a(-1). The ocean tide gravity effects (OTGEs) were estimated from NAO. 99b, FES2004, and CSR4.0, and their amplitudes agree with the SG observations at the submicroGal level, but their phases differ from the observations up to 10 degrees. The Newtonian effect of ocean tide contributes 20% to the total OTGE at HS, and it is larger at islands in the Taiwan Strait. The inelastic body tide model of Dehant et al. (1999) is more consistent with the SG observations than the elastic model. Modeled gravity-atmosphere admittances based on an exponential distribution of air mass explain well the observed admittances. The average gravity-atmosphere admittance during typhoons is 30% larger than that in a nontyphoon time. A list of coseismic gravity changes from T48 caused by earthquakes over 2006-2007 is given for potential studies of fault parameters. The modeled effects of atmospheric pressure, groundwater, soil moisture, and polar motion explain the FG5 observed gravity trend to 1.1 mu Gal a(-1). Seasonally, the groundwater-induced gravity change contributes the most to the SG residual gravity, but its phase leads the latter by 63 days.
Sediment-Mass Accumulation Rate and Variability in the East China Sea Detected by GRACE
(2016-09-01)
Liu, Ya-Chi
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Hwang, Cheinway
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Han, Jiancheng
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Kao, Ricky
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Wu, Chau-Ron
The East China Sea (ECS) is a region with shallow continental shelves and a mixed oceanic circulation system allowing sediments to deposit on its inner shelf, particularly near the estuary of the Yangtze River. The seasonal northward-flowing Taiwan Warm Current and southward-flowing China Coastal Current trap sediments from the Yangtze River, which are accumulated over time at rates of up to a few mm/year in equivalent water height. Here, we use the Gravity Recovery and Climate Experiment (GRACE) gravity products from three data centres to determine sediment mass accumulation rates (MARs) and variability on the ECS inner shelf. We restore the atmospheric and oceanic effects to avoid model contaminations on gravity signals associated with sediment masses. We apply destriping and spatial filters to improve the gravity signals from GRACE and use the Global Land Data Assimilation System to reduce land leakage. The GRACE-derived MARs over April 2002-March 2015 on the ECS inner shelf are about 6 mm/year and have magnitudes and spatial patterns consistent with those from sediment-core measurements. The GRACE-derived monthly sediment depositions show variations at time scales ranging from six months to more than two years. Typically, a positive mass balance of sediment deposition occurs in late fall to early winter when the southward coastal currents prevail. A negative mass balance happens in summer when the coastal currents are northward. We identify quasi-biennial sediment variations, which are likely to be caused by quasi-biennial variations in rain and erosion in the Yangtze River basin. We briefly explain the mechanisms of such frequency-dependent variations in the GRACE-derived ECS sediment deposition. There is no clear perturbation on sediment deposition over the ECS inner shelf induced by the Three Gorges Dam. The limitations of GRACE in resolving sediment deposition are its low spatial resolution (about 250 km) and possible contaminations by land hydrological and oceanic signals. Potential GRACE-derived sediment depositions in six major estuaries are presented.
Time-varying land subsidence detected by radar altimetry: California, Taiwan and north China
(2016-06-21)
Hwang, Cheinway
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Yang, Yuande
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Kao, Ricky
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Han, Jiancheng
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Shum, C. K.
Contemporary applications of radar altimetry include sea-level rise, ocean circulation, marine gravity, and icesheet elevation change. Unlike InSAR and GNSS, which are widely used to map surface deformation, altimetry is neither reliant on highly temporally-correlated ground features nor as limited by the available spatial coverage, and can provide long-term temporal subsidence monitoring capability. Here we use multi-mission radar altimetry with an approximately 23 year data-span to quantify land subsidence in cropland areas. Subsidence rates from TOPEX/POSEIDON, JASON-1, ENVISAT, and JASON-2 during 1992-2015 show time-varying trends with respect to displacement over time in California's San Joaquin Valley and central Taiwan, possibly related to changes in land use, climatic conditions (drought) and regulatory measures affecting groundwater use. Near Hanford, California, subsidence rates reach 18 cm yr(-1) with a cumulative subsidence of 206 cm, which potentially could adversely affect operations of the planned California High-Speed Rail. The maximum subsidence rate in central Taiwan is 8 cm yr(-1). Radar altimetry also reveals time-varying subsidence in the North China Plain consistent with the declines of groundwater storage and existing water infrastructure detected by the Gravity Recovery And Climate Experiment (GRACE) satellites, with rates reaching 20 cm yr(-1) and cumulative subsidence as much as 155 cm.