Browsing by Author "Masson, Frederic"

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Absolute gravity change in Taiwan: Present result of geodynamic process investigation
(2017-12-01)
Kao, Ricky
;
Hwang, Cheinway
;
Kim, Jeong Woo
;
Ching, Kuo-En
;
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).
Quantifying sediment mass redistribution from joint time-lapse gravimetry and photogrammetry surveys
(2020-06-22)
Mouyen, Maxime
;
Steer, Philippe
;
Chang, Kuo-Jen
;
Le Moigne, Nicolas
;
Hwang, Cheinway
The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment mass redistribution over a 1 km(2) area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out three joint gravimetry-photogrammetry surveys, once a year in 2015, 2016 and 2017, over a 1 km(2) area in southern Taiwan, featuring both a wide meander of the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface displacements and air pressure variations. Then, we inverted the density of the sediment with an attempt to distinguish the density of the landslide from the density of the river sediments. We eventually estimate an average loss of 3.7 +/- 0.4 x 10(9) kg of sediment from 2015 to 2017 mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long surveys, new instrumentation currently being developed will enable dense and continuous measurements at lower cost, making the method that has been developed and tested in this study well-suited for the estimation of erosion, sediment transfer and deposition in landscapes.