Absolute gravity change in Taiwan: Present result of geodynamic process investigation

  • Author(s): Ricky Kao, Cheinway Hwang, Jeong Woo Kim, Kuo-En Ching, Frédéric Masson, Wen-Chi Hsieh, Nicolas Le Moigne, and Ching-Chung Cheng
  • DOI: 10.3319/TAO.2017.06.13.01
  • Keywords: Absolute gravity, Moho deepening, Typhoon Morakot, Pingtung earthquake, Volcano, Taiwan
  • Absolute gravity change in Taiwan are used to investigate four geodynamic processes
  • Declining gravity values at Tatun, Lanyu and Ludao volcanoes imply retreating magma
  • Decreasing gravity valuesin central and eastern Taiwan indicate Moho deepening
Abstract

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 μGal near Sinwulyu and Laonong Rivers are caused by typhoon Morakot, leading to estimated volumes of 6.0×105 m3 and 3.6×105 m3 in deposited debris. The observed co-seismic gravity change near the epicenter of the M6.9 Pingtung earthquake (December 26, 2006) is 3.12 ± 0.99 μGal, consistent with a dislocation-based gravity change at the μGal 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 μGal/yr and -0.50 μGal/yr, 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 and a combined Moho deepening and plate subduction at the rates of -0.18, -2.03 and -1.34 cm/yr.

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