Abstract
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 μGal 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 ∼280 μGal) along with field observations and satellite images.
Highlights
The study of temporal gravity changes enables to investigate processes that involve mass redistributions and vertical ground motions
4.2.1 Debris flow deposits As a new application of gravity, here we show that we can determine the thickness of a debris flow deposit or of a landslide by coupling the gravity change at a given site with its surrounding surface geometry obtained from satellite images and field observations (Fig. 13)
We explained temporal gravity changes observed in southern Taiwan from 2006 to 2010 by vertical ground motions, local hydrology and massive mass transfers due to landslides and debris flow deposits
Summary
The study of temporal gravity changes enables to investigate processes that involve mass redistributions and vertical ground motions. This method is applied to several research areas such as volcanology Jacob et al 2010) and tectonic deformation The change of gravity combines all the processes due to mass transfers and deformation. Their respective parts must be identified prior to the analysis of one process in particular. The difficulty is that the part of each process can seldom be traced from gravity data only. Complementary observations and models are needed to identify them
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