Absolute gravity monitoring in the Taiwan Orogen

Transcription

1 Absolute gravity monitoring in the Taiwan Orogen 1 M. Mouyen 1, F. Masson 1, C. Hwang 2, C.C. Cheng 2, N. Le Moigne 3, C.W. Lee 4, R. Kao 2,4, W.C. Hsieh 2,4 1 : Institut de Physique du Globe de Strasbourg, CNRS Strasbourg, France 2 : National Chiao Tung University Hsinchu, Taiwan 3 : Géosciences Montpellier, CNRS Montpellier, France 4 : Industrial Technology Research Institute Hsinchu, Taiwan

2 Introduction 2 Taiwan Plate boundary

3 Introduction 3 Taiwan Plate boundary Active tectonic context

4 Introduction Absolute gravity (AG) measurements repeated once a year (November) using FG5 4

5 Gravity Network 5 Tainan Taitung

6 Short time-range study : 4 years of measurements No trend analysis

7 Why does g change with time? Erosion Tectonics Hydrology 7 Ground movements Deep processes Global Local

8 Why does g change with time? Erosion Tectonics Hydrology 8 Ground movements Deep processes Global Local Erosion rates (Dadson et al., 2003) Converted to gravity Negative effect (mass loss)

9 Why does g change with time? Erosion Tectonics Hydrology 9 Ground movements Deep processes Global Local Example : TUNH GPS station GPS stations and GPS LAB solutions (Kuo, 2010) Converted to gravity Negative effect if surrection

10 Why does g change with time? Erosion Tectonics Hydrology 10 Ground movements Deep processes Global Local Frontal accretion (Yamato et al., 2009) Underplating (Simoes et al., 2007) WNW ESE WNW ESE Density Density Velocity field Velocity field Gravity change rate Gravity change rate

11 Why does g change with time? Erosion Tectonics Hydrology 11 Ground movements Deep processes Global Local AG measure Global hydrological loading computed from GLDAS/Noah model (Rodell et al., 2004) Effect < 1 microgal Example at AG01 site

12 Why does g change with time? Erosion Tectonics Hydrology 12 Ground movements Deep processes Global Local may account for the strongest part of the signal, even if measurements are done at the same season each year but no detailed hydrological monitoring or model near sites

13 13 Measurement correction Measure

14 14 Measurement correction Measure Model : Erosion + Tectonic + Global hydr. loading

15 15 Measurement correction Measure Model : Erosion + Tectonic + Global hydr. loading Residues (= measure model) Hypothesis : Residues are local hydrological changes

16 How to link gravity residues and local hydrology? Main water input in local hydrology : rainfalls Rainfalls gauges exist near absolute gravity sites 16 AG sites Rainfalls gauges Correlation between gravity residues and rainfalls? Correlation between gravity residues and accumulated water before the g measure

17 How to link gravity residues and local hydrology? 17 Hypothesis: The gravity residues are the expression of rainfall regime before the absolute gravity (AG) measure of site properties (slope, soil, vegetation)

18 Principle for each site 18 Variation of gravity residues in comparison with 2006 (first measurement) 1 = = = =

19 Principle for each site 19 Variation of gravity residues in comparison with 2006 (first measurement) Variation of cumulated rainfalls during T days before AG measure in comparison with AG measure = = = = 2009 T

20 Principle for each site Variation of gravity residues in comparison with 2006 (first measurement) 20 Find a time range T for which a correlation appears between the gravity residuals and the accumulated rainfalls during T days before AG measure. Variation of cumulated rainfalls during T days before AG measure in comparison with AG measure = = = = 2009 T

21 Application Example at AG07 21 Find a time range T for which a correlation appears between the gravity residuals and the accumulated rainfalls during T days before AG measure. AG measure T : constant at the same site but can be different between two sites 1=2006 ; 2=2007 ; 3=2008 ; 4= T=70 days 4 3 1

22 Application Example at AG07 22 Find a time range T for which a correlation appears between the gravity residuals and the accumulated rainfalls during T days before AG measure. AG measure T : constant at the same site but can be different between two sites 1=2006 ; 2=2007 ; 3=2008 ; 4= T=70 days T=100 days α= µgal/mm 4 3 α 2 1 1

23 Application Example at AG07 23 Find a time range T for which a correlation appears between the gravity residuals and the accumulated rainfalls during T days before AG measure. AG measure T : constant at the same site but can be different between two sites 1=2006 ; 2=2007 ; 3=2008 ; 4= T=70 days T=100 days T=140 days 2 α= µgal/mm 4 3 α

24 Two parameters to consider Accumulation time T (days) Regression slope α (µgal.mm -1 ) AG03 : affected by height change of a close river AG05 and AG09 : residues already close to 0 24

25 Interpretation 25 Topography (DEM) Topographic index : i x ln a tan x AG04 x a= total surface draining x β = slope at x Quinn et al., 1991 x AG07 x

26 Interpretation 26 Topography (DEM) Topographic index : i x ln a tan x AG04 x x x i= -0.1 i= 2.6 Accumulation of water more encouraged in AG07 than in AG04 AG07

27 Interpretation 27 Topography (DEM) Topographic index : i x ln a tan x AG04 x x x i= -0.1 i= 2.6 Accumulation of water more encouraged in AG07 than in AG04 T= 40 days T= 100 days AG07

28 Interpretation 28 From absolute gravity measures From topography analyze

29 Interpretation 29 From absolute gravity measures From topography analyze

30 Interpretation 30 From absolute gravity measures From topography analyze Topography has a first order role

31 α (µgal.mm -1 ) Interpretation What does α represent? also effect of topography redundant information 31 0,06 0,04 0, , Topographic index

32 Conclusion Good measurements, showing several microgals of changes 32 Explained as the combined effect of vertical movements, erosion and hydrology Deep tectonic (deep rocks movements) need more time to be separated Empirical approach : correlation of gravity with local hydrology could be investigated in terms of : Slope effect : possible link shown using topographic index Soil properties Evapotranspiration

33 Thank you for your attention Morakot typhoon effect near AG03 (August 2009)

34 Two parameters to consider Accumulation time T (days) Regression slope α (µgal.mm -1 ) AG03 : affected by height change of a close river AG05 and AG09 : residues already close to 0 34

35 gravity change (µgal) AG03 35 Correlation between river depth and gravity change AG km -20 river level (m) 3D modelling of river level change returns observed gravity changes