tectonic deformations inferred from absolute gravity measurements in belgium and across the roer...
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Tectonic deformations inferred from absolute gravity Tectonic deformations inferred from absolute gravity measurements in Belgium and across the Roer Grabenmeasurements in Belgium and across the Roer Graben
Michel Van Camp & Thierry Camelbeeck
Royal Observatory of Belgium, Seismology
Ås, 15 March 2006
1) Based on the Membach experiment, ability of AGs to monitor gravity rate of change
2) Results of the AG measurements in Belgium to constrain tectonic deformation
This presentationThis presentation
The Membach station: The Membach station: Superconducting (drift corrected)Superconducting (drift corrected)
and Absolute gravity measurementsand Absolute gravity measurements
40 nm/s²or 4 µGal
1 year
MaintenancesMaintenances
« Set up » noise : « Set up » noise : difference [SG-AG]difference [SG-AG]
112 AG meas. [1996-2005]: nm/s²
- 1 ≤ 71 % ≤ 1 - 2 ≤ 97 % ≤ 2 - 3 ≤ 97 % ≤ 3
Slightly more AG data are lower than SG: poor alignment of the verticality or the test and ref. beams, …
“setup noise” ~ 15 nm/s² AG Instrumental setup noise is white
Histogram of [SG-AG]
[nm/s²] Van Camp, Williams, Francis (JGR 2005)Van Camp, Francis (J. Geod. 2006-submitted)
AG and SG spectra in MembachAG and SG spectra in Membach
~ f -2.5: power law
~ f -1.2 : fractional Brownian noise
10 days 1 day100 days
27 µGal d d27 µGal d d7 µGal d to d7 µGal d to d5 µGal d to d5 µGal d to d
High microseismic noise:aliasing
0.08 µgal per day or7 µGal drop to drop (10 s)
5 µGal d to d5 µGal d to d
Power law processesPower law processesCommon for many type of geophysical signalCommon for many type of geophysical signal
Effect on the estimated slope and the associated uncertainty !
= -2 f-2 : random walk (Brownian)
First-order Gauss-Markov
= -1 f-1 : flicker
f
P(f)
White noiseAG (f > 1 cpd)
Cross over frequency
Annual Semi-annual
Flicker f -1 15 13
Fractional f -1.2 25 23
Time (years) to measure a slope with an uncertainty of 1 nm/s²/yr ( 0.5 mm/yr)
1.0E-005 1.0E-004 1.0E-003 1.0E-002 1.0E-001
F req u en cy [H z]
1.0E+002
1.0E+003
1.0E+004
1.0E+005
1.0E+006
1.0E+007
1.0E+008
1.0E+009
[(n
m/s
²)²/
Hz]
1.0E-005 1.0E-004 1.0E-003 1.0E-002 1.0E-001
F req u en cy [H z]
1.0E+002
1.0E+003
1.0E+004
1.0E+005
1.0E+006
1.0E+007
1.0E+008
1.0E+009
[(n
m/s
²)²/
Hz]
AG noise at AG noise at high frequencieshigh frequencies ( (ff > 1 cpd) at > 1 cpd) atindustrial and coastal stationsindustrial and coastal stations
1.0E-005 1.0E-004 1.0E-003 1.0E-002 1.0E-001
F req u en cy [H z]
1.0E+002
1.0E+003
1.0E+004
1.0E+005
1.0E+006
1.0E+007
1.0E+008
1.0E+009
[(n
m/s
²)²/
Hz]
0.08 Gal daily or0.4 µGal hourly or7 µGal drop to drop (10 s)
1 µGal daily or4 µGal hourly or75 µGal drop to drop
Jülich noisy 1 / 5 sJülich noisy 1 / 10 sJülich quiet 1 / 5 sJülich quiet 1 / 10 s
Ostend 1 / 10 sOstend 1 / 5 s
POL 1 / 10 s (average of 200 PSDs)
T ²2
Summary: HF High noise : a problem ?Summary: HF High noise : a problem ?
10 days No, provided that :- higher sampling rate and/or - longer measurement time
Low microseismic noise : quiet enough to see the (white) instrumental noiseLow microseismic noise : quiet enough to see the (white) instrumental noise ?
10 days 1 day100 days
[Hz]
AG :• Setup noise ~1.5 µGal• When microseismic noise is low, instrumental (white) noise dominates, specific
to each instrument• When the microseismic noise is high: clear aliasing effect• Uncertainty on the trend depends on the noise structure• If 2 measurements/yr: 0.1 µGal/yr [ 0.5 mm/yr] after 13-23years
(Flicker)
... even in noisy stations such as Jülich (industrial) or Oostende (coastal), if
measurements taken carefully
GPS :• Continuous measurements; (x, y, z)• At mid-latitudes, precision = 1 mm/yr [ 0.2 µGal/yr] after 6 to 8 years
(vertical component, reference system problems not taken into account)
The AG is an accurate tool to monitor vertical deformations The AG is not reference-dependent: very useful for slow
deformation rates (peripheral bulge [PGR], intraplate, …)
Ability of AGs : Conclusions
Intraplate seismicity in N-W EuropeIntraplate seismicity in N-W Europe
[Camelbeeck & Meghraoui, GJI, 1998]
Feldbiss fault zone
Coseismic displacement
~ 80 cmMw ~ 6.5
Vanneste et al., J. Seis, 2001Camelbeeck et al., GSA book, 2006
• GEOLOGY : 1) Roer Graben:
Long-term view of a cumulated deformation (~104-5 yr) + possible information on individual large event Deformation rate ~ 0.1 mm/yr (Late pleistocene - Holocene)2) Ardenne: River sinking ~ 0.1 mm/yr uplift but not uniform
• SEISMOLOGY :
Cumulative released seismic moment for the known seismicity
! incomplete history (typical of intraplate context). Deformation rate ~ 0.01 mm/yr ≠ 0.1 mm/yr (geology)
2 possible explanations:1) Aseismic faulting2) Occurrence of large earthquakes
Comforted by historical seismicity in N-W Europe
Suggested by paleoseismic investigations
• GEODESY : Measure a small part of the deformation cycle – could characterize the deformation (seismic – aseismic)
Tectonic deformation: Tectonic deformation: summarysummary
Profile across the Ardenne and Profile across the Ardenne and the Roer Grabenthe Roer Graben
Are the present deformation linked to active faults (elastic Are the present deformation linked to active faults (elastic rebound) in the Ardenne and (or) bordering the Roer Graben ?rebound) in the Ardenne and (or) bordering the Roer Graben ?
AG profileAG profile
PGRPGR
+ Hydrogeological investigations in Membach (Van Camp + Hydrogeological investigations in Membach (Van Camp et al., Meurers et al., 2006, submitted);et al., Meurers et al., 2006, submitted);
London
Brussels
The Hague
Sprimont
Membach
Sohier
Werpin
Manhay
Bensberg
Monschau
Jülich
Paris
Luxembourg
~ 140 km
2 campaigns / yr
Roer Graben
+ Ostend
GPSGPS
Absolute gravity profile across the Ardenne and the Absolute gravity profile across the Ardenne and the Roer Graben since September 1999 (8 stations) + GPSRoer Graben since September 1999 (8 stations) + GPS
GPS measurements closed to border faults
Absolute gravity measurements along a profile across the Ardenne and the Roer Graben to infer vertical movements (deformation rates and wavelength) and to formulate hypothesis on their cause :
linked to active faults (elastic rebound) ? to PGR ?
19992000
20012002
20032004
2005
-10.0
-5 .0
0.0
5.0
10.0
-10.0
-5.0
0.0
5.0
10.0
-10.0
-5.0
0.0
5.0
10.0
-10.0
-5 .0
0.0
5.0
10.0
19992000
20012002
20032004
2005
-10.0
-5.0
0.0
5.0
10.0
-10.0
-5 .0
0.0
5.0
10.0
-10.0
-5 .0
0.0
5.0
10.0
-10.0
-5.0
0.0
5.0
10.0
Sprimont ( - 22.5 km)
Manhay ( - 35 km)
Werpin ( - 44 km)
Bensberg ( + 65 km de Membach)
Jülich ( + 42.5 km)
Monschau ( + 14 km)
g va
riat
ion
(µ
gal)
Membach ( 0 km) Sohier ( - 72 km)
Rate:2.90.7 µGal/yr
Absolute gravity profile : resultsAbsolute gravity profile : results
01-1
996
12-1
996
12-1
997
12-1
998
12-1
999
12-2
000
12-2
001
12-2
002
12-2
003
12-2
004
12-2
005
D ate (Y ear)
-8 .0
-4 .0
0.0
4.0
8.0
+ Ostend (tide gauge)+ Ostend (tide gauge)
Profile + GPS: present conclusionsProfile + GPS: present conclusions
Absolute Gravity :Absolute Gravity :
No vertical crustal deformation larger than 6.5 mm/yr (2 in the Ardenne (1 µGal = 5 mm)Uncertainties can be improved by correcting for hydrological effects (Membach)
GPS: GPS:
5-years continuous GPS-measurements : the relative vertical movement of the two crustal blocks separated by the western border fault of the Roer Graben is less than that previously estimated from repeated leveling (estimating ~1 mm/yr) [Camelbeeck et al. JGR 2003].
In agreement with:
VLBI, SLR & GPS (Ward [1994, 1998]), GPS (Nocquet & Calais [2003, 2004]) : Central Europe (East of Rhine Graben, north of the Alps and Carpathians, south of Scandinavia) rigid at the 0.4
mm/year level In particular: 0.6 mm/year across the Rhine Graben.
PGR : Measurements around 50°N: peripheral PGR : Measurements around 50°N: peripheral zonezone
GlaciationDeglaciation
Peripheral bulgePeripheral bulge
PGR effects on the peripheral bulge predicted by models based on GPS measurements in Fennoscandia : -0.9 mm/year in Belgium (Milne et al., 2001)
Presently not (yet) constrained by measurements Absolute measurements could help
GPS and gravity : ice changes in Antarctica and Greenland information present-day fluctuations, on a radius of 500 km (van Dam et al. [2000]).
GPS/AG combination : separate present-day deformation (elastic deformation) from PGR signals (viscoelastic contribution due to past changes)
Viscoelastic contribution : dh/dg = -6.5 mm/µGal (Wahr 1995)
GPS/AG in Fennoscandia could help checking this value( Ekman & Mäkinen: -5 mm/µGal; Upper mantle density: -6 mm/µGal )
PGR andPGR andpresent-day ice fluctuationspresent-day ice fluctuations
AG and PGR: the futureAG and PGR: the future
Constrain models by taking into account AG measurements on the peripheral bulge:
POL, NERC (Herstmonceux), NPL (UK)ROB (Belgium)BKG (Germany)ECGS (Luxemburg)EOST (France)BIPM (Paris)…