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)…