Tectonic deformations inferred from absolute gravity measurements 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 presentation

The Membach station: Superconducting (drift corrected) and Absolute gravity measurements 40 nm/s² or 4 µGal 1 year Maintenances

« Set up » noise : difference [SG-AG] 112 AG meas. [ ]:  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 submitted)

AG and SG spectra in Membach ~ f -2.5 : power law ~ f -1.2 : fractional Brownian noise 10 days1 day 100 days 27 µGal d d 7 µGal d to d 5 µGal d to d High microseismic noise: aliasing 0.08 µgal per day or 7 µGal drop to drop (10 s) 5 µGal d to d

Power law processes Common 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 noise AG (f > 1 cpd) Cross over frequency AnnualSemi-annual Flicker f Fractional f Time (years) to measure a slope with an uncertainty of 1 nm/s²/yr (  0.5 mm/yr)

AG noise at high frequencies (f > 1 cpd) at industrial and coastal stations 0.08 Gal daily or 0.4 µGal hourly or 7 µGal drop to drop (10 s) 1 µGal daily or 4 µGal hourly or 75 µGal drop to drop Jülich noisy 1 / 5 s Jülich noisy 1 / 10 s Jülich quiet 1 / 5 s Jülich quiet 1 / 10 s Ostend 1 / 10 s Ostend 1 / 5 s POL 1 / 10 s (average of 200 PSDs)

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 noise Low microseismic noise : quiet enough to see the (white) instrumental noise ? 10 days1 day 100 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 Europe [Camelbeeck & Meghraoui, GJI, 1998] Feldbiss fault zone Coseismic displacement ~ 80 cm Mw ~ 6.5 Vanneste et al., J. Seis, 2001 Camelbeeck et al., GSA book, 2006

GEOLOGY : 1) Roer Graben: Long-term view of a cumulated deformation (~ 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 faulting 2) 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: summary

Profile across the Ardenne and the Roer Graben Profile across the Ardenne and the Roer Graben Are the present deformation linked to active faults (elastic rebound) in the Ardenne and (or) bordering the Roer Graben ?  AG profile  PGR  + Hydrogeological investigations in Membach (Van Camp et al., Meurers et al., 2006, submitted);

~ 140 km 2 campaigns / yr Roer Graben + OstendGPS Absolute gravity profile across the Ardenne and the Roer 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 ?

Sprimont ( km) Manhay ( - 35 km) Werpin ( - 44 km) Bensberg ( + 65 km de Membach) Jülich ( km) Monschau ( + 14 km) g variation (µgal) Membach ( 0 km) Sohier ( - 72 km) Rate: 2.9  0.7 µGal/yr Absolute gravity profile : results

+ Ostend (tide gauge)

Profile + GPS: present conclusions 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: 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 zone Glaciation Deglaciation Peripheral 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 and present-day ice fluctuations

AG 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) …