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Geodesy and Geodynamics By Christophe Vigny National Center for scientific Research (CNRS) &

Geodesy and Geodynamics By Christophe Vigny National Center for scientific Research (CNRS) & Ecole Normale Supérieure (ENS) Paris, France http://www.geologie.ens.fr/~vigny. SEISMIC CYCLE. Elastic accumulation and rupture on a fault. Example on a Strike-slip fault and a Subduction fault

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Geodesy and Geodynamics By Christophe Vigny National Center for scientific Research (CNRS) &

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  1. Geodesy and Geodynamics By Christophe Vigny National Center for scientific Research (CNRS) & Ecole Normale Supérieure (ENS) Paris, France http://www.geologie.ens.fr/~vigny DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  2. SEISMIC CYCLE • Elastic accumulation and rupture on a fault. Example on a Strike-slip fault and a Subduction fault • Size of an earthquake • Time dependent station motion and earthquake cycle : READ and Wallace models • Pre-seismic, co-seismic and post-seismic motions • Triggering of earthquake • Precursors ? DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  3. Elastic accumulation and rupture Because the fault is locked, and the 2 plates want to move, they will deform. Deformation will accumulate (arctangent shape) until the accumulation is too much for the fault to resist. It then brakes : it is an earthquake earthquake Keep going earthquake DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  4. Seismic cycle in subduction context 100s years seconds -> minutes months- -> years DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  5. Uy = 2.V0/P arctang (x/h) Arctang profiles DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  6. Time & space dependant station motion Because the fault is locked, a station located close to the fault (blue line) is not moving for a long period of time (when deformation is accumulating). Then the station jumps suddenly, the exact distance that has been accumulated since many years. On the opposite, a station far from the fault (red curve) will move steadily with plate tectonic velocity DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  7. Earthquake cycle : Read and Wallace models The simplest model (READ) is : the same size earthquake (called characteristic earthquake) repeats regularly, every n hundred years (200 on the plot). This time interval is called recurrence time interval. READ modified, suggests an earthquake may occur at regular time interval, but different size each time. WALLACE model, suggests that different size earthquakes may occur in sequences (called clusters) and be separated by different times in a more chaotic way. It is only over a very long period of time and after many earthquakes that the average motion correspond to the plate tectonic velocity DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  8. Size of an Earthquake Earthquake « size » or released Moment M0, is proportional to : - Quantity of slip (U) fault velocity (V) x time between earthquakes Dt - Size of ruptured surface (S) Length of rupure (L) x Locking depth of fault (d) => M0 = mxS x U = mxLxd x V xDt Magnitude of an Earthquake (Hanks & Kanamori): Mw ~ Log(M0) = 2/3 Log (M0) – 10.7 DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  9. Scaling Laws: Gutenberg-Richter The number of Eq. of magnitude M is proportional to 10-bM (and b~1) (log N = a –b.M) DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  10. Scaling Laws: others….. The length of the rupture is proportional to the magnitude The slip on the fault is also related to the length of the rupture A fault of given length should give an earthquake of given magnitude and given slip …….when time is right… DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  11. Difficulty of earthquake prediction Even though a given fault can have a characteristic Earthquake repeating itself over a characteristic time, earthquake prediction is difficult because : • Those values can be unknown, especially if the characteristic time is very long • The earthquakes may occur at recurrence time interval, plus or minus many decades (or centuries) • Physical and/or rheological conditions may change with time and in particular affected by earthquakes themselves Only lower bound of future Earthquake magnitude can be given, assuming : time of latest event Current velocity on fault Locking depth of fault DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  12. Zooming around earthquakes • An earthquake may release in only a couple of seconds 100% of the accumulated deformation. • But some slip may occur before and after the earthquake, the earthquake releasing only part of the accumulated deformation • Slip can then be : • pre-seismic • Co-seismic • Post-seismic DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  13. Post-seismic : K. HEKI, Nature 1997 Silent fault slip following an interplate thrust earthquake at the Japan trench Horizontal coordinate time series before and after the 1994 Sanriku-haruka-Oki earthquake observed at three GPS stations : Mutsu, Aomoriand and Kuji. Dots denote north and east components. Black lines are the model curves (stationary for t < 0, logarithmic decay for t > 0, discontinuity for t= 0). DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  14. Sanriku-Haruka-Oki sequence In that example, it is very clear that the earthquake released only ½ of the accumulated energy. The other ½ was released later (over approximately a year) in a silent and continuous way DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  15. 20+ years of Post seismic in Afar Rift Local velocities in the rift show it is opening today at 16 mm/yr, and then the velocity is decreasing in space to 13 mm/yr, then the far field 11 mm/yr is reached 30 km further. It cannot be like that for ever, therefore the rift opening must slow down in the future. Far field velocities show plate tectonics long term rate of the red sea opening is 11 mm/yr DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  16. 20+ years of Post seismic in Afar Rift Co-seismic (in 1979) = 1.5 m Viscous relaxation = 50 cm Present day velocity : +6.1 mm/yr Far field velocity : +11 mm/yr Accumulation : ~5 mm/yr => Next crisis in 200 years ? DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  17. GPS time series in deforming area near a subduction DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  18. Post seismic relaxation model over 10 years DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  19. Klotz et al., EPSL 2001 d’Antofagasta (Mw=8, 1995) Post-seismic (10 years) Valdivia (Mw=9,5, 1960) Post-seismic (40 years) DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  20. Silent slip on Cascadian subduction zone Short term transients Dragert et al., Science, 292, May 2001 DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  21. Jump in time series DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  22. Silent slip on subduction interface DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  23. McGuire and Segall, G.J.Int. ,2003. Maps of the estimated slip-rate as a function of time and station distribution (black triangles). DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  24. DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  25. AREQ after Arequipa Eq. (23-june-2001, Mw 8.4) Ruegg et al., 2001, seismological research letters DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  26. UAPF after Tarapaca Eq. (13-june-2005, Mw 7,7) Peyrat et al., 2006, Geophysicalresearch letters DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  27. Tongoy before (??-????-2???, Mw 7?,?) Vigny et al., submitted to PEPI DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  28. TORG during Aysen Eq. (21-april-2007, Mw 6,2)Not published… yet…. DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008 Page 28

  29. BTON during Tocopilla Eq. (14-November-2007, Mw 7,8)Not published… yet…. DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008 Page 29

  30. Thai/Malay/Indonesian sites during Sumatra Eq. (25-Dec-04, Mw 9,2)Vigny et al, Nature, 2005 Rupture propagation Seismic surface waves propagation (3.7 km/s) GPS stations displacements Rupture Propagation: 3.7 km/s initially (South) 30s stop ~ 8° lat 1.8 km/s onward (North) DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  31. 34mm/yr - 12° INDE SONDE 35mm/yr - 17° 53mm/yr - 9° Ride 90°E AUSTRALIE Modeled Observed 67 mm/yr - 15° Inversion of slip on faultVigny et al., Nature, 2005 Triple junction DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  32. Complexity of rupture IS IMPORTANT for Tsunami modellingPietrzak et al., EPSL, 2007 DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

  33. END OF CHAPTER DGF – Santiago, Chile – curso 764_1 – Ch. Vigny - 2008

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