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Variations in GPS-derived displacements and stresses during the 2003 Miyagi earthquakes

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  • 1. Variations in GPS-derived displacements and stresses during the 2003 Miyagi earthquakes Ali O. Oncel and Omer Aydan Geological Survey of Japan Tokai University, Department of Marine Civil Engineering
  • 2. Contents of Presentation
    • Seismotectonic aspects of Japan
    • GPS Earth Observation Network (GEONET)
    • Variations in GPS-derived regional strain and stress
    • Directivity of inferred deformation for July 26 event
    • Aftershocks of July 26 event and GPS-local stations
    • Computation rates of stress and strain
    • Variations in the rates of stress and strain
    • Disturbing stress changes and asperity
  • 3. Seismotectonic aspects of Japan (Oncel, Wilson and Nishizawa, JGR-2001) Negative Correlations :Stress suddenly released of larger magnitude seismicity on interconnected faults of larger total surface . . Positive Correlations: Stress is gradually released by lower magnitude seismicity on smaller fault strands. Tohoku Events: July 26 event (M=6.2) were located in a positive correlation (Area III) noted to be anomalously quiescent. 5.00 to 6.00 6.00 to 7.00 1998-2003 M=6.2 2003.07.26 M=5.5 2003.07.26
  • 4. GPS Earth Observation Network (GEONET) displacement rate GPS stations almost 1000 stations in Japanese Islands daily measurement of location with accuracy of 1[cm] No spatial filtering to get rid of measurement noise FEM with triangle element
  • 5. Variations in regional strain (Hori, 2003) displacement rate GPS stations volumetric max. shear
  • 6. Variations in regional stress (Hori, 2003) displacement rate GPS stations volumetric max. shear
  • 7. Miyagi Event and GPS-stations Directivity of inferred deformation for July 26 event (Mw=6.2)
  • 8. Aftershocks AND GPS-local stations
  • 9.
    • Strain rates
    average strain rates computed by using 1 st -order derivative from displacements Coordinate system and the definition of displacement rates for a triangular element Displacements Strain rates Strain rates (Aydan 2000, 2003)
  • 10. stress rate tensor computed by using Hooke’s law from the computed strain rates for elastic materials. Maximum Shear Stress + Mean Stress Rate * = Disturbing Stress Rate Friction of coefficient ( =0.8) Lame’s constants, which are generally assumed to be 30 GPa [Fowler, 1990]. Stress rates (Aydan 2000, 2003)
  • 11. Estimated displacements for about three months M6.1 M6.3 M6.1 M6.3 M6.1 M6.3 1.5cm 2.7 cm 2.7cm 3.4 cm 1.4 cm 0.7 cm 1.7cm 1.0cm Variations in Displacements
  • 12. Variations in Stress Rates M6.1 M6.3
  • 13. Disturbing stress changes and asperity
  • 14. Changes in GPS measurement seems to be partially related to the fault patches of seismic (asperity) and aseismic (creep). Especially, GPS velocity vectors is smaller in northern strand of NAFZ indicating higher seismic hazard while observing larger in its southern strand. Suggested asperities and GPS