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Disputable n on-D C c omponents of s everal s trong e arthquakes

Disputable n on-D C c omponents of s everal s trong e arthquakes. Petra Adamov á Jan Šílený. Motivation. For some strong events a large non-DC component is reported by agencies

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Disputable n on-D C c omponents of s everal s trong e arthquakes

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  1. Disputable non-DCcomponents of several strong earthquakes Petra Adamová Jan Šílený

  2. Motivation • For some strong events a large non-DC component is reported by agencies • This component is often questionable (large events, tectonic origin)  it can be false due to source finiteness (strong event is modeled as point source)

  3. Introduction to second degree moments  a ‘dot’ Standard moment tensor Point source: couple of planes

  4. Introduction to second degree moments  Inside the ‘dot’ “Finite source” parameters from point source approximation • Detailed modeling of slip on the fault plane: • - costly • - often data not available (near the fault) • 2nd degree moments are advantageous alternative • geometry of the source duration of the source process average slip on the fault spatial and temporal centroid rupture velocity vector

  5. Theory Zero degree moment tensor (standard MT) Moments till degree 2, Doornbos (1982) Temporal centroid Spatial centroid Rupture propagation Source process duration Source ellipsoid

  6. Inverse scheme: full waveform inversion Standard MT Estimation of second degree moments exclusion of the non-physical solutions  inversion is faster

  7. Applications: Location of earthquakes

  8. Standard focal mechanisms zero degree moments

  9. Jack-knife test - Izmit and Kobe

  10. Kobe – 2nd degree moments Ide et al. (1996) 10 km

  11. Example of seismogram fit Black: observed seismograms Red: synthetic seismograms

  12. Izmit – 2nd degree moments Clevede et al. (2004)

  13. Synthetic experiment: Estimate of error due to noise for the Izmit earthquake 5 samples of random noise of 20% and 30% of maximum amplitude superimposed on the records  Level 20%: stable solution, small deviation, for source ellipsoid orientation – max. deviation 5 degrees Level 30%: unstable solution, large deviation for source ellipsoid orientation – max. deviation 17 degrees Black ellipsoid: extreme deviation with 20 % noise Blue ellipsoid: extreme deviation with 30 % noise

  14. Focal mechanisms Standard MT • computing standard MT • estimation of 2nd degree moments • removing 2nd degree moments from the data • computing new standard MT Standard MT without second degree effects

  15. Conclusions • Second degree moments bring additional information above standard MT: source ellipsoid, duration of the source process, average rupture velocity • Neglect of source finiteness, which remains in the data even after an essential low-pass filtering may originate spurious non-DC components in the mechanism. • The procedure suggested reduces spurious non-DC components substantially

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