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Magnitudes

Magnitudes. Magnitudes in SEISAN. ML, local, less than 1500 km Mc, local, less than 1500 km mb, distance 20-100°, less than M=7 mB, distance 20-100°, less than M=8 Ms, distance 20-160°, less than M=8 MS, distance 2-160°, less than M=9 MW, any distance and size. Amplitude based magnitudes.

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Magnitudes

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  1. Magnitudes

  2. Magnitudes in SEISAN • ML, local, less than 1500 km • Mc, local, less than 1500 km • mb, distance 20-100°, less than M=7 • mB, distance 20-100°, less than M=8 • Ms, distance 20-160°, less than M=8 • MS, distance 2-160°, less than M=9 • MW, any distance and size

  3. Amplitude based magnitudes

  4. Moment magnitude

  5. Amplitude reading ML • Calculate displacement seismogram • Convolve with Wood Anderson seismograph response • Apply 8 pole high-pass filter at 1.25 Hz • Read maximum S-wave amplitude on vertical channel

  6. Instrument simulation (MSOP)

  7. Local magnitude ML

  8. -log A0

  9. ML scale • -log A0 scaled so thatML=3 at 100 km has 1 mm amplitude on WA seismometer

  10. Inversion for ML scale parameters

  11. Example of ML scale: Norway • ML=log A + 0.91 log R + 0.00087 R -1.68 • Reference distance of 60 km • At 60 km, amplitude 1 mm / 2080 * 1e6 ML = log 480 + 0.91 log 60 + 0.00087 * 60 -1.68 = 2.68 • Result is the same as same amplitude at 60 km in California

  12. Different ML scales

  13. Energy magnitude Me E

  14. Conclusion • Many different magnitude scales are in use • Local scales have to be calibrated • All scales except for MW and Me suffer from saturation • Broadband magnitudes for teleseismic body and surface waves better than traditional scales

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