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Earthquake Sources and Magnitude

Earthquake Sources and Magnitude. Annabel Kelly USGS Menlo Park, CA. What is an Earthquake ?. An Earthquake : Instrumentally recorded (or felt) ground shaking. An Earthquake Source : A sudden change in stress in the Earth that generates seismic waves. Seismic Sources. Fault movement

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Earthquake Sources and Magnitude

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  1. Earthquake Sources and Magnitude Annabel Kelly USGS Menlo Park, CA

  2. What is an Earthquake ? An Earthquake : Instrumentally recorded (or felt) ground shaking An Earthquake Source : A sudden change in stress in the Earth that generates seismic waves

  3. Seismic Sources • Fault movement • Volcanic activity (magma movement or eruptions) • Ocean storms (microseisms) • Cave collapse, rock fall, etc • Manmade sources – explosions, vibrators, cultural noise BGS recordings of an explosion at an oil storage depot near London Dec 16, 2005. Equivalent to M2.4 earthquake

  4. Tectonic earthquakes Elastic Rebound Theory Reid (1910) 8.5 feet offset in San Andreas fault from 1906 earthquake. Marin County

  5. Failure • Build-up of stress (strain energy) • Difficult to predict time and place • Breaks at weakest point • Sometimes hear precursors • Sound of breaking same as seismic waves Comparing an earthquake to the breaking of a chopstick

  6. Types of faults Normal fault Thrust (Reverse) fault Strike-slip fault Images courtesy of IRIS

  7. Strike-Slip Faults

  8. 1979 Imperial Valley, California (M=6.5) Photo by D. Cavit, USGS

  9. March 28, 2005 Sumatra Earthquake Complicated Slip Distributions

  10. Rupture Sumatra earthquake, Dec 28, 2004 Haskell, 1964 Sumatra Ishii et al., Nature 2005 doi:10.1038/nature03675

  11. Asperity = a region of a fault with higher strength than its surroundings The evolution in time and space of the 1985 Michoacán, Mexico, earthquake. Note that this is almost 2 separate earthquakes, one in the south and one in the north, separated by ~10 sec and ~100 km. Bormann 2002. New Manual of Seismological Observatory Practice.

  12. Seismic Moment (Mo) 15 km Area (A) 10 Slip (S) 5 Seismic Moment = (Rigidity)(Area)(Slip) 0 M4 M5 M6 Courtesy of Jim Mori

  13. 2004 Sumatra 1100 x 1027 dyne-cm Mw 9.3 Seismic moments and fault areas of some famous earthquakes Courtesy of Jim Mori

  14. d Point source approximation - Equivalent Body Forces Couple (Single Couple) Double Couple Bormann 2002, New Manual of Seismological Observatory Practice

  15. Moment tensor: dipoles and couples 9 components, but symmetric matrix so 6 are independent USGS

  16. Moment Tensor for an Explosion USGS

  17. Moment Tensor for Fault Slip Double Couple Fault - Slip USGS

  18. Magnitude – a measure of how large an Earthquake is. Incomplete for 2006 Average: ~1 M8 / year Average: 13-15 M7 / year Average: 130-150 M6

  19. M = log(Ad/T)max + σ(Δ,h) + Cr + Cs Regional correction for source directionality Optional station correction Based on velocity therefore proportional to energy Distance correction Types of Magnitude Traditional magnitudes based on amplitudes of recorded data.

  20. Local magnitude - Ml Charles Richter 1900-1985 USGS, NEIC Ml = log Amax – log A0 Defined using horizontal, short period seismometer. Therefore no period consideration. Log A0 correction taken from published tables and related to distance (< 600km) The ~1 sec period response of the seismometer is similar to many small buildings, therefore still useful for engineers. Bruce Bolt. Earthquakes. WH Freeman and Company

  21. Surface wave magnitude - Ms • First defined by Gutenberg 1945. • IASPEI Standard: Distances 2 degrees < Δ < 160 degrees.Depth h < 50 km.Any surface wave period measured on horizontal and vertical components • NEIC:Limit periods to 18 < T < 22 sec and only use vertical component.Distances from 20 degrees < Δ < 160 degrees Ms = log (A/T)max + σS(Δ) = log (A/T)max + 1.66 log Δ + 3.3

  22. Body wave magnitude - mb Distance correction from Gutenberg 1945 • Calculated from P wave displacement amplitude. Commonly reported but very variable calculation methods: • Fairly standard features of measurement: distance 20 deg < Δ < 100 deg, period T < 3 sec. mb = log (A/T)max + Q(Δ,h) • IASPEI Standard: measure Amax from whole recorded P wave; vertical or horizontal max. • NEIC: vertical P only, measure max amp in first 10 cycles (~10-20 sec), or manually extended to 60 sec for large earthquakes. • China and the CTBTO: measure only first 5-6 seconds.

  23. Saturation • Ml, Ms and mb all suffer from saturation. • Occurs for 2 reasons: Time window saturation: The magnitude is calculated for a time window that is less than the duration of the rupture (particularly effects mb) Spectral saturation: The wavelength of the wave is too short to “see” all of the rupture (effects mb, Ml, and Ms) Kanamori 1983

  24. Types of Magnitude Do not saturate and physically meaningful. But more complicated to calculate

  25. Moment Magnitude - Mw • Calculated from seismic moment (Mo). Therefore related to fault slip not energy released as waves. More relevant for tsunamis, less relevant for damage from ground shaking. • Harvard CMT and NEIC calculate Mw from the moment tensor solution. • Fit shape and amplitude of long period surface waves to synthetics to model moment tensor and Mo. Stein and Wysession, “An Introduction to seismology, earthquakes and Earth structure”

  26. Energy Magnitude Me • Calculates the energy released as seismic waves. • Done by integrating radiated energy flux in velocity-squared seismograms over the duration of the rupture.

  27. These two earthquakes in Chile had the same Mw but different Me Earthquake 1: 6 July 199730.0 S 71. W Me 6.1, Mw 6.9No fatalities, no houses destroyed. Earthquake 2: 15 October 199730.9 S 71.2 W Me 7.6 Mw 7.17 people killed, more than 300 people injured. 5,000 houses destroyed. Landslides and rockslides in the epicentral region. Courtesy of George Choy

  28. Types of Magnitude

  29. Mwp ∫uz(t)dt ∝ Mo Mo = Max |∫uz(t)dt| 4pra3r/Fp Mw = (logMo/1.5) – 10.73 ・ Quick magnitude from P wave ・ Uses relatively long-period body waves (10-60 sec) ・ Some problems for M>8.0 Courtesy Jim Mori

  30. Mantle Magnitude - Mm Source Correction Mm = log10(X(w)) + Cd + Cs – 3.9 Distance Correction Spectral Amplitude ・ amplitude measured in frequency domain ・ surface waves with periods > 200 sec Courtesy of Jim Mori

  31. Magnitudes for tsunami warnings • Want to know the moment (fault area and size) but takes a long time (hours) to collect surface wave or free oscillation data and calculate Mw • Magnitude from P waves (mb) is fast but underestimates moment, so: • If have time (hours), determine Mm from mantle waves or Mw from long period surface wave. • For quick magnitude (seconds to minutes), determine Mwp from P waves Courtesy of Jim Mori

  32. Magnitudes for the Sumatra Earthquake Courtesy of Jim Mori

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