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Forecasting Magnitude from Fault Geometry Bill Ellsworth, USGS Menlo Park, CA

Forecasting Magnitude from Fault Geometry Bill Ellsworth, USGS Menlo Park, CA. Magnitude – Area Relations M = log(A) + k. Wells and Coppersmith (W&C, 1994) widely used in hazard analysis. Good agreement between W&C and kinematic rupture models derived from seismic waves.

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Forecasting Magnitude from Fault Geometry Bill Ellsworth, USGS Menlo Park, CA

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  1. Forecasting Magnitude from Fault GeometryBill Ellsworth, USGS Menlo Park, CA

  2. Magnitude – Area RelationsM = log(A) + k • Wells and Coppersmith (W&C, 1994) widely used in hazard analysis. • Good agreement between W&C and kinematic rupture models derived from seismic waves. • Application of W&C to WG02 fault model overpredicts historical seismicity rate. • WG02 adopted 3 relations for large earthquakes: M = 3.98 + 1.02 log(A) (W&C) M = 4.2 + log(A) (Ellsworth) M = 3.03 + 4/3 log(A) (Hanks & Bakun) where A = Length x Width x R (seismic coupling factor)

  3. Magnitude – Area RelationsM = log(LWR) + k • Length (L): easy • Width (W): difficult; disagreement between seismic and geodetic rupture models • Aseismic slip factor (R): shallow creep – do-able; brittle-ductile transition – hard • Trade-off between W and R: M = log(L) + log(WR) + k

  4. M(A) and the Earthquake Cycle

  5. Seismologists observe the coseismic rupture M = log(LW) + k

  6. Forecast models must account for the total slip budget M = log(LWR) + k

  7. In this example R = 0.7 or log(R) = -0.15

  8. If the coseismic rupture is described by M = log(LW) + 4.0 the forecast rupture is M = log(LWR) + 4.15

  9. WG02 Approach to Determining W and R Define W as the depth of the brittle-ductile transition determined from seismicity and thermal data Use geodetic data to determine R given W

  10. Depth of Seismicity and Depth to Brittle-Ductile transition in the San Francisco Bay AreaColin Williams USGS, Menlo Park

  11. WG02 Fault L, W and R Values R factor accounts for creep but not for aseismic slip at the brittle-ductile transition If great earthquakes rupture into the brittle-ductile transition W and R will be incorrect

  12. 2002 Mw 7.9 Denali Fault, Alaska Earthquake

  13. Denali Aftershocks LocationsRathkoviski et al.

  14. Near-Fault Displacements from GPS Survey and Rupture Depth of a Uniform Dislocation Fault displacement vectors along the Trans-Alaskan Pipeline Best model: 5.95 m slip from 0 to 11.6 km

  15. Comparing Depth of Rupture EstimatesObtained from Geodesy and Seismology Depth range of geodetic models Depth range of seismic rupture models

  16. Concluding Remarks • Different approaches to the definition of W lead to different M(A) relations. • The trade-off between W and R will be difficult to resolve with available data. • The discrepancy between M(A) relations derived from earthquake slip models and those derived from earthquake cycle considerations can be explained by the R-factor. • If rupture in large magnitude earthquakes routinely extends below the depth of complete locking the R-value in the current Working Group model will need to be modified if the W&C relation is used.

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