Predicting Earthquake Shaking and hazard

1. Predicting Earthquake Shaking and hazard John N. Louie, Nevada Seismological Lab. with UNR undergraduate interns: Will Savran, Brady Flinchum, Colton Dudley, Nick Prina and Geology B.S. graduate Janice Kukuk

2. Last Week’s Earthquakein Christchurch, New Zealand • Magnitude 6.3 aftershock of M 7.1 in Sept.

3. Unexpectedly Intense Ground Shaking • Horizontal accelerations >2 times gravity

4. What Happens with Such Intense Shaking? • >200 deaths, 1/3 of city’s buildings destroyed Stuff.co.nz

5. Could It Happen Here? Photos by Marilyn Newton, Reno Gazette-Journal It Already Did! Wells, Nevada, Feb. 2008

6. How Do We Protect Nevada’s People and Economy from Earthquakes? • Stiffen building codes to strengthen buildings everywhere? • But, would make construction too costly • Improve our understanding of earthquake shaking • What areas have high hazard? Put resources there. • Don’t waste money reinforcing safer areas

7. Three Elements to Predicting Shaking (1) Where are the earthquake sources? • Discover and locate faults with seismic monitoring and surveying • Characterize faults with geology and seismic surveying (2) How will the waves propagate from the sources? • Characterize basins with gravity and seismic surveying (3) How will the soils under your property react? • Seismic microzonation with Parcel Mapping • Scenario predictions with “Next-Level ShakeZoning” • Use physics and geology to get realistic shaking predictions for likely earthquakes • Combine predictions with probability of each earthquake • Nevada researchers are working on these challenges.

8. Adding Fault Geology Black Hills Fault in Google Earth with USGS Qfaults trace

9. Adding Geology & Geotechnical Data Black Hills Fault in Google Earth with USGS Qfaults trace Earthquake Magnitude from Fault Size M0 = μAd μ = 3x1011 dyne/cm2 A = Fault Area (cm2) = (9 km length)(105 cm/km) (9 km width)(105 cm/km) d = fault displacement = 200 cm (from geologists)

10. Adding Geotechnical Data ShakeZoning GeotechMap Obtained by Clark Co. and City of Henderson 10,721 site measurements

11. Adding Physics 2nd-order PDE controls P(x,y,z) wave’s evolution in time Uses Laplacian to get spatial derivatives Use definition of derivative to compute a Finite Difference (don’t take limit)

12. Wave Computation on a 3D Geological Grid Fine grid gives accurate FD estimate of derivatives Finer grid takes longer to compute, higher cost Finer grid for higher shaking frequencies

13. Adding Physics • Black Hills M6.5 event • Short trace but 4-m scarps noted • Viscoelastic finite-difference solution • 0.5-Hz frequency • 0.20-km grid spacing • A few hours on our small cluster • Map view of waves • Mode conversion, rupture directivity, reverberation, trapping in basins

14. Showing 3-DVector Motions • 3 computed components of the ground particle velocity vector: • (x, y, z) • 3 components of color on your computer screen: • (R, G, B) • red, green, blue

15. Showing 3-D Vector Motions • 3 computed components of the ground particle velocity vector: • (x, y, z) • 3 components of color on your computer screen: • (R, G, B) • red, green, blue from MathWorks.com

16. Showing 3-DVector Motions • 3 computed components of the ground particle velocity vector: • (x, y, z) • 3 components of color on your computer screen: • (R, G, B) • red, green, blue

17. Showing 3-DVector Motions • 3 computed components of the ground particle velocity vector: • (x, y, z) • 3 components of color on your computer screen: • (R, G, B) • red, green, blue

18. Showing 3-DVector Motions • 3 computed components of the ground particle velocity vector: • (x, y, z) • 3 components of color on your computer screen: • (R, G, B) • red, green, blue

19. Showing 3-DVector Motions • Add the color components to get a perceived color • Color depends on strength and direction of wave vibration

20. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • Timelapse animation • 60 seconds wave propagation compressed to 16.6 sec video • Time compression factor of 3.6

21. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 0 seconds after rupture begins on the Black Hills fault (9 km down) • Las Vegas basin in shadedrelief FM LV H BH

22. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 2.2 seconds after rupture begins on the Black Hills fault • Seismic waves reach the surface in Eldorado Valley FM LV H BH

23. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 6.9 seconds after rupture begins on the Black Hills fault • P wave in Las Vegas, small (dark yellow) • Intense surface waves funneling into Henderson FM LV H BH

24. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 13.4 seconds after rupture begins on the Black Hills fault • Rayleigh wave in W. Las Vegas, large (red-blue) • Like ocean wave: vertical in between radial motions FM LV H BH

25. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 23.9 seconds after rupture begins on the Black Hills fault • Rayleigh wave carrying energy to Pahrump • Much energy left behind in soft geologic basins FM LV H BH

26. Adding Physics • Cue up and play: BH-ClarkCo-0.5Hz.m4v • 45.2 seconds after rupture begins on the Black Hills fault • Rock areas like FM insulated from shaking • Shaking trapped in basins, radiating out FM LV H BH

27. Black Hills M6.5 Scenario Results • Max Peak Ground Velocity (PGV) >140 cm/sec • PGV over 60 cm/sec (yellow) bleeds into LVV by Railroad Pass • Large event for a short fault • Geologists are divided on likelihood • Need to know how likely

28. Frenchman Mountain Fault M6.7 Scenario Possible Scarp in Neighborhood Event Inside the LVV Basin

29. Frenchman Mountain Fault M6.7 Scenario Event Inside the LVV Basin • Cue up and play: FMF_ClarkCo_0.5Hz_24fps.m4v • Timelapseanimation • 60 seconds wave propagation compressed to 24 sec video • Time compression factor of 2.5

30. 2-Segment Frenchman Mtn. Fault M6.7

31. 2-Segment Frenchman Mtn. Fault M6.7 • All of Las Vegas Valley shakes as hard as Wells in 2008 (20 cm/s) • Higher shaking in areas of refraction and focusing • Less shaking in west Valley: stiffer soil

32. We Are Computing Dozens of Scenarios

33. Combine the Scenarios Probabilistically λ = annual frequency of exceeding ground motion u0 rate(M, sourcej) = annual rate of occurrence for an earthquake with magnitude M at source locationj P = probability of ground motions u≥ u0at sitei, if an earthquake occurs at source locationjwith magnitude M

34. US Geological Survey Hazard Maps • On line at http://earthquake.usgs.gov/hazards/ • Mostly from past earthquakes • No wave physics

35. Fault

36. Model Setup • Two Basin-Thickness Datasets: • Widmer et al., 2007 Washoe Co. gravity model • Saltus and Jachens 1995 gravity model • Two Geotech Datasets: • Pancha 2007 ANSS station measurements • Scott et al., 2004 shallow shear-velocity transect • Scenario Fault (like 2008 Wells): • Strike: N-S • Motion: Normal- down to the west • Length: 7.58 km • Mw: 5.94 (Anderson et al., 1996) • Frequency: 0.1 Hz and 1.0 Hz

37. Physics-Based Wave Propagation 0.1 Hz Model 1.0 Hz Model • Cue up and play DowntownReno-1Hz-5.04M.m4v • The basin amplifies and traps seismic shaking • Wave propagation unaffected by basin dataset boundaries in the 0.1 Hz Model • Wave propagation is affected by basin dataset boundaries in the 1.0 Hz Model- but not in basin

38. Peak Ground Velocities (PGV) Max PGV: 22 cm/s Max PGV: 46 cm/s