Predicting Earthquake Shaking and hazard

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

J. Louie, NMSLC 3/3/2011

Last Week’s Earthquakein Christchurch, New Zealand

Magnitude 6.3 aftershock of M 7.1 in Sept.


Unexpectedly Intense Ground Shaking Horizontal accelerations >2 times gravity


What Happens with Such Intense Shaking? >200 deaths, 1/3 of city’s buildings destroyed

J. Louie, NMSLC 3/3/2011Stuff.co.nz

Photos by Marilyn Newton, Reno Gazette-Journal

Could It Happen Here? It Already Did! Wells, Nevada, Feb. 2008

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


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.


Adding Fault GeologyBlack Hills Fault in Google Earth with USGS Qfaults trace


Adding Geology & Geotechnical DataBlack Hills Fault in Google Earth with USGS Qfaults trace

Earthquake Magnitude from Fault Size

M0 = μAdμ = 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)


Adding Geotechnical

Data

ShakeZoning Geotech Map

• Obtained by Clark Co. and City of Henderson• 10,721 site measurements


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)


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


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


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


Showing 3-D Vector Motions 3 computed

components of the ground particle velocity vector: (x, y, z)




from MathWorks.com

















Add the color components to get a perceived color

Color depends on strength and direction of wave vibration


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


Adding Physics


0 seconds after rupture begins on the Black Hills fault (9 km down)

Las Vegas basin in shaded relief

LVH

FM

BH


Adding Physics


2.2 seconds after rupture begins on the Black Hills fault

Seismic waves reach the surface in Eldorado Valley

LVH

FM

BH


Adding Physics



P wave in Las Vegas, small (dark yellow)

Intense surface waves funneling into Henderson

LVH

FM

BH


Adding Physics Cue up and play:BH-ClarkCo-0.5Hz.m4v


Rayleigh wave in W. Las Vegas, large (red-blue)

Like ocean wave: vertical in between radial motions

LVH

FM

BH




Rayleigh wave carrying energy to Pahrump

Much energy left behind in soft geologic basins

LVH

FM

BH




Rock areas like FM insulated from shaking

Shaking trapped in basins, radiating out

LVH

FM

BH


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


Frenchman Mountain Fault M6.7 ScenarioPossible Scarp in Neighborhood

Event Inside the LVV Basin

J. Louie, NESC 2/9/2011

Frenchman Mountain Fault M6.7 ScenarioEvent Inside the LVV Basin

Cue up and play:FMF_ClarkCo_0.5Hz_24fps.m4

v

Timelapse animation 60 seconds wave

propagation compressed to 24 sec video

Time compression factor of 2.5


2-Segment Frenchman Mtn. Fault M6.7


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


We Are Computing Dozens of Scenarios



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 location j

P = probability of ground motions u ≥ u0 at site i, if an earthquake occurs at source location j with magnitude M

US Geological Survey

Hazard Maps On line at http

://earthquake.usgs.gov/hazards/

Mostly from past earthquakes

No wave physics


http://earthquake.usgs.gov/hazards/




Fault

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

Physics-Based Wave Propagation0.1 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

Peak Ground Velocities (PGV)

Max PGV: 22 cm/s Max PGV: 46 cm/s

Documents

Predicting Earthquake Shaking and hazard