Lg Q Across the Continental US
Dan McNamara and Rob Wesson
with Dirk Erickson, Arthur Frankel and Harley Benz
Talk OutlineWhat is Lg?
What is Lg used for?
Regional Lg Q Studies1/Q=attenuation
Lg Q Tomography
Summary
Trinidad CO EventM4.6 9/05/2001 10:52:07311 km to ISCO
What is Lg?
Multiply reflected shear-wave in crustal waveguide
Travels at average crustal shear-wave velocity 3.5km/s
Observed from regional earthquakes~120-2000km
Trinidad CO Event, M4.6 9/05/2001 10:52:07
Vr=8.2km/s
Lg travels at 3.5km/s
PgPn
What is Lg used for?
Crustal StructureLg amplitude sensitive to tectonic structuressuch as ocean basins, mountain ranges, faults.
Explosion discriminationExplosion sources do not generate shear energy.
At the USGS:Q modeling to predict ground shaking for:
Shakemap. National hazard map.ANSS network magnitude thresholds.
Lg amplitudes are used todiscriminate between nuclear explosions and naturally occurring earthquakes.
Discrimination becomes difficultin areas with structures that attenuateLg amplitude, such as the deep basinof the Mediterranean Sea.
Lg attenuation models required foraccurate discrimination.
USGS Shakemap
Shakemap systems provide rapidshaking and potential damage information to emergency responseagencies.
USGS plans to implement Shakemapsystems in several major urban areas in the US with significantearthquake hazard.
USGS Shakemap
Attenuation models are usedto predict ground shaking whereseismic station coverage is sparse,such as central Alaska.
USGS National Hazard Map
Attenuation models are used to calculateground shaking to estimate potential hazard.
National hazard map producedusing attenuation models foreastern US, and western US.
More detail needed to betterpredict ground shaking andpotential hazard.
Talk OutlineWhat is Lg?
What is Lg used for?
Regional Lg Q Studies
Lg Q Tomography
Summary
Trinidad CO EventM4.6 9/05/2001 10:52:07311 km to ISCO
Advanced National Seismic System (ANSS)
Trinidad CO Event, M4.6 9/05/2001 10:52:07
Vr=8.2km/s
Lg travels at 3.5km/s
PgPn
Regional Lg Q StudiesColorado Plateau
150 Lg raypaths usedto calculate a single regional frequency dependent Q model
for the Colorado Plateau.
For each raypath Lg amplitude is measured in 5 separated filteredpassbands.
Individual Frequency Q inversion Method
CO Raypaths
3Hz Q fit
Q(f)=Qo(f)
Frequency dependent Q modeled as a power law of the form:
Frequency Dependent Lg Q
Regional Lg Q Results Across Continental US
Q higher in east, lower in west
Unable to model regions where seismic stations and earthquakesare sparse.
Talk OutlineWhat is Lg?
What is Lg used for?
Regional Lg Q Studies
Lg Q Tomography
Summary
Trinidad CO EventM4.6 9/05/2001 10:52:07311 km to ISCO
Frequency Dependent Lg Q Tomography
Set up automated procedureTo measure Lg amplitudes.
Quality assigned based on Energy ratios.
Pn Lg
We have collected 2300 paths from 230 earthquakesrecorded at 110 ANSS stations.
Tomography Model
€
ijA =ij
−γ
R jS ( f )iG
−πf
βijkr
kQk∑e
WhereAij(f) is the observed Amplitude for frequency fRij is the distance in km from source j to station i is the geometric spreading exponent, 0.5.Sj(f) is the spectrum of source j.Gi(f) is the site response for station j. is the average crust shear-wave velocity, 3.5km/srijk is the distance in cell k along path from source j
to station k (Rij = rijk)Qk is the quality factor in cell k.f is the center frequency of the data.
In each frequency band, the source, Sj, station, Gi, and Qk-values were determinedusing an SVD procedure.
Best fits were determined that also minimized the roughness of the solution.
The RMS of the amplitude residuals, solution roughness and magnitude of the singular values were inspected to determine the appropriate number of singular valuesto include in the solution.
2.5x5 degree Q model grid
Resolution was tested by invertingsynthetic Lg amplitudes calculatedwith our raypath distribution througha checkerboard with alternating 5x5 degreesquares with Q values of 500 and 1500.
Resolution and reliability of the results arecorrelated with the cell path length
Cumulative raypath length can givea sense of the reliability of the inversion results.Coverage is best in the BRP andintermountain west where earthquake andstation distribution is most dense.
Resolution
Results
Tomography results closely matchdiscrete regional Lg Q studies.
Q higher in the East
Q lower in the west
Colorado shows transition
Results
Tomography results closely matchdiscrete regional Lg Q studies.
Q higher in the East
Q lower in the west
Colorado shows transition
Implications for Hazard in Colorado
National Hazard Map Hazard maps have used EUS Q values for CO.
New CO Q lower than EUS Q.
New CO Q predicts weaker ground shaking than calculated for hazard maps using EUS Q.
Summary
Q higher in the east and lower in the west.
Mountain regions (CO) show transition from east to west.
Additional data is currently being collected to improve coverage andmodel resolution along west and east coast.
Also additional data should allow us to reduce grid cells size and betterimage the mid-continent Q transition.
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