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.