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Lg Q Across the Continental US. Dan McNamara and Rob Wesson with Dirk Erickson , Arthur Frankel and Harley Benz . Talk Outline. What is Lg? What is Lg used for? Regional Lg Q Studies 1/Q=attenuation Lg Q Tomography Summary. Trinidad CO Event M4.6 9/05/2001 10:52:07 311 km to ISCO.

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lg q across the continental us

Lg Q Across the Continental US

Dan McNamara and Rob Wesson

with Dirk Erickson, Arthur Frankel and Harley Benz

talk outline
Talk Outline

What is Lg?

What is Lg used for?

Regional Lg Q Studies

1/Q=attenuation

Lg Q Tomography

Summary

Trinidad CO Event

M4.6 9/05/2001 10:52:07

311 km to ISCO

what is lg
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

slide4

Trinidad CO Event, M4.6 9/05/2001 10:52:07

Lg travels at 3.5km/s

Pn

Pg

Vr=8.2km/s

what is lg used for
What is Lg used for?

Crustal Structure

Lg amplitude sensitive to tectonic structures

such as ocean basins, mountain ranges, faults.

Explosion discrimination

Explosion sources do not generate shear energy.

At the USGS:

Q modeling to predict ground shaking for:

Shakemap.

National hazard map.

ANSS network magnitude thresholds.

slide6

Lg amplitudes are used to

discriminate between nuclear

explosions and naturally

occurring earthquakes.

Discrimination becomes difficult

in areas with structures that attenuate

Lg amplitude, such as the deep basin

of the Mediterranean Sea.

Lg attenuation models required for

accurate discrimination.

usgs shakemap
USGS Shakemap

Shakemap systems provide rapid

shaking and potential damage

information to emergency response

agencies.

USGS plans to implement Shakemap

systems in several major urban

areas in the US with significant

earthquake hazard.

usgs shakemap8
USGS Shakemap

Attenuation models are used

to predict ground shaking where

seismic station coverage is sparse,

such as central Alaska.

usgs national hazard map
USGS National Hazard Map

Attenuation models are used to calculate

ground shaking to estimate potential hazard.

National hazard map produced

using attenuation models for

eastern US, and western US.

More detail needed to better

predict ground shaking and

potential hazard.

talk outline10
Talk Outline

What is Lg?

What is Lg used for?

Regional Lg Q Studies

Lg Q Tomography

Summary

Trinidad CO Event

M4.6 9/05/2001 10:52:07

311 km to ISCO

slide12

Trinidad CO Event, M4.6 9/05/2001 10:52:07

Lg travels at 3.5km/s

Pn

Pg

Vr=8.2km/s

regional lg q studies colorado plateau
Regional Lg Q StudiesColorado Plateau

For each raypath Lg amplitude is

measured in 5 separated filtered

passbands.

150 Lg raypaths used

to calculate a single regional

frequency dependent Q model

for the Colorado Plateau.

slide15

Frequency Dependent Lg Q

Frequency dependent Q modeled as a power law of the form:

Q(f)=Qo(f)

slide16

Regional Lg Q Results Across Continental US

Q higher in east, lower in west

Unable to model regions where

seismic stations and earthquakes

are sparse.

talk outline17
Talk Outline

What is Lg?

What is Lg used for?

Regional Lg Q Studies

Lg Q Tomography

Summary

Trinidad CO Event

M4.6 9/05/2001 10:52:07

311 km to ISCO

slide18

Frequency Dependent Lg Q Tomography

Set up automated procedure

To measure Lg amplitudes.

Quality assigned based on

Energy ratios.

We have collected 2300 paths from 230 earthquakes

recorded at 110 ANSS stations.

Lg

Pn

tomography model
Tomography Model

2.5x5 degree Q model grid

  • Where
  • Aij(f) is the observed Amplitude for frequency f
  • Rijis 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/s

rijk 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 determined

using 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 values

to include in the solution.

slide20

Resolution

Cumulative raypath length can give

a sense of the reliability of the

inversion results.

Coverage is best in the BRP and

intermountain west where earthquake and

station distribution is most dense.

Resolution was tested by inverting

synthetic Lg amplitudes calculated

with our raypath distribution through

a checkerboard with alternating 5x5 degree

squares with Q values of 500 and 1500.

Resolution and reliability of the results are

correlated with the cell path length

slide21

Results

Q higher in the East

Q lower in the west

Colorado shows transition

Tomography results closely match

discrete regional Lg Q studies.

slide22

Results

Q higher in the East

Q lower in the west

Colorado shows transition

Tomography results closely match

discrete regional Lg Q studies.

implications for hazard in colorado
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
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 and

model resolution along west and east coast.

Also additional data should allow us to reduce grid cells size and better

image the mid-continent Q transition.