# Presentation: 062951

## 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

## 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

- Pg

- Pn

## 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.

## 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

- 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 Shakemap

- Attenuation models are used
- to predict ground shaking where
- seismic station coverage is sparse,
- such as central Alaska.

## 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 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

## 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

- Pg

- Pn

## Regional Lg Q StudiesColorado Plateau

- 150 Lg raypaths used
- to calculate a single regional
- frequency dependent Q model
- for the Colorado Plateau.

- For each raypath Lg amplitude is
- measured in 5 separated filtered
- passbands.

## Individual Frequency Q inversion Method

- CO Raypaths

- 3Hz Q fit

## Frequency Dependent Lg Q

*Q(f)=Q**o**(f)***

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

## 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 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

## Frequency Dependent Lg Q Tomography

- Set up automated procedure
- To measure Lg amplitudes.
- Quality assigned based on
- Energy ratios.

- Pn

- Lg

- We have collected 2300 paths from 230 earthquakes
- recorded at 110 ANSS stations.

## Tomography Model

- Where
*A**ij**(f) *is the observed Amplitude for frequency *f*
*R**ij** *is the distance in km from source* j* to station *i*
-  is the geometric spreading exponent, 0.5.
*S**j**(f) *is the spectrum of source *j.*
*G**i**(f) *is the site response for station *j.*
-   is the average crust shear-wave velocity, 3.5km/s
*r**ijk *is the distance in cell *k* along path from source *j*
- 	to station *k* (Rij =  rijk)
*Q**k* is the quality factor in cell k.
*f* is the center frequency of the data.

**In each frequency band, the source, *****S******j*****, station, *****G******i*****, and *****Q******k*****-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.**

- 2.5x5 degree Q model grid

## Resolution

- 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

- 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.

## Results

- Tomography results closely match
- discrete regional Lg Q studies.

- Q higher in the East
- Q lower in the west
- Colorado shows transition

## Results

- Tomography results closely match
- discrete 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 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.