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/ :52: 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 ~ km

Trinidad CO Event, M4.6 9/05/ :52:07 Vr=8.2km/s Lg travels at 3.5km/s PgPn

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/ :52: km to ISCO

Advanced National Seismic System (ANSS)

Trinidad CO Event, M4.6 9/05/ :52:07 Vr=8.2km/s Lg travels at 3.5km/s PgPn

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

Q(f)=Q o (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 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/ :52: 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 (R ij =  r ijk ) 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 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 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. Resolution

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.