Université Joseph Fourier Grenoble, France The nature and promise of broadband surface-wave measurements from the random wavefield Nikolai Shapiro Michael Ritzwoller Michel Campillo Laurent Stehly Université Joseph Fourier Grenoble, France University of Colorado at Boulder

Main goal: to optimize the surface-wave imaging in context of USArray earthquakes with M>4.5 occurred during 18 months (01/01/2001-07/01/2002) Main problem: low regional seismicity for most of USA

Diffuse field vs. ballistic waves localized lateral sensitivity samples all directions source independent may allow many short-period measurements Alternative solution: making measurement from random wavefield (ambient seismic noise) May improve resolution traditional approach: using teleseismic surface waves source extended lateral sensitivity sample only certain directions source dependent difficult to make short-period measurements Consequence: limited resolution

Extracting Green functions from the random wavefield by field-to-filed correlation: theoretical background seismic noise is excited by randomly distributed ambient sources (oceanic microseisms and atmospheric loads) modal representation of the random field: - eigenfunctions - eigenfrequencies - modal excitations, uncorrelated random variables: - spectral energy density cross-correlation between points x and y : differs only by an amplitude factor F() from an actual Green function between x and y

Cross-correlations from ambient seismic noise: ANMO - CCM (from Shapiro and Campillo, GRL, 2004) cross-correlations from 30 days of continuous vertical component records (2002/01/10-2002/02/08) prediction from global group velocity maps of Ritzwoller et al. (2002) frequency-time analysis of the broadband cross-correlation

Cross-correlations from ambient seismic noise at US stations (from Shapiro and Campillo, GRL, 2004) frequency-time analysis of broadband cross-correlations computed from 30 days of continuous vertical component records

Cross-correlation from ambient seismic noise in North-Western Pacific (from Shapiro and Campillo, GRL, 2004) broadband cross-correlation computed from 30 days of continuous vertical component records

Cross-correlation from ambient seismic noise in North-Western Pacific (from Shapiro and Campillo, GRL, 2004) broadband cross-correlation computed from 30 days of continuous vertical component records

Cross-correlations from ambient seismic noise in California (from Shapiro and Campillo, GRL, 2004) cross-correlations of vertical component continuous records (1996/02/11-1996/03/10) 0.03-0.2 Hz 3 km/s - Rayleigh wave

correlations computed over four different three-week periods PHL - MLAC 290 km band- passed 15 - 30 s repetitive measurements provide uncertainty estimations band- passed 5 - 10 s

correlations computed over four different three-week periods PHL - MLAC 290 km band- passed 15 - 30 s band- passed 5 - 10 s repetitive measurements provide uncertainty estimations

Conclusions Cross-correlations computed from the ambient seismic noise can provide new surface-wave dispersion measurements that have numerous advantages relative to traditional measurements made from teleseismic waves: Measurements possible for every pair of stations No source related errors Localized sensitivity zones Measurements can be extended to shorter periods Repetitive measurements provide uncertainty estimations Those measurements may be particularly useful in context of dense arrays of seismometers, such as USArray