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Velocity Structure of the Western US from Surface Wave Phase
Velocity Measurements
Anna Foster (LDEO, Columbia University), Göran Ekström (LDEO, Columbia University), Vala Hjörleifsdóttir (Instituto
Geofísica, UNAM)
Knowledge of the velocity structure of the crust and upper mantle can improve source studies and aid investigations into
mantle dynamics. Using an initial data set of single-station phase measurements of surface waves recorded on the USArray
Transportable Array, we investigate the phase-velocity structure of the western United States at discrete periods between 25-100
s using two different methods. First, we estimate the local phase velocity at a station by subdividing the array into a set of miniarrays,
defined to include all stations within 2° radius of the station of interest. This is done for each event. We perform a grid
search over back azimuth, and for each trial location, a phase velocity is determined in the least-squares sense that best predicts
the observed phase measurements at all stations within the mini-array. The optimal local phase velocity corresponds to the back
azimuth yielding the smallest misfit to the observed phase. This back azimuth also provides an estimate of the arrival angle of
the energy at the mini-array. Local phase-velocity results from all events for a single station are averaged to produce the map
shown in figure 1a. Our second method is based on the difference in phase anomaly between two stations that record the same
event and lie roughly along the same great-circle path. The resulting phase anomaly is assigned to the inter-station path. This is
done for all events, and data comprised of the median measurement for each path are inverted to obtain a velocity map at a given
period. These results can be further improved by correcting for the estimated arrival angle of the wave energy, since off-greatcircle
arrivals bias the two-station method to higher velocities. We calculate the arrival angle using the previously described
method. The arrival angle is used to correct the inter-station distance to which the two-station phase anomaly is assigned prior
to inversion, resulting in a small change in the final velocity (up to 2%). This correction has been done for the inversion results
shown in figure 1b. Most anomalies observed in both models correspond with well-known geologic features. The strong similarities
between these two maps indicate that the methods are robust, and provide encouragement for the ongoing investigation
into earth structure.
Acknowledgements: This work is funded by NSF award EAR-0952285 under the EarthScope program.
Figure 1: Phase velocity maps for Rayleigh waves at 25 s period. a) Averaged local phase velocity estimates from the mini-array method. b) Results of the inversion of
two-station phase anomaly measurements that were corrected for the wave’s arrival angle.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-171