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Development of a Velocity Model for Black Mesa, Arizona, and the Southern Colorado Plateau from Multiple Data Sets

¹ Weston Geophysical Corporation
3919 Essex Lane, Suite 232
Houston, Texas 77027
mleidig{at}westongeophysical.com
 (M.R.L.)

² Weston Geophysical Corporation
4000 S. Medford, Suite 10W
Lufkin, Texas 75901
bonner{at}westongeophysical.com
 (J.L.B.)

³ Weston Geophysical Corporation
57 Bedford Street, Suite 102
Lexington, Massachusetts 02420
delaine{at}westongeophysical.com
 (D.T.R.)

During the summer of 2003, a consortium of scientific institutions conducted a set of seismic experiments on Black Mesa, Arizona, in the southern Colorado Plateau to determine the velocity structure and crustal thickness below the mesa. We detonated a series of explosions, which were recorded by 130 near-source, vertical-component sensors and 25 broadband seismometers. The broadband stations, deployed in a linear array to the south of the mesa, also recorded earthquakes at regional and teleseismic distances during the duration of the deployment. Prior to the explosion series, we conducted a shallow refraction study at the site by using 20 three-component geophones. We utilized the multiple data sets recorded by the experiment to develop a new velocity model for Black Mesa.

We analyzed the shallow refraction and explosion data to generate P- and S-velocity models for the upper crust. The P velocities () were determined by examining first-arrival times as a function of distance from the source. In addition, we performed ground-roll and surface-wave studies to develop a shear-wave velocity (ß) structure for the upper crust. The results indicate an upper-crustal velocity structure that consists of very slow velocity sediments ( < 3 km/sec; ß < 1.5 km/sec) to a depth of 1.8 km. At depths below 1.8 km, the velocities increase to approximately 6 km/sec and 2.7 km/sec for and ß, respectively. For the deeper-crustal and upper-mantle velocity, we used refraction data from broadband stations and inverted surface-wave dispersion data. We were not able to confidently determine the depth of the Moho, as our results indicate it to be between 37 and 46 km. The velocity model developed for Black Mesa was validated using comparisons between observed and synthetic waveforms and small ground-truth explosion location studies.

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