In Figure E-3, the Intercept terms (a) are again set equal, but the 

 spectral slopes (b) of the component signals differ. The north-south 

 component has spectral slope b = -1.5, while the east-west component has 

 a spectral slope b = -1.0. The artificial surface was designed to mimic 

 the bathymetry of the Mendocino Fracture Zone. The north-south signal 

 dominates the surface, as reflected in an east-west trending contour 

 chart. The wavelength associated with the Intercept term a, was arbi- 

 trarily selected to correspond to a wavelength of two data points. 

 Because of the higher angle of negative slope in the spectrum of the 

 north-south profile, this trend contains higher amplitudes in all fre- 

 quencies lower than one-cycle-per- two-data intervals. Only at the very 

 southern limit of the surface, where the north-south signal is rela- 

 tively constant, can the orthogonal trend be detected. 



At higher frequencies, the spectra of the two component signals 

 cross over, and the east-west signal contains higher amplitudes and dom- 

 inates the surface topography. This somewhat complicated set of circum- 

 stances Is not outside geological experience. For a terrestrial exam- 

 ple, envision a long ridge of several kilometers width and perhaps one 

 thousand meters height, oriented east-west. On the side of this ridge 

 are a series of north-south trending streams and gullies with relief of 

 tens-of -meters which shed the runoff from the ridge. If one were to 

 travel in the north-south direction, the effect of the streams would be 

 minimal, and the long wavelength shape of the ridge would present the 

 only obstacle. If one were to travel in the east-west direction on the 

 face of the ridge, the obstacles in the terrain would be dominated by 

 the lower amplitude, but higher frequency streambeds. This difficulty 

 of travel represents in a very direct sense the concept of surface 



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