The constrained linear least-squares perturbations to the function were found to be very stable and well represented by a sixth order Chebyshev polynomial at hypocentral distances from 8 km to 500 km. At shorter distances, it is approximated by a line with a slope close to 2. In this study, we use hypocentral distance, rather than epicentral distance as originally used by *Richter* (1935), to accurately represent variation in the attenuation function at close distances. This form was adopted and a CISN algorithm was developed and used in all subsequent inversions for the SNCL adjustments. A plot of the CISN function is shown in Figure 2.26.

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