code: def _get_deltas(self, sites):
          
          vs30 = sites.vs30
          delta_C = np.zeros(len(vs30))
          delta_C[(vs30 >= 360) & (vs30 < 760)] = 1
  
          delta_D = np.zeros(len(vs30))
          delta_D[vs30 < 360] = 1
  
          return delta_C, delta_D
docs: Return delta's for equation 4
          delta_C = 1 for site class C (360<=Vs30<760), 0 otherwise
          delta_D = 1 for site class D (Vs30<=360), 0 otherwise