code: def _compute_sigma_0(self, C, mag, vs30measured):
          
          s1 = np.zeros_like(vs30measured, dtype=float)
          s2 = np.zeros_like(vs30measured, dtype=float)
  
          idx = vs30measured == 1
          s1[idx] = C['s1mea']
          s2[idx] = C['s2mea']
  
          idx = vs30measured == 0
          s1[idx] = C['s1est']
          s2[idx] = C['s2est']
  
          return self._compute_std_0(s1, s2, mag)
docs: Equation 27, page 82.