code: def _compute_inter_event_std(self, C, C_PGA, pga1100, mag, vs30):
          
          tau_0 = self._compute_std_0(C['s3'], C['s4'], mag)
          tau_b_pga = self._compute_std_0(C_PGA['s3'], C_PGA['s4'], mag)
          delta_amp = self._compute_partial_derivative_site_amp(C, pga1100, vs30)
  
          std_inter = np.sqrt(tau_0 ** 2 + (delta_amp ** 2) * (tau_b_pga ** 2) +
                              2 * delta_amp * tau_0 * tau_b_pga * C['rho'])
  
          return std_inter
docs: Compute inter event standard deviation, equation 25, page 82.