code: def _compute_site_response_term(self, C, imt, sites, pga1100):
          
          site_resp_term = np.zeros_like(sites.vs30)
  
          vs30_star, _ = self._compute_vs30_star_factor(imt, sites.vs30)
          vlin, c, n = C['VLIN'], self.CONSTS['c'], self.CONSTS['n']
          a10, b = C['a10'], C['b']
  
          idx = sites.vs30 < vlin
          arg = vs30_star[idx] / vlin
          site_resp_term[idx] = (a10 * np.log(arg) -
                                 b * np.log(pga1100[idx] + c) +
                                 b * np.log(pga1100[idx] + c * (arg ** n)))
  
          idx = sites.vs30 >= vlin
          site_resp_term[idx] = (a10 + b * n) * np.log(vs30_star[idx] / vlin)
  
          return site_resp_term
docs: Compute and return site response model term, that is the fifth term
          in equation 1, page 74.