code: def _compute_median_z1pt0(self, vs30):
          
          z1pt0_median = np.zeros_like(vs30) + 6.745
  
          idx = np.where((vs30 >= 180.0) & (vs30 <= 500.0))
          z1pt0_median[idx] = 6.745 - 1.35 * np.log(vs30[idx] / 180.0)
  
          idx = vs30 > 500.0
          z1pt0_median[idx] = 5.394 - 4.48 * np.log(vs30[idx] / 500.0)
  
          return np.exp(z1pt0_median)
docs: Compute and return median z1pt0 (in m), equation 17, pqge 79.