code: def global_tau(imt, mag, params):
      
      if imt.name == "PGV":
          C = params["PGV"]
      else:
          C = params["SA"]
      if mag > 6.5:
          return C["tau4"]
      elif (mag > 5.5) and (mag <= 6.5):
          return ITPL(mag, C["tau4"], C["tau3"], 5.5, 1.0)
      elif (mag > 5.0) and (mag <= 5.5):
          return ITPL(mag, C["tau3"], C["tau2"], 5.0, 0.5)
      elif (mag > 4.5) and (mag <= 5.0):
          return ITPL(mag, C["tau2"], C["tau1"], 4.5, 0.5)
      else:
          return C["tau1"]
docs: 'Global' model of inter-event variability, as presented in equation 5.6
      (p103)