code: def _get_phi(self, C, mag):
          
          if mag < 5.5:
              return C["phi1"]
          elif mag < 5.75:
              return C["phi1"] + (C["phi2"] - C["phi1"]) * ((mag - 5.5) / 0.25)
          else:
              return C["phi2"]
docs: Returns the magnitude dependent intra-event standard deviation (phi)
          (equation 15)