  We investigate the sources of the Hubble-induced mass for a flat direction in
supersymmetric theories and show that the sign of the Hubble-induced mass
generally changes just after the end of inflation. This implies that global
cosmic strings generally form after the end of inflation in most supersymmetric
models, including the Minimal Supersymmetric Standard Model. The cosmic strings
emit gravitational waves whose frequency corresponds to the Hubble scale, until
they disappear when the Hubble parameter decreases down to the soft mass of the
flat direction. As a result, the peak frequency of gravitational waves is
related to the supersymmetric scale. The observation of this gravitational wave
signal will give us information of supersymmetric scale and reheating
temperature.
