Tsunami earthquake produces anomalously strong tsunamis for their surface wave magnitude. To issue accurate early tsunami warnings for tsunami earthquakes, it is critical to identify them as tsunamigenic events as early as possible. Here we show that a pulse width of P-wave, which we use to calculate broadband moment magnitude Mwp, gives accurate estimates of source duration time. We find that we may identify 1992 Nicaragua earthquake as a possible tsunamigenic earthquake by using a combination of both Mwp and the source duration. Our result suggests that the combination of Mwp and the source duration will provide an effective tool to issue early tsunami warnings even for tsunami earthquakes which have anomalously long source duration. ¿ 2000 American Geophysical Union