An analysis of regional broadband seismograms of moderate and large subduction-zone earthquakes in Mexico shows that earthquakes which occur near the trench are abnormally depleted in high-frequency radiation. This observation leads to a simple and fast method to assess regional tsunami potential from earthquakes which occur along the Pacific coast of Mexico. A significant advantage of the method is that a single broadband seismograph is sufficient for the purpose. The method is based on the ratio of the total energy to the high-frequency energy (between 1 and 5 Hz), ER, computed from the seismograms. For earthquakes with the same seismic moment, ER is an order of magnitude greater for a source area near the trench as compared to those near the coast. The same seismograms are used to compute energy magnitude, ME, which is tied to the moment magnitude, MW. A regional tsunami may be expected along the coast if ME≥6.5 and ER corresponds to a source near the trench. If, however, ER corresponds to a near-coast source, then ME may have to be greater than about 7.3 for a tsunami of similar size to occur along the coast. The method holds promise for a fast regional tsunami warning in many Pacific basin countries which lack an adequate seismic network. ¿ 1998 American Geophysical Union