The short-term spreading velocity in the Andaman Sea, a marginal basin inboard of the Western Sunda Arc, is calculated using strain rate tensor which, in turn, is obtained from summing seismic moment tensors of normal-faulting earthquakes. A velocity of displacement is also obtained for the right-lateral Sagaing Fault, which is the northern continuation of the Andaman spreading system. Using earthquakes in a 22-year period (1964--1986), the maximum spreading velocity in the Andaman Sea is found to be oriented in a N 19¿ W direction, with a speed of 0.05 cm/yr. The north-south relative velocity along the Sagaing Fault is 0.5 cm/yr, for strike-slip earthquakes between 1971 and 1992. Historical earthquakes in the Andaman Sea do not significantly add to the short-term spreading velocity because there are no earthquakes with Ms≥6.0. For the Sagaing Fault, however, when moment tensors from earthquakes since 1908 are used, the relative velocity is 5.7 cm/yr, a figure close to the 3.72 cm/yr long-term (13 ma) rate of opening of the Andaman Sea. These results suggest that strain due to the opening of the Andaman Sea is seismic along the Sagaing Fault and aseismic along the Andaman Sea Spreading System, probably because the spreading centers and transform faults in the Andaman Sea are short and cannot accommodate large earthquakes. ¿American Geophysical Union 1993