A seismological reassessment of the source of the 1946 Aleutian ‘tsunami’ earthquake

S U M M A R Y We present a re-evaluation of the seismological properties of the Aleutian ‘tsunami earthquake’ of 1946 April 1, characterized by a deceptively low conventional magnitude (7.4) in view of its catastrophic tsunami, both in the near and far fields. Relocation of 40 aftershocks show that the fault zone extends a minimum of 181 km along the Aleutian trench, in a geometry requiring a bilateral rupture from the original nucleation at the epicentre. Their spatial and temporal distribution are typical of the aftershock patterns of a large earthquake, and rule out the model of a landslide source exclusive of a dislocation. The analysis of the spectra of mantle waves favours the model of a large seismic source, with a static moment of 8.5 × 1028 dyn-cm, making the event one of the ten largest earthquakes ever recorded (hence the destructive tsunami in the far field), and of a slow bilateral rupture, at an average velocity of only 1.12 km s−1, hence the destructive interference in all azimuths for all but the longest mantle waves. The exceptionally slow character of the earthquake is confirmed by a deficiency in radiated seismic energy expressed by the lowest value measured to date of the energy-tomoment ratio. The earthquake appears as an end member in the family of ‘tsunami earthquakes’, resulting from the combination of anomalous, but not unprecedented, parameters, such as low stress drop and rupture velocity.