The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event

Abstract

A moment tensor solution is derived for the great 1938 Banda Sea earthquake, one of the largest events for which no definitive focal geometry has been published. We apply the method of preliminary determination of focal mechanism, which consists of inverting for a moment tensor using exclusively the spectral amplitudes of mantle Rayleigh and Love waves, while discarding the phase information which can be affected by uncertainties in relative timing between historical records. From a dataset of 17 records at seven stations, we obtain a robust solution, featuring a very large moment of 8.37×10 28 dyn-cm, the centroid depth being constrained around 60 km by minimizing the rms residual of the inversion. The method involves an inherent two-fold 180° indeterminacy on the orientations of strike and slip, which is lifted by the examination of the polarization of long-period body waves read individually on original records, resulting in a mostly thrust-faulting mechanism (strike: 276°; dip: 63°; slip: 70°). This approach, coupled with more traditional relocation efforts, allows a rare glimpse of this exceptional event, confirmed here to rank among the 10 largest moments ever published. Within the extremely complex regional plate tectonics pattern, the 1938 event took place in a region of sparse seismicity, and away from the presumed locations of block boundaries. The deeper than normal focus of the event agrees well with the generally minor level of reported damage, and with the generation of a relatively benign tsunami. Available modern focal mechanisms in its vicinity are too few and too scattered to allow any meaningful comparison, but the 1938 event shares its compressional axis with that of the smaller and deeper 1963 shock to the southwest, which probably expresses coherence in the regional contortion of the subducting Australian plate lithosphere.