Possible sources of the tsunami observed in the northwestern Indian ocean following the 2013 September 24 Mw 7.7 Pakistan inland earthquake

Abstract

We report and analyse the tsunami recorded in the northwestern Indian Ocean at the Makran region following the Mw 7.7 Pakistan inland strike-slip earthquake on 2013 September 24. We analyse eleven tide gauge records as well as one DART record of this tsunami and perform numerical modelling of the tsunami that would be triggered by a range of possible sources. The tsunami registered a maximum wave height of 109cm at the Qurayat tide gauge station (Oman). The dominant period of the tsunami was around 12min, although wavelet analysis showed that pars of the tsunami energy were partitioned into a slightly wider period range of 7 and 16min. Tsunami backward ray tracing showed that the tsunami source was possibly located offshore Jiwani (Pakistan) and that the tsunami was most likely triggered by the main shock. The aftershocks are distributed in the inland region and the coseismic vertical and horizontal displacements are also limited inland implying that the tsunami was generated by secondary sources triggered by the earthquake. Different possible tsunami sources including a mud volcano at the location of the newly generated island, and a mud volcano or diapir at offshore deep water were examined through numerical modelling and all failed to reproduce the observed waveforms. Numerical modelling showed that a submarine slump with a source dimension of about 10-15km and a thickness of about 100m located at 61.49{ring operator}E and 24.62{ring operator}N, that is, about 60-70km off the Jiwani coast (Pakistan), seems capable of reasonably reproducing the wave amplitudes and periods of the observed tsunami waveforms. This event was the second instrumentally recorded tsunami in the region, after the Makran tsunami of 1945 November, and provides evidence for a hazard from landslide/slump-generated waves following seismic activity in the area.