Misattributed tsunami 3: the Mw 7.7 2013.9.24 SE Pakistan earthquake

Abstract

The Mw 7.7 earthquake that struck SE Pakistan on 24 September 2013 at 11.29.48 UTC was a sinistral strike-slip event on a branch of the Ornach-Nal-Chaman fault system which hereabouts separates the Eurasian Plate from the Indian Plate. Although the focus was at a depth of 15km and 400km inland the earthquake was accompanied by the emergence of an island off the Makran coast and the generation of a tsunami with a peak amplitude of 27cm at Muscat (Oman) and 20cm at Chah Bahar (Iran). At DART marine buoy 23228 in the Indian Ocean 500km to the south a series of seismic Rayleigh waves about 4min after the main shock was followed 54min later by a tsunami with a peak amplitude of 1cm. The Rayleigh series is here attributed to seafloor vibration during accelerated subduction of the Arabian Plate beneath the Eurasian Plate, and the tsunami to the development or reactivation of one or more reverse faults on the seaward portion of the Makran imbricate fan. As in the 2010.2.27Mw 8.8 Maule (Chile), the 2004.12.26Mw 9.2 Sumatra–Andaman, the 2005.3.28Mw 8.7 Nias (Indonesia) and the 2011.3.11Mw 9.0 Tohoku (Japan) earthquakes, the link between tsunami generation and slip on the megathrust is thus very indirect, to the detriment of attempts to mitigate coastal hazards using teleseismic data when nearshore seafloor monitoring would probably prove more effective.