Ratio of radiated seismic energy and moment to determine source mechanism of the 2010 Mentawai tsunami earthquake

The 25 October 2010 Mentawai earthquake has magnitude Mw 7.7 and shallow depth (12 km). Although the local people of Sikakap - the closed island near epicenter (78.33 km) only felt low shaking (II-III Mercally Intensity), it rised tsunami 7.4 m height causing casualties 447 people died. We suggested the phenomenon is the tsunami earthquake. Therefore we calculate the source mechanism by using W-phase method to determine its parameter. The result yield good value due to reference of GCMT solution with strike 340.3°, dip 8.3°, and slip 116.9°. The cross section diagram of hypocenter show the event located on the beginning of subduction. We obtain the values of seismic radiated energy 1.76×1014 Nm, seismic moment 5.37×1027 Nm, and its ratio is Θ = −6.1. It counted as the threshold of tsunami earthquake. The rupture process has long duration 82 s. The facts proved that the 2010 Mentawai as a tsunami earthquake.