An 8.4 magnitude earthquake occurred off the coast of southern Sumatra on September 12, 2007, followed by a 7.9 magnitude aftershock. This caused 25 deaths and 161 injuries. Seismograph recordings from stations in Washington state, like the one near the LIGO detector, showed arrivals of seismic P and S waves around 21 minutes after the earthquake began. This provides evidence that the gravitational waves detected by LIGO at this time were caused by this Sumatra earthquake.