A magnitude 8.6 earthquake struck North Sumatra, Indonesia, at 2:38 p.m. local time today. According to the U.S. Geological Survey, ground zero for the earthquake was 22.9 km (14.2 miles) below the Earth's crust and 434 kilometers (270 miles) southwest off the coast of Banda Aceh, the capital of Indonesia's Aceh province (which was tragically devastated by the 2004 Indian Ocean earthquake and tsunami). The earthquake was followed by a magnitude 8.2 aftershock 617km (383mi) southwest of Banda Aceh at 4:43 p.m. local time.
Thankfully, there was no big tsunami this time. The region's tsunami defenses have been much improved after the 2004 tragedy that killed over 200,000 people, but they still can be very dangerous, especially if they are bigger than the designers of the safety mechanisms expected (like the tsunami that hit Japan in 2011).
Why No Tsunami This Time?Why no huge tsunami this time despite the pretty big seismic event? Probably because the shock did not deform the sea floor in a way that caused a big vertical displacement of water. There's going to be some pretty big waves for sure, but nothing as dangerous as what we've seen in 2004 or in Japan in 2011. The USGS has a series of images that illustrate this:
It's not only about depth, because the 2004 earthquake took place at a depth of 30 km (19 miles), while this one was at a depth of 22.9 km (14.2 miles). Magnitude matters, but also the local geological formations and how the energy is released.