The 2004 Indian ocean earthquake and tsunami was one of the deadliest natural disasters in human history, killing almost one quarter of a million people spread over an area of tens of thousands of miles. The earthquake is commonly referred to as the Sumatra earthquake since its epicenter was located off the coast of Sumatra, Indonesia. One of the Indian Ocean plates subducted under another, and the result was that the ocean floor surface was quickly raised up by several meters. The resulting displacement of ocean water was massive and cause a huge wave (tsunami) to emanate from the epicenter in multiple directions. The countries most affected by the tsunami were Indonesia, India, Sri Lanka, and Thailand.
There are several reasons that the tsunami was so much more devastating than the earthquake. First, the earthquake was centered in the open ocean and not under a populated land mass. If the same earthquake had been located in southern California, for instance, the devastation would be almost unthinkable.
Second, the earthquake itself is more of a point source, meaning that it originates in and its effects are felt in a relatively contained area on the planet's surface. The rippling of the Earth's crust doesn't spread out too far on a global scale. The tsunami, on the other hand, emanates from a point source and radiates outward in numerous directions. It is estimated that the largest part of the tsunami produced a 98 foot wave. So while the earthquake would have only really affected western Indonesia, the tsunami is able to spread out much more readily and affected tens of thousands of miles of coastline across numerous countries.
Finally, there is almost nothing in nature more powerful than a large mass of moving water. There is nothing that can withstand a wave of that magnitude. No man made structure or seawall can possible survive it. Since coastal areas tend to be highly populated across the world, this is a recipe for a large scale disaster with many, many deaths.