From pp. 39-40 of David Keller’s “Great Disasters: The Most Shocking Moments in History“:
In East Pakistan, in 1970, [a tsunami] killed almost half a million people. East Pakistan (it’s now called Bangladesh) had been hit with gigantic waves before, but none with the force and destruction of this one.
It all started with a cyclone roaring through the Bay of Bengal in the Indian Ocean. Winds blew at over 100 miles per hour. The storm was so fierce that it churned up a huge surge of water, aimed directly at East Pakistan. For hours the people had watched as the winds whipped and the rain fell in sheets. Then the wave struck. Twenty feet high and miles wide, it swept over the Pakistani coast and flooded over 3,000 square miles of land.
Thousands of people were crused or pulled out to sea to drown. On the largest island off the coast, 200,000 people were killed. On many smaller islands, no one was left alive at all.
One of the reasons that there were so many deaths is that Pakistan had so many people: nearly 1,000 for every square mile of land. Most of the country is on land not much higher than the ocean itself, so there was no high ground to run to. Another reason is that the peopl were unprepared for the storm. There had been another storm just a few weeks before, and many people fled the coast. But the storm died out and nothing happened. This time many people thought the same thing would happen again and stayed where they were. By the time the wave hit, it was too late to run.