The correct answer is option 1: directly proportional to the depth of the sea.

When a tsunami is generated, it is usually caused by an underwater earthquake or other geological event. These events cause a displacement of water, creating a large wave that travels across the ocean. The speed of these waves is determined by the depth of the sea.

As the depth of the sea increases, the speed of the tsunami waves also increases. This is because deeper water allows the waves to travel more freely and with less resistance. Therefore, the speed of the waves is directly proportional to the depth of the sea.

The devastating impact of a tsunami occurs when these fast-moving waves approach the coast. The high speed, combined with the large volume of water, can cause significant destruction. It is important to note that the distance between the coast and the epicenter of the earthquake does not directly affect the speed of the waves. Instead, it is the depth of the sea that plays a crucial role in determining the speed and intensity of a tsunami.

In summary, the speed of tsunami waves is directly proportional to the depth of the sea. This understanding helps explain why tsunamis can cause such devastation near the coast.