Correct Option
The correct option isA rapid withdrawal of water from the coastal region.
Explanation
Tsunamis are high-energy seismic sea waves caused by the displacement of a large volume of water, typically due to undersea earthquakes, volcanic eruptions, or landslides. Unlike wind-driven waves, tsunamis involve the movement of the entire water column from the surface to the ocean floor.
Detailed Analysis
- Mechanism of Drawback: A tsunami wave train consists of a series of crests (high water) and troughs (low water). In many instances, the trough of the wave arrives at the coast before the first crest.
- Rapid Withdrawal (Correct Indication): When the trough arrives first, it causes the sea level to drop significantly. This results in a rapid and abnormal withdrawal of water from the shore, often exposing the seabed, coral reefs, and marine life. This phenomenon is known as "drawback" and serves as the first natural warning sign of an impending tsunami.
- Subsequent Surge: The withdrawal is followed minutes later by the arrival of the wave crest, which manifests as a massive surge of water or a sudden rise in sea level . Therefore, the rise in sea level is the destructive event that follows the initial indication.
- Meteorological Factors: Strong winds and dense fog are atmospheric phenomena. Tsunamis are geological and hydrodynamic events; they are not indicated by changes in local weather conditions like wind or fog.
Key Takeaway
The rapid withdrawal of water (drawback) occurs when the trough of the tsunami wave reaches the coast before the crest, serving as the primary natural warning sign before the destructive water surge arrives.