How strong does an earthquake have to be to cause a tsunami

An earthquake with the magnitude of more than 7.5 on the Richter scale can cause to a destructive Tsunami.

Explanation:

According to International Tsunami Information center, the cause of the most destructive tsunami is shallow and large earthquake with the epicenter or fault line on the ocean floor or near the ocean floor.When the tectonic plates (are pieces of earth's crust and uppermost mantle) are collided with each other then the earthquake occurs. This results in the displacement of large area of ocean floor from few km to 1000 km or more. This displacement of ocean floor in the formation of tsunami waves.Like in the 1960, the tsunami in Chile was resulted due to earthquake with the magnitude of 9.5 on Richter scale with rupture area of over 1000 km. This tsunami not only cause destruction in Chile but also affected the other countries like Japan.