Rising seabed will cause tsunamis

A tsunami is likely to occur if an earthquake causes the sea floor to move vertically.

View of a house on Pagan Utara Island, Mentawai Islands, Indonesia after the tsunami hit on October 26.(Photo: AFP).

The earthquake on October 25 caused tsunamis in Indonesia that left at least 311 people dead, hundreds missing and about 20,000 people homeless. With a magnitude of 7.7 on the Richter scale, this earthquake is in the group of earthquakes capable of creating tsunamis.

Don Blakeman , a geophysicist at the US National Earthquake Information Center, confirmed that earthquakes with magnitudes less than 7.5 on the Richter scale often do not cause tsunamis. However, he admits that sometimes a 6-magnitude seismic can cause tsunamis in a small area, with relatively low devastating power.

Experts say tsunamis only occur after earthquakes thanks to a combination of the following three factors: seismic intensity, soil displacement direction and seabed topography .

If the seismic earth makes the land along the fault lines (milling line) at the bottom of the sea move vertically - up or down - the tsunami will likely appear. Due to the vertical movement of the seabed, all the water above will also rise or fall, creating waves of high energy. The power of the waves will increase to a terrible level if the epicenter of the earthquake lies on the ocean floor.

In the case of an earthquake that only causes the ocean floor to move horizontally, the tsunami rarely occurs because the water only rises or falls relatively lightly.

The stronger the sea floor moves vertically, the higher the height of the tsunami. Any change in the topography of the sea floor can increase or decrease the height of the tsunami as it moves. According to Blakeman, when spreading on the surface of the ocean, tsunamis often reach close speeds by jets, ie from 800 to 1,000 km / h. But when approaching the mainland the tsunami speed decreases.

" The sudden and sharp drop of water from the shore is a sign that the tsunami is coming ," Blakeman said.

There are earthquakes that also create double tsunamis. Indonesia's Meteorological and Geophysical Agency said in April a 7.7-magnitude earthquake in the country triggered two consecutive tsunamis.

Tsunami takes power from the movement of the ocean floor. So all weather conditions do not affect its power.