Tsunami can be caused different things but the ones such as the Japanese and Sumatran tsunami were caused by the sudden uplifting of a section of the ocean bed which in turn were caused by sudden movements of tectonic plates.
Opposing plates move in opposite directions, effectively colliding head on. Over time, one plate is deformed and is forced beneath the upper plate under immense stresses and strains. Ultimately the forces become so great that the deformed edge of the plate springs back up. In doing so it rips through the ocean floor at several hundred miles per hour and rapidly rises upward, often by several metres.
It’s this sudden rising upward that lifts the water in the overlying ocean and so the makings of a tsunami are formed. The wave travels outward from the epicentre at speeds of up to 500mph. When such an event happens close to shore there is very little time for people to evacuate and this is one of the reasons why the human toll in the Indian Ocean was so high, and why, as appears likely, the loss of life in Japan will also be very high.
To understand why this happens you can think of the Earth as a molten mass with a thin crust covering it, this crust is broken into what are termed ‘plates’, some are very large and others are comparatively small. These plates push against, and pull away from, each other. It’s along these cracks, or faults, that most earthquakes and volcanoes occur.
I’m not sure what the link between global warming and earthquakes is supposed to be. I know some people have speculated that the additional burden of meltwater from the polar regions is increasing pressure on tectonic plates and that this is leading to more earthquakes, but such a notion just doesn’t add up.
The mass of water in the Earth’s oceans is approx 1,330,000,000,000,000,000 tons, the amount being added from melting of the ice-caps is about 550,000,000,000 tons each year. That’s an annual increase of just one 2½ millionth by mass / volume. This amount is infinitely smaller than the burden that is imposed by normal tidal movements linked to the Sun, the Moon and weather.
Furthermore, these major earthquakes we’ve witnessed in recent years have largely occurred along subduction zones – areas in which tectonic plates are moving horizontally into, over and under one another. Any loading from introduced meltwater would cause a vertical pressure component that would actually alleviate some of the pressure.
When we look back to the last occasion when there was massive increases in the amount of water in the seas and oceans – namely the end of the last glacial maximum (ice age), we find that there was some 45 quintillion tons of water added to the seas and oceans over a period of about 10,000 years. This equates to a sea-level rise about 6 times as fast as is happening now. Despite this much greater volume of water, there is no geological evidence of tectonic activity outwith the normal parameters.
An argument could be made that an increased mass of sea-water acted as the trigger for some kind of tectonic activity but this would be nothing more than the straw that broke the camels back. Such an event would have happened anyway, the additional mass of water may have caused it to occur a few minutes earlier (or later – it works both ways), but it would have happened anyway.
The bottom line is that creating a link between increased earthquakes and global warming makes for good headlines but it’s bad science.
Further, the idea that drilling oil causes earthquakes, volcanoes, tsunami is somewhat tenuous to say the least. Not least because oil reserves are generally in different places and at different depths to these seismic events. The fracturing or dissolution of limestone or sandstone in order to extract the oil does cause some instability in the rock but the injection of high pressure material into the void compensates for this.