Thousands of earthquakes shook the ground yesterday but you probably didn't feel a single one. The massive quake that killed over a thousand people in Taiwan was but one of these, becoming noteworthy only because of its strength and because it took place near an urban centre where deaths would be likely.
Earthquakes aren't becoming more commonplace, rather, the Earth's rising population makes it more likely that a tremor will affect a built-up area, explained Prof Brian Jacob, head of the department of geophysics at the Dublin Institute for Advanced Studies.
Seismologists who study earthquakes and the effects they cause might record up to 3,000 magnitude-five quakes each year, he said. "Most of these are in places where nobody pays any attention." They occur up to 700 km underground or in remote areas where their passing is hardly noted.
Several magnitude-eight quakes can be expected in any given year and between 10 and 20 magnitude seven, about as strong as the quake in Turkey. And for every strong quake there usually follows a swarm of smaller aftershocks, minor tremors that can persist for many weeks and months afterwards. The minor quake that shook Wales in 1984 has been followed by 1,000 small aftershocks since then, Prof Jacob said.
Prof Jacob was able to read data on the Taiwanese quake captured by a number of seismic recording stations based here and run by the institute. The original shock was close to a magnitude eight, he said, larger than the recent quakes in either Turkey or Greece. This was followed within 30 minutes by no less than four magnitude-six aftershocks. "As aftershocks go it was quite brutal," he said. "Quite often the aftershocks do considerable damage to buildings already in a parlous state."
About 80 per cent of the world's quakes occur in a wide band known as the "Ring of Fire" that traces the perimeter of the Pacific Ocean. The ring marks out the violent point of contact between drifting chunks of the Earth's crust. Other active earthquake areas exist in Italy and between Greece and Turkey. The crust is not a uniform structure. It is broken up into massive slabs known as plates. Some butt up against one another, others slide past one another and in some cases, as in the Pacific Ocean near Taiwan, one is forced down under the other.
Evidence of the tremendous forces involved can be seen in mountain ranges such as the Himalayas, still growing as the plate which carries India on its back crunches northwards up into the south-eastern flank of the Eurasian plate. Italy's quakes are caused at the point where the African plate meets the plate carrying western Europe.
Less obvious is the pincer movement being conducted by the Eurasian and North American plates on the enormous Pacific plate. The Eurasian plate travels eastward between seven and eight centimetres a year, sliding up over the edge of the Pacific, a movement matched by the North American plate which is drifting westward.
In effect the Pacific Ocean is shrinking as these plates drift towards one another. The opposite effect is taking place in the Atlantic where the eastern US and Europe are drifting apart.
Where plates make contact earthquakes are the result. Friction prevents the plates from moving smoothly past one another so the pressure builds until it becomes too great and the plates slip, creating shock waves that move away like ripples on a pond.
These waves emerge at the surface, shaking the ground and producing side-to-side or up-and-down movement. Buildings are shaken apart and collapse. Rigid facades and concrete roadways crumble as the severity of movement exceeds their very limited ability to flex.