Causes: Meteorologically, the island of Java in Indonesia is famous for being the most thundery location on the planet, as reckoned by the average number of days a year on which thunderstorms occur. At any one spot on that island you are likely to hear thunder on 223 days out of the 365.
But as we have been reminded in recent times, both by the threatening rumblings of Mount Merapi, and now by the earthquake which has devastated Yogyakarta, Indonesia is also one of the most seismically active regions in the world.
Earthquakes and volcanoes are related. The most vulnerable spots for both are located along the joints between the vast tectonic plates that make up the Earth's crust, large expanses of solid rock like the fragments of a cracked eggshell floating on the molten "white" beneath. The edges of these plates, which we know as "faults", define the major earthquake zones. In some regions, the plates slide past each other smoothly and without consequence; in others they do so in a kind of "stick-slip" fashion, adjacent segments perhaps "sticking" for several decades before slipping suddenly by several metres to produce a tremor; and in yet others, periodic earthquakes are a consequence of one plate being forced by the pressures of collision to slide down underneath an adjacent counterpart.
The entire west coast of the Americas is a high-risk zone. So too is the west coast of the Pacific Ocean, with the relevant fault running close to the archipelago of Japan. And a fault line in the Mediterranean continues eastwards into Asia and then south to form the great, curved boundary, 100km or so southwest of Java and Sumatra, between the huge Indo-Australian and Eurasian plates. The tremor that caused the tsunami of December 2004 occurred along this fault, near the northern tip of Sumatra, and so too, did Java's earthquake around dawn on Saturday.
Although we understand the mechanisms in which they have their origins, we are a long way from being able to predict earthquakes in any useful way. Indeed, seismologists have, now and then, been likened to economists; critics have unkindly pointed out that both professions are good at explaining how and why a particular event has taken place - but always after it has happened.
Many scientists distinguish nowadays between earthquake forecasting and earthquake prediction. The former is based on the accurate measurement of small movements of the ground in the vicinity of faults, either by terrestrial instruments or by satellites, and on analysis of historical records of seismic occurrences.
The results are estimates of the long-term probability of tremors of specific magnitudes at certain places, and are widely regarded as useful and acceptably reliable. Earthquake prediction, on the other hand, is concerned with anticipating a major tremor at a certain place anything from hours to months ahead, and despite the optimism that existed a quarter of a century ago, scientists are a long way from being able to do so in any useful way.
Some have placed their faith in what they call "empirical precursors", observed phenomena that may precede a major tremor for reasons not fully understood. The Chinese have documented the strange behaviour of some animals prior to major earthquakes; water levels in wells are often seen to change as fractures open up below the ground; radon gas is commonly expelled before a tremor; and magnetic and electrical anomalies are common.
Sometimes minor tremors indicate an imminent earthquake; and paradoxically, another symptom of the worst to come is silence, a "seismic quiescence", where seismic activity in the region remains at a low level for several years.
But none of these criteria is unambiguous. The best hope of success is to measure the build-up of stress adjacent to large faults, which allows scientists to predict that an earthquake may happen within a few decades. But prediction in any real sense - telling in advance the place, the time and magnitude of the event - has yet to be achieved.
Brendan McWilliams is a meteorologist and writes the Weather Eye column in The Irish Times.