In reply to Trangia:
It's been a long time but...
Yes, the Himalaya are one of the most active seismic areas in the world but there are many others. Essentially earthquakes will normally be caused by stresses in the crust which typically arise from plate tectonic movements. So you will see a lot of eartquakes at plate boundaries.
For example, the Pacific ring of fire is essentially a zone of earthquakes and volcanoes ringing the Pacific where the Pacific oceanic plate is subducting under or generally moving against surrounding plates.
As you probably know, the Himalaya were created when the Indian continental plate drove north into the Eurasian plate. It's still moving northwards, albeit slowly, but may create another 1,000km+ of compression. Therefore the mountains are still being created (even whilst erosion is happening very actively) and the crust incredibly thickened as the Indian plate drives under the Eurasian plate. This creates huge stresses and creates fault lines which occasionally release and trigger earthquakes. Earthquakes are of course just one trigger for avalanches, and probably quite a rare trigger in reality. It's interesting that we have the two effects happening at the same time i.e. plate movements forcing the mountains up even higher whilst very active erosional forces (glaciers, massive rivers etc) is countering that to some extent.
In geological terms, the Himalaya are quite new. The ocean which used to separate the Indian plate and the Eurasian plate only disappeared when the two continents collided some 50 million years or so ago.
Another example is the San Andreas fault which is caused by the Pacific ocean plate moving roughly northwards whilst the N American continental plate moves roughly south. What you get there isn't compression (and huge mountains) so much as a sideways tearing effect. The "big one" is supposedly overdue and may well level large parts of California when it does. Unfortunately, in terms of human casualties, that may dwarf even the disaster in Nepal when it does eventually happen (and it will).
Sometimes earthquakes are very closely associated with volcanic activity either because of the magma movements or because of associated plate movements (which may be causing the volcanic activity).
It's sobering to think that the Scottish Highlands were caused by a similar event (termed the Caledonian orogeny) which happened around 400-500 million years ago when another ocean (Iapetus) closed causing continents to collide across northern Europe and into what is now N America. At the time, all this was located in the southern hemisphere. Anyway, what are now the Scottish Highlands, extending either waty into N America and N Europe (which has all moved again since) may once have rivalled the Himalaya in scale. Subsequent plate movements and erosion have had their impact over the following 400 million years or so and the Scottish mountains aren't quite what they were....
