Compiled by Murray McGowan, Guralp Systems Ltd.

Three sets of recordings are available at this time for analysis:

• Launch venue Science Museum, London
• Clatford Primary school, Andover, Hampshire
• Wolverton Seismic Vault, Hampshire

Seismic equipment needs to record time in a universal manner, that is consistent all round the world, so that the recording from an earthquake in Australia (for example) can be matched to a recording from the UK.

The time-reference for all seismic equipment around the world is based on the global standard of Universal Time (UT), which is the same as Greenwich Mean Time (GMT). So when the giant jump at 11am British Summer Time (BST), the GMT was actually 10am (since we are +1 hour at the moment for British Summer Time). You can see this in the times shown in the plots.

The recordings shown here are all from "Tri-axial" seismometers. That is, there are three separate sensors within the seismometer (tri-), each one measuring the movement on one of the three axis of movement: up/down, forward/back, and left/right. By convention, the seismometer is pointed north, so these axes become Vertical, North-South, and East-West, shown top-to-bottom in Green, Red and Blue respectively.

The recordings for each of these locations is shown below. For easy comparison, all seismographs are set to the same scale (the 1:1024 in the top left corner):


Science Museum, London


Clatford Primary School, Hampshire


Wolverton Seismic Vault, Hampshire

It is clear to see that the first two locations clearly picked up the people jumping locally. Looking at the recording from the seismic vault, the first thing that is evident is that the 'background' noise is much, much lower. This seismometer is located away from buildings, roads, people, machinery, and other man-made sources of noise. So it is in a much better location to pick up any faint traces of seismic activity. We regularly record earthquakes from all round the world in this vault. Constantly updated seismograph readings from this vault are available on our web site.

What can we say from all this? The seismometers in the Science Museum and Clatford recorded what are known as 'micro-tremors' that were caused by the people at those locations jumping. These are minature earthquakes that happen around us all the time, like a lorry driving over a pot-hole, or a wrecking-ball smashing down a wall.
But was there any 'national' earthquake? For that, we need to look at the seismometer that was far away from any of these micro-tremors, so that the only measurement it would make is of a signal that would be big enough to be seen all over the country. Looking at the Wolverton results, we see no evidence of the jump. In other words, the combined effect of all the people jumping at the same time was not big enough for the signal to travel very far.

Why Not?

Ask your teacher...?

There are two basic reasons:

1. The shock waves take time to travel through the earth. It would take 2-3 minutes for the vibrations from the jump in Glasgow to reach London (even if they were big enough). By that time, the vibrations in London would have died down. So there is no way for the vibrations to add together - something that would be necessary to cause a national earthquake.

2. The size of the vibrations drops as they travel away from the centre (like ripples on a pond). This is called the "inverse square law", which means that the size of the signal is inversely proportional to the distance from the source. Very quickly the signal fades away and is less than all the other 'noise' caused by people (cars, factories, machinery, etc). Even the wind in the trees or the waves crashing on the shore cause the ground to shake.
   
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