Seismometer (PLUS only)

Public summary: 

Find out about the structure of the earth, and how we have found it out despite only digging down 0.2% of its depth.

A seismometer and lots of explanation
Useful information
Kit List: 

In this box there should be:
• Lego Seismometer (including magnets, weight, coil, spring and lego structure)
• aux cord
• micro-usb to usb cord
• seispy converter
• 4/5 tennis balls (under development)
• one magnet in labelled sealed box (under development)

Packing Away: 

Be careful to make sure that any exposed circuitry is protected. Unplug the cords from the device so that it would be less easy for connection ports to be ripped out.

Use foam casing to store the electrical component of the seismometer. Do not remove from this casing. Be careful when packing away the lego not to bend the metal strip..

Frequency of use: 

The following are some suggested talking points for the seismometer experiment. You do not need to do all of them, but there should be enough here for an interested person. A lot of it is not directly related to the seismometer, but the larger field of seismology. Drawing diagrams and asking questions is a good way to keep it interactive. Keep the seismometer set up and refer to and use it throughout the experiment.

Start off by using the tennis balls to discuss how we might work out what is inside something without just opening it. Get participants to experiment with the tennis balls; e.g. shake, compare weights, test with the magnet in the box, to think about what might be inside them. Then start discussions about what kind of techniques we can use to see inside the earth.

What is the earth’s structure? (figures 1 and 2)

There are two dominant classification schemes:

• Crust, mantle, outer core, inner core are the popular terms. The boundaries between these are defined by chemical compositional changes, i.e. the rock type changes from one side to the other. The crust/mantle boundary is called the 'moho boundary'.

• You can also describe the crust and mantle by the lithosphere (which covers the crust and upper part of the mantle) and the asthenosphere, which is everything deeper up to the outer core (mesosphere is sometimes also included). The lithosphere and asthenosphere are different in elasticity. Across the boundary, the temperature increases, making rock more ductile and less rigid (A famous example of the importance of this transition is the failure of the titanic: In the icy temperatures, the metal hull became brittle and was subject to a brittle failure).

The earth structure classifications are therefore chemical and physical respectively.

How do we know what the earth’s structure is?

How far do you think we’ve drilled? Earth’s radius is 6371km. We certainly haven’t drilled down very far: Kola Superdeep Borehole in Russia continent went 12.3km. The earth’s crust is ~8km deep in the ocean, and ~27km deep in the continents, so we haven’t even reached the bottom.
Why? Calculate what the pressure might be at 12.3km depth (continental crust density = 2.83* that of water, ocean crust even denser ~3.3*water). This is a hydrostatic pressure, so there is the same pressure pushing upwards. Quite hard to push down.

We don’t have samples from very deep: ~400km. The peridotite in the 'Rocks and Fossils' experiment will be from a similar depth. This is only partly into the mantle! How do we have this if we haven’t drilled down to it? 'Rocks and Fossils' has a rock cycle explanation.

The solution: Earthquakes!!! They naturally probe the earth for us.
What is an earthquake? Earthquakes occur by ‘elastic rebound’. Over time, tectonic processes (possible digression to plate tectonics here, much higher pressures at convergent plate boundaries) build energy. Once this overcomes friction, rupture occurs, and seismic waves radiate off. Deepest earthquakes are around 300km and occur almost exclusively at convergent plate boundaries. Why are none found deeper? Ans: refer back to discussion of ductile vs brittle, hot rock flows better and stresses are dissipated.

How do waves move through the earth? (figures 3 and 4)

As we get deeper, waves get quicker (wave speed is a function of density and elastic moduli). Now for a diffraction explanation, maybe huygens wave principle? As they get deeper, waves get slower, and they curve! So waves emitted initially away from the surface come back.

Arrival Times (figures 5 and 6)

We use different arrival times to figure out where an earthquake originated, called the hypocentre (you may have heard of the epicentre: that is the point on the earth’s surface above the earthquake’s origin). Can you figure out roughly how fast they are going on the seismogram above (assuming direct travel...)?
Hawaii → Albuquerque ~5000km. Therefore wave speed is O(10km/s). This is not too far wrong (see figure 6 below). Why is this not right? We haven’t considered the different paths of the waves, varying speeds etc.

There are different types of wave, P is longitudinal, S is transverse. P is faster. Finally, there are surface waves which appear at the boundaries, such as earth’s surface. Why can’t we separate the different wave types on our seismogram? Use the speed result above.

Evidence for Earth’s Structure (figure 7)

Now for structure. Waves are reflected and transmitted at surfaces (maybe relate to 'Waves at Boundaries' experiment if that’s out). The reflected bits return to earth’s surface, so we can figure out where the major changes in earth’s structure are. These are due to basically anywhere the structure dramatically changes. E.g. Crust/Mantle Lithosphere/asthenosphere, core/mantle, core/inner core. See above diagram for shadow zones.

Iron inner core:
At the temperature and pressure we predict for the earth’s core, an iron composition would replicate the wave speeds we observe.
Iron is abundant in the solar system due to its high binding energy
We have samples of iron meteorites from proto-planets which we think have a similar specified structure to the earth.
We need a magnetic element to generate the earth’s magnetic field

Liquid Outer Core:
No s-waves pass through this region. Transverse waves can’t pass through fluids. (NB: the mantle is not therefore molten as lots of people seem to think. It does flow, but on much longer timescales, so to an earthquake, it appears solid)
The earth’s magnetic field needs a flowing magnetic material. A-level physicists will probably have heard of faraday’s law.

Solid Inner Core:
The sharp wave speed change here us not likely possible in a purely fluid region.
There is a small amount of evidence for inner core s-waves, generated at the inner core/outer core boundary. They have small amplitudes, so we’re not sure we trust the observations yet.

Transition Zones:
Generally due to solid-state phase transitions, and therefore their height is temperature sensitive [related by the Clausius-Claypeyron relation].
The low velocity zone between lithosphere and asthenosphere may be due to some degree of melt.

It would be good to see whether geophysics is possible to explain without all the jargon.
Possible ideas:
P waves --> ~sound or pressure waves
S waves --> transverse waves
Astenosphere --> weak solid layer
Lithosphere --> strong but brittle solid layer

Risk Assessment
Date risk assesment last checked: 
Wed, 05/02/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Thu, 06/02/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Lego Seismometer and Laptop

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Jumping Whilst jumping a child may injure themselves. 3 2 6 Do not ask them to jump if the floor seems dangerous or they do not seem steady on their feet.
In case of injury call a first aider.
1 2 2
Lego Parts/ tennis ball fillings Small parts can be swallowed 2 3 6 Keep the seismometer in one piece. If small children are watching, no touching. Keep careful watch on tennis ball fillings and don't allow very small children to open the tennis balls. 1 3 3
Sharp seismometer Parts Some parts are sharp or could trap fingers 2 2 4 If small children are watching, no touching. 1 2 2
Filled tennis balls Tennis balls could be thrown which may cause minor injury 2 2 4 Be aware of what children are doing with tennis balls and restrict thier use if necessary 1 2 2
Magnet to test tennis ball innards Small, choking hazard. Risk of trapping fingers. Could affect pacemakers etc. 2 4 8 Keep magnet in labelled, sealed box at all times. Do not allow magnet to leave experiment area. Warning sign regarding presence of magnet. 1 4 4
This experiment contains mains electrical parts, see separate risk assessment.