Build a Seismograph That Draws the Shaking

The clever thing about a seismograph is not that it moves. It is that one part of it refuses to. Hang a heavy weight on strings, attach a pen to it, and shake the frame: the frame moves, the paper moves, and the pen stays more or less where it was. The line it draws is the difference between a thing that moved and a thing that didn’t. This DIY seismograph experiment takes about forty minutes to build, and the two details that make it work rather than scribble are the mass of the weight and how lightly the pen touches the paper.

Key Takeaways

• You’ll hang a heavy cup with a felt pen taped to it inside a cardboard frame, pull a paper strip through underneath, and record real shaking.
• It teaches inertia as a measuring principle, and why magnitude scales are logarithmic.
• Best for ages 9 to 15.
• The usual failure is a pen pressing too hard, which drags the pen along with the paper and erases the whole effect.
• Each whole step up in magnitude is ten times the wave amplitude and about thirty-two times the energy.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Sturdy cardboard box, open on one side	1, roughly 30 by 30 by 40 cm	Delivery box	A stool with a crossbar, or two stacked crates
Paper or plastic cup	1	Kitchen	A small yoghurt pot
Coins, pebbles or washers	300–500 g	Pocket and garden	Dry rice is too light. You need real mass
Fine felt-tip pen	1	Stationery drawer	A soft pencil sharpened flat. Not a ballpoint, which needs pressure
String	1 m	—	Strong thread, doubled
Wooden skewer or pencil	1, to bridge the box	—	A chopstick
Paper strip	1 m long, 8 cm wide	Cut from a roll of till paper, lining paper or taped A4 sheets	The back of a wallpaper offcut
Tape and scissors	—	—	An adult does any box-cutting

Time: prep 40 min, experiment 20 min, cleanup 5 min Cost: everything is probably in your house Mess level: low

Do the Experiment

Step 1 — Make the frame and the paper slot

Stand the box on its side so one large face is open toward you. Cut a horizontal slot about 10 cm wide in the bottom panel, near the back, and another at the front, so a paper strip can be pulled through from back to front across the floor of the box. An adult does the cutting.

Step 2 — Load the cup and hang it on two strings, not one

Fill the cup with coins or pebbles until it weighs 300 to 500 grams. Punch two holes on opposite sides of the rim, thread a single length of string through both, and hang the two ends from the skewer laid across the top of the box so the strings form a narrow V. This is the detail most instructions skip. A single string lets the cup swing in any direction and the trace becomes a scribble; a V constrains it to swing along one axis, which is what makes a readable record.

Step 3 — Attach the pen so it barely touches

Tape the felt pen under the base of the cup, pointing straight down, with the tip just kissing the paper. Hang the cup so the tip touches with almost no pressure. Too much pressure and friction drags the pen along with the paper, which is the single most common reason this build fails.

Step 4 — Take the prediction

Ask: “When I shake the box, will the pen move or will the paper move?” Most children say the pen. Write that down.

Step 5 — Pull the paper at a steady speed and shake

One person pulls the paper slowly and evenly, about 2 cm per second, while another taps or rocks the box side to side. Vary it: light taps, then heavier rocking, then a steady rhythm.

What you should see: a wobbly line. While nothing shakes, the pen draws a straight line down the middle of the strip, because the paper is moving under a stationary pen. When you shake the box, the line develops side-to-side wiggles whose width tracks how hard you shook. Light taps give a few millimetres; a good rock gives a centimetre or more.

Step 6 — Calibrate it and read it back

Mark the paper every five seconds as you pull, then measure the maximum wiggle width for a light shake, a medium one and a hard one.

What you should see: amplitude that rises with the strength of the shake, and a record you can interpret afterwards without having seen the shaking. Hand the strip to someone who was out of the room and ask them to tell you how many times you shook it and which was hardest. If they can, you have built an instrument.

What’s Actually Going On

A seismograph is a recording system plus a ground-motion sensor called a seismometer, and as the IRIS consortium’s fact sheet puts it, seismometers operate on the principle of inertia. Inertia is the tendency of a mass to keep doing what it was already doing. Shake the frame quickly and the suspended mass, which is only weakly connected to the frame through a flexible string, has no time to follow. So the frame moves and the mass does not, and the pen records the relative motion between them.

That is why the mass matters. A light cup is pushed around by the string, by air currents and by the pen’s own friction, and it follows the frame instead of ignoring it. A heavy cup resists all of those. You are not measuring the weight of the cup; you are buying the pen enough stubbornness to stay put.

The pendulum has a natural period, and this is the real limitation of your instrument. For a simple pendulum, period equals 2π times the square root of length divided by gravity, so a 30 cm suspension swings with a period of about 1.1 seconds. Shaking much faster than that is recorded well, because the mass cannot keep up and genuinely stays still. Shaking much slower than that and the mass swings along with the frame, recording almost nothing. Real research instruments go to extraordinary lengths to achieve very long natural periods, which is how they capture the slow, long-wavelength motions of a distant large earthquake. By comparison, IRIS notes that modern research seismometers can detect ground movements as small as one ten-millionth of a centimetre at quiet sites.

Now the magnitude question, where the popular version is wrong in two ways. The USGS explains that magnitude scales are logarithmic: each whole-number increase represents a tenfold increase in measured amplitude on a seismogram and about 32 times more energy released. A magnitude 7 is not slightly worse than a magnitude 6. It shakes the ground ten times as far and releases roughly thirty-two times the energy.

The first error in the popular account is the phrase “the Richter scale”. Charles Richter introduced magnitude at Caltech in 1935, and his definition only held for Californian earthquakes within 600 km of one particular instrument, the Wood-Anderson torsion seismograph. For very large earthquakes, amplitude-based scales saturate, so seismologists now use moment magnitude, which is based on the physical size of the fault rupture rather than the height of a squiggle. The second error is the idea that the scale “goes up to 10”. It has no upper bound. The largest earthquake instrumentally recorded is the magnitude 9.5 Valdivia event in Chile on 22 May 1960, which heads the USGS list of the twenty largest earthquakes since 1900. Nothing in the arithmetic stops a bigger number; the limit comes from how long a fault can be.

How to Explain It to Your Kid

Ages 5–8: The heavy part doesn’t want to move

Put a coin on a card balanced on a cup and flick the card out: the coin stays and drops in. Same idea. Tell them the weight in the cup is lazy and wants to stay where it is, so when the box jiggles, the box moves and the pen stays, and that is how the line gets its wiggles. Then let them do the shaking, which is the job every child wants.

Ages 9–12: Pull at a steady speed and you have invented time

The great trick in the design is the moving paper. Pulling it at a constant speed turns distance along the strip into time, which is why the record is readable at all. Have them mark the paper every five seconds and then work out, from a finished strip, exactly when each shake happened and how long it lasted. Then introduce the logarithm: a magnitude 6 moves the needle ten times as far as a magnitude 5.

Ages 13+: Natural period, and why your instrument has a blind spot

Give them T = 2π√(L/g) and have them compute the period for their suspension, then test it by timing ten free swings. Now get them to shake the box much faster and much slower than that period and compare the traces. They should find a clear frequency response: good at high frequency, nearly deaf at low. Then the design question: what would you change to make it sensitive to slow motion, and why is that hard?

The question to ask: “If two earthquakes happen and the second one draws a line four times wider on our paper, how much bigger is its magnitude? And is four times wider a big difference or a small one?”

Why Teaching This Early Pays Off

Logarithmic scales are everywhere and almost never taught as a concept before they are needed. Magnitude, decibels, pH, stellar brightness and the loudness controls on every device a child owns are all logarithmic, and a child who has measured amplitude on a paper strip has something physical to attach to the idea that “one step up is ten times”.

The second payoff is the idea of a reference frame. A seismograph works because it holds one thing still and measures everything else against it. That same move underlies accelerometers in phones, gyroscopes in drones, inertial navigation in aircraft and the stabilisation in a camera gimbal. It is one of the genuinely reusable ideas in engineering, and a cup of coins on two strings makes it visible.

Alfieri and colleagues found that guided discovery improves learning while unguided exploration does not (Alfieri et al., 2011), which matters here: the build is fiddly enough that a child left alone will produce a scribble and conclude that the idea does not work. Stay with them for Step 3. And set the expectation honestly: a cardboard seismograph will not detect a real distant earthquake, and no study claims a build like this changes school outcomes.

Before You Start: What Parents Should Know

Best age	9–15. Below 9 the construction is frustrating, though shaking the finished one is not
Supervision	Adult does all cutting. Adult also decides what gets shaken, and where
Real hazards	Craft-knife cuts when slotting the box. Coins and small washers are a choking hazard for under-3s, so count them out and back in. A hot-glue gun, if used, causes real burns. A heavy cup falling off its strings can hurt a foot, so test the knots with a tug before loading
Don’t	Do not shake a table with anything breakable, electrical or hot on it, and do not shake a shelf. Do not fill the cup with water instead of coins: it sloshes, which adds its own oscillation and ruins the trace

When It Doesn’t Work

• The pen draws nothing, or tears the paper. Pressure. The tip should barely touch. Raise the cup a millimetre or two and re-test with a gentle tap.
• The trace is a scribble with loops. The cup is swinging in two directions. Switch to the two-string V suspension in Step 2, which confines it to one axis.
• The pen moves with the box, so the line stays straight. The mass is too light or the string is too stiff. Add coins until the cup is at least 300 g, and use soft string rather than wire.
• The line drifts steadily to one side. The paper is not being pulled straight, or the cup is hanging off-centre. Tape a cardboard guide channel for the paper and re-centre the suspension.
• Everything works but the record is unreadable. You pulled the paper too slowly. Faster paper spreads the wiggles out in time. Aim for about 2 cm per second and keep it steady.

Take It Further With the DIY Seismograph Experiment

1. Measure the frequency response. Shake the box at a counted rhythm: one shake per two seconds, one per second, two per second, four per second. Measure the trace amplitude at each. Plot amplitude against shaking rate. You have measured your instrument’s sensitivity curve, which is a real piece of engineering practice.
2. Change the pendulum length. Run the same shaking with a 15 cm and a 45 cm suspension. Longer means a slower natural period and better response to slow shaking. Predict first, then measure.
3. Compare building responses. Build two paper towers of different heights on the box floor, with a coin taped on top of each, and shake at different rates. Different heights fall at different frequencies, which is the heart of why earthquake engineering in Mexico City is such a hard problem. Pair this with the tablecloth trick, which is the same inertia in one second flat.

What to Watch For Over the Next 3 Months

• Week 4: they describe a news magnitude correctly, saying a 6.8 is much worse than a 5.8 rather than slightly worse.
• Month 2: they use the word inertia about something unrelated, usually in a car or on a bus.
• Month 3: they ask how a phone knows it has been turned over. That question leads to accelerometers, which are the same principle shrunk to a few micrometres of silicon.

Frequently Asked Questions

Will this detect a real earthquake?

Almost certainly not. Your instrument has no amplification, a pen with real friction, and a natural period around one second, so it responds only to shaking strong enough to feel anyway. Real seismometers are electronic and amplify ground motion enormously. What you have built is a correct demonstration of the principle, not a sensitive instrument.

Why two strings instead of one?

A single string allows the cup to swing in any horizontal direction, and the pen then draws loops rather than a line. Two strings hung as a narrow V restrict swinging to a single axis. Real horizontal seismometers also constrain their mass to one direction and use three separate sensors for three axes.

Is the Richter scale still used?

Not for large earthquakes. Richter’s 1935 definition applied to a particular instrument and a particular region, and amplitude-based scales saturate for very big events. Agencies now report moment magnitude, based on the physical size of the fault rupture. News reports often still say “Richter” out of habit.

Does the magnitude scale stop at 10?

No. There is no upper limit in the mathematics. The practical limit is the length and area of fault that can rupture at once. The largest instrumentally recorded earthquake is the 1960 magnitude 9.5 event in Chile.

What school topic does this prepare?

Forces and inertia, plate tectonics, and logarithmic scales. The National Research Council’s framework names “designing solutions” and “analysing and interpreting data” as core practices, and this project is both, since you build the instrument and then have to read its output (NRC, 2012).

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. IRIS (Incorporated Research Institutions for Seismology). “How Does a Seismometer Work?” https://www.iris.edu/hq/inclass/fact-sheet/how_does_a_seismometer_work
2. U.S. Geological Survey. “Earthquake Magnitude, Energy Release, and Shaking Intensity.” https://www.usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity
3. U.S. Geological Survey. “20 Largest Earthquakes in the World Since 1900.” https://www.usgs.gov/programs/earthquake-hazards/science/20-largest-earthquakes-world
4. Kious, W. J., & Tilling, R. I. This Dynamic Earth: The Story of Plate Tectonics. U.S. Geological Survey. https://pubs.usgs.gov/gip/dynamic/dynamic.html
5. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
6. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165