What it does
Every smartphone has an accelerometer: a tiny sensor that measures acceleration along three axes, about 50 to 200 times a second. Lying still on a table it reads gravity, about 9.8 m/s² on the axis out of the screen, plus small wobbles from anything that shakes the table. This page turns your phone into a toy seismometer:
- Records the three components after you press Start (your phone may ask for permission first; the page never starts by itself) and draws the last 10 seconds live.
- Resamples the readings evenly (phones deliver them at slightly uneven times) and removes gravity and slow tilt with a zero-phase Butterworth highpass filter.
- Measures the peak ground acceleration (PGA) on each component, in m/s² and as a percentage of g, plus the 3-D and horizontal peaks.
- Shows the amplitude spectrum and the dominant frequency of each component.
- Saves recordings in this browser only, and downloads them as CSV or as a SAC file to open in the Seismogram viewer or seismology software.
It is a toy for learning, not an earthquake detector. A phone is thousands of times less sensitive than a real seismometer, its sensor drifts, and footsteps or a closing door look like earthquakes. It gives no warnings. For real earthquake information follow your national geological survey or civil protection agency.
How to use it
- Lay the phone flat on a solid surface, screen up, and press Start recording. On iPhones, allow Motion & Orientation access when asked.
- Don’t touch the phone. Make something shake: walk past, drop a book on the floor nearby, or record a washing machine.
- Press Stop (or wait for the time limit). The analysis appears below.
- Adjust Remove below (the highpass corner; 0.5 Hz removes gravity and slow tilt) and optionally Remove above to cut fast noise. Higher filter orders cut more steeply.
- Save this recording to keep it in this browser, or download the CSV or SAC.
On a computer (which has no accelerometer), open a CSV recording from a phone app, or press Use the demo recording.
Worked example
The demo is a made-up 30-second recording of a phone on a table, with readings about every 16 ms (62.5 a second, slightly uneven) resampled to 63 Hz. A lorry passes from 8 to 16 s and a door is slammed at 21 s. With the default 0.5 Hz highpass, the page reports PGA 0.314 m/s² (3.20 % g) on z, at 21.06 s: the door. The dominant frequency on z is 3.17 Hz (the door’s knock) and on y 12.0 Hz (the lorry’s hum, which shakes y most). Set Remove above to 5 Hz and the lorry almost disappears from y, because its 12 Hz is filtered out.
Good to know
- The axes are the phone’s own: x across the screen, y along it, z out of the screen. Lying flat, z is vertical.
- Phones deliver readings unevenly and some stop when the screen locks; keep the screen on. The page states the spread of the intervals.
- A sharp tap on the table gives a huge PGA: that is why it says “check your recording” for large values.
FAQ
Is my recording sent anywhere? No. Readings stay on this page. Saved recordings are kept in this browser’s storage on this device until you delete them.
Why is the PGA not zero when nothing happens? The sensor has noise of a few thousandths of a m/s², plus building vibration.
Can I use it for a science fair? Yes: compare floors of a building, machines, or how a heavier table damps vibration.
Where the numbers come from
- Motion readings from the W3C DeviceOrientation Event specification (devicemotion, accelerationIncludingGravity in m/s²; device axes x across the screen, y along it, z out of the screen).
- Linear resampling, Butterworth highpass by the bilinear transform with zero-phase filtering, Tukey taper and amplitude spectrum: standard signal processing written by fibub, checked against NumPy (interp, rfft) and SciPy 1.18 (butter, sosfiltfilt, detrend, tukey). Standard gravity g = 9.80665 m/s².
- The words for how strong shaking feels are fibub's rough guide for scale, not an intensity scale. The demo recording is made up by fibub.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.