How it works
A strong motion accelerograph combines two parts: an accelerometer and a recorder. The accelerometer senses acceleration, the rate of change of ground or structural velocity. The recorder digitises that signal, timestamps it, and stores or streams it for analysis.
Unlike a broadband seismometer, which is built for extreme sensitivity to detect faint, distant tremors, an accelerograph is built for the opposite priority: dynamic range. It needs enough headroom to record a Magnitude 7 earthquake at close range without the signal running off scale, while still capturing enough detail to be useful for engineering analysis.
What it measures
- Acceleration, typically in units of g (where 1g is the acceleration due to gravity)
- Frequency content across the range relevant to earthquake engineering, generally from below 1 Hz to around 50 Hz
- Three axes of motion: two horizontal and one vertical, usually from a single triaxial sensor
Modern accelerographs increasingly use digital MEMS sensors. Closed-loop (servo) MEMS designs now match traditional force-balance sensors for stability, while being smaller, lower cost, and stable down to DC, which allows factory calibration against gravity as a known reference.
Strong motion accelerograph vs. seismograph
The two terms are often used loosely, so it helps to be precise.
A seismograph is the general term for any instrument that records earthquake ground motion. Most seismographs use a seismometer, a highly sensitive sensor designed to detect small or distant events, sometimes from thousands of kilometres away.
A strong motion accelerograph is a specific type of seismograph. Instead of a seismometer, it uses an accelerometer, a sensor with a much wider dynamic range but lower sensitivity. This trade-off is deliberate: an accelerograph is designed to stay on scale during strong, damaging shaking, where a sensitive seismometer would clip.
| Seismometer (in a seismograph) | Accelerometer (in an accelerograph) | |
|---|---|---|
| Best for | Detecting small or distant earthquakes | Recording strong, potentially damaging shaking |
| Sensitivity | Very high | Lower, by design |
| Dynamic range | Narrow | Wide |
| Typical use | Regional earthquake detection, seismic networks | Structural monitoring, dam safety, critical infrastructure |
Where strong motion accelerographs are used
Accelerographs are the standard instrument wherever the question is “how did this structure respond to shaking,” rather than “did an earthquake happen.” Typical applications include:
- Buildings and structures
- Dams and hydro
- Infrastructure and lifelines
- Energy and industrial
- Research and networks
- Heritage and monuments
Because signals too weak to register clearly on an accelerograph are generally well below any level of concern for structural integrity, a strong motion accelerograph alone is usually sufficient for these applications, without needing a separate seismometer.
Choosing a strong motion accelerograph
Not all accelerographs are built the same way. Sensor type, timing architecture, data format, and dynamic range all affect whether an instrument stays useful for the life of a monitoring programme.
Canterbury Seismic builds two strong motion accelerographs. The EQR-90 and the research-grade EQR-120 combine sensor, digitiser, recorder, precise timing and an on-instrument web interface in a single box, and consolidate into one synchronised record via the EQR-C central recorder when you grow.
Read our guide to choosing a strong motion accelerograph.
Canterbury Seismic Instruments designs and manufactures strong motion accelerographs and structural array recording systems from Christchurch, New Zealand.