Acceleration
The rate at which motion changes. A seismic sensor measures acceleration over time so the shape and intensity of building movement can be recorded.
Terminology
Seismic monitoring glossary
Definitions for the main sensing, earthquake and engineering terms used on this site.
Accelerometer
A sensor that measures acceleration. In a seismic device it records how a surface speeds up and slows down along one or more axes.
Ambient vibration
Small, continuous vibration caused by wind, traffic and machinery. It can reveal a building's natural frequencies without an earthquake.
Array
A group of sensors installed at several points, for example on different floors, so their records can be compared.
Building motion
The movement of a building during vibration or an earthquake. It is not the same as a structural safety assessment.
Event recording
A time-stamped series of sensor measurements captured around a detected shaking event for later technical review.
MEMS
Micro-electro-mechanical system: a microscopic mechanical structure on a silicon chip. MEMS accelerometers are small, low-cost sensors that measure acceleration.
Modal analysis
A way to describe a structure's vibration as separate modes, each with its own frequency and shape. Engineers use it to interpret how a building moves.
Mounting
How a sensor is attached to a structure. A loose or flexible mount can add noise or its own resonance to the measurement.
Natural frequency
A frequency at which a structure tends to vibrate when disturbed. It depends on mass, stiffness and geometry, and it can shift with temperature, occupancy and the amplitude of shaking.
Noise floor
The smallest signal a sensor can tell apart from its own background electrical noise.
P wave
The primary seismic wave. It normally travels faster than other earthquake waves and may arrive first at a sensor.
S wave
The secondary seismic wave. It normally arrives after the P wave and moves material perpendicular to its direction of travel.
Seismic monitoring
The continuous or event-based measurement of ground or building motion. Monitoring supports observation; it does not certify that a building is safe.
Seismic network
A set of instruments, or phones, spread over an area to detect and record shaking.
Sensor
The measurement component that converts physical building movement into digital data.
Sensor placement
The choice of where and how sensors are installed in a building. It affects data quality and what the data can show.
Strong motion
Shaking strong enough, as in an earthquake, that it is recorded by instruments built not to saturate at high acceleration.
Structural engineer
A qualified engineer who evaluates how structures behave and can interpret measurements together with inspections, plans and other evidence.
Time history
A record of a measured quantity, such as acceleration, over time. Also called a time series.