Measuring Earthquake Strength
Know that seismometers record ground movement, earthquake waves carry energy, and magnitude describes an earthquake's overall size rather than the shaking at one place
What a learner can do afterwards
- Explain that a seismometer is an instrument that detects and records ground shaking
- Describe earthquake waves carrying energy outward and compare their arrival times at recording stations
- Interpret magnitude as a measure of an earthquake's overall size, related to the energy released
1 · Read
A seismometer senses ground movement and makes a record of when it happens and how much the ground moves. It can record shaking too small to feel. Scientists use these records to help estimate an earthquake's magnitude. They need working instruments and complete records. A recorder that stops partway through a quake misses information.
Sudden rock movement releases energy as earthquake waves. A fault is a crack along which rocks can move. Waves travel outward through rock and soil. They carry energy to other places; the rocks at the fault do not travel with them to distant towns. Shaking usually weakens with distance, but ground conditions also affect local shaking.
A magnitude 5 earthquake is larger and releases more energy than a magnitude 2 earthquake. Magnitude describes the whole earthquake. It does not count people who felt it. One earthquake can cause different shaking in different towns. A high magnitude does not mean every town shakes strongly. Magnitude numbers are not energy ratios: magnitude 4 does not mean twice the energy of magnitude 2.
Compare wave arrival times measured from the same starting time. A wave reaches one station 8 seconds after a quake starts and another at 13 seconds. Subtract the earlier time from the later time: 13 - 8 = 5. It arrives 5 seconds later at the second station.
Seismometers record ground movement. Waves carry energy outward. Magnitude describes the whole earthquake; local shaking can differ.
Readings and practice questions written for this learning dataset.
More to read
Keeping a Ground-Movement Record
A person's report tells scientists what that person noticed. An instrument can add observations that people missed.
A seismometer senses ground movement over time. Its record can include shaking too small to feel. Scientists study records from several places to learn about an earthquake.
A missing part of a record is missing evidence. It does not prove the ground was still. Scientists can seek other working instruments that recorded the same time.
How Energy Reaches Another Town
A sudden slip at a fault releases stored energy. Earthquake waves carry energy through the surrounding ground.
When waves reach a distant town, the ground there moves. The rocks that slipped at the fault have not travelled to the town. Energy has reached it through waves.
Shaking usually weakens with distance. The kind of rock and soil also matters, so distance alone cannot tell scientists exactly how strongly each town will shake.
Comparing Arrival Times
Stations record when earthquake waves arrive. To compare two arrival times, use the same starting time.
Suppose a wave reaches one station 12 seconds after the quake starts and another at 19 seconds. Subtract: 19 - 12 = 7 seconds. It reaches the second station 7 seconds later.
This difference is an interval of time. It is not a difference in magnitude, and it does not mean the stations recorded two separate earthquakes.
Rock Moves as Waves Pass
Earthquake waves travel through material such as rock and soil. As a wave passes a place, the material there moves.
The wave can continue onward while that rock remains near its own location. Energy reaches the next place without the source rocks travelling all the way there.
This explains how a town can shake during an earthquake that began elsewhere. The movement is passed through the ground between the source and the town.
Size and Local Effects
Magnitude answers a question about the earthquake as a whole: how large was it? Local shaking answers a question about one place: what happened here?
One earthquake may rattle dishes in one town while people elsewhere barely notice it. These different reports can describe the same event.
Scientists use instrument records and reports of local effects for different purposes. Magnitude alone does not tell them exactly how much each building moved or how much damage it suffered.
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