Measuring Earthquakes: Seismographs, Richter & Moment Magnitude: Sixth Grade Science
Earthquakes are recorded and quantified through seismological instruments, measuring ground motion, total seismic energy release, and structural damage intensity.
The Seismograph: Recording Earth's Vibrations
A seismograph is a scientific instrument designed to detect, measure, and record the ground vibrations produced by seismic waves. A modern electronic seismograph operates on the principle of inertia:
- The base of the instrument is anchored firmly into solid bedrock so that it moves along with the ground during an earthquake.
- Inside the instrument, a heavy mass (pendulum) is suspended by a delicate spring or wire.
- When an earthquake strikes, the ground and the outer housing shake violently, but the heavy suspended mass remains stationary due to its large inertia.
- A recording pen or electronic optical sensor measures the relative motion between the vibrating frame and the stationary mass, producing a paper or digital waveform recording called a seismogram.
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| ANATOMY OF A SEISMOGRAM TRACE |
| |
| Quiet Bedrock P-Wave Arrival S-Wave Arrival Surface Waves |
| (Flat Baseline) (Small Jiggle) (Larger Waves) (MASSIVE AMPL) |
| |
| ------------------/\/\/\/\-----------/\/\/\/\/\/\/\-----\/\/\/\/\/\/\- |
| ^ ^ ^ |
| | | | |
| t = 0s t = 45s t = 90s |
| |
| <--- S-P Lag Time = 45 Seconds ---> |
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Triangulating an Earthquake Epicenter
A single seismograph station cannot determine where an earthquake occurred; it can only determine how far away the epicenter was. To find the exact location of an epicenter, seismologists use the triangulation method using data from at least three separate seismograph stations.
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| EPICENTER TRIANGULATION MAP |
| |
| Station A (Radius = 1,200 km) |
| /-----------\ |
| / \ |
| / ( * ) <===== EPICENTER (Exact Intersection)|
| / / \ \ |
| Station B \ / \ / Station C |
| (Rad = 800 km) \----+---------+ (Rad = 1,500 km) |
| \ / |
| \-----/ |
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The Three Steps of Triangulation
- Step 1: Calculate S-P Lag Time: Measure the exact time difference (in seconds) between the arrival of the first P-wave and the first S-wave on the seismogram.
- Step 2: Determine Distance: Use a standardized seismic travel-time graph to convert the S-P lag time into an exact distance in kilometers from the station to the epicenter.
- Step 3: Draw Intersecting Circles: On a geographical map, draw a circle around each seismograph station where the radius of the circle equals the calculated distance.
- The Intersection: A single station produces an infinite number of possible locations along a circle. Two stations produce two intersecting points. A third station provides the unique, single intersection point where all three circles meet, pinpointing the exact epicenter!
Quantifying Earthquakes: Magnitude Versus Intensity
Geologists use distinct scientific scales to measure the size, power, and destructiveness of earthquakes:
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| EARTHQUAKE MEASUREMENT SCALES |
+---------------------+-------------------+-----------------------------------+
| Measurement Scale | What It Quantifies| Scale Range & Characteristics |
+---------------------+-------------------+-----------------------------------+
| Moment Magnitude | Total physical | Logarithmic scale (0 to 10+); |
| Scale (Mw) | energy released | Modern scientific standard; |
| | by the fault | based on rock rigidity and slip |
+---------------------+-------------------+-----------------------------------+
| Richter Magnitude | Maximum ground | Logarithmic scale; based on peak |
| Scale (ML) | wave amplitude | seismograph pen jiggle; inaccurate|
| | | for massive or distant quakes |
+---------------------+-------------------+-----------------------------------+
| Modified Mercalli | Observed structural| Roman numerals I to XII; |
| Intensity Scale | damage & human | subjective; varies by location; |
| | perception | highest at epicenter / soft soils |
+---------------------+-------------------+-----------------------------------+
The Logarithmic Scaling of Moment Magnitude
The Moment Magnitude Scale is logarithmic:
- Ground Shaking: Each whole number increase on the scale represents a ten-fold (10x) increase in recorded ground wave amplitude. A magnitude 7.0 quake produces 10 times more shaking than a magnitude 6.0 quake, and 100 times more shaking than a magnitude 5.0 quake!
- Total Energy Release: Each whole number increase represents an exponential 32-fold (32x) increase in total seismic energy released. A magnitude 7.0 earthquake releases 32 times more energy than a 6.0, and approximately 1,000 times (32 x 32 = 1,024x) more energy than a magnitude 5.0!
Exercises and Solutions
Exercise 1: Calculating Logarithmic Energy Multipliers. Compare a major magnitude 8.0 subduction earthquake to a moderate magnitude 6.0 fault earthquake. (a) How many times greater is the ground wave shaking amplitude of the magnitude 8.0 quake compared to the magnitude 6.0 quake? (b) How many times more total seismic energy is released by the magnitude 8.0 quake compared to the magnitude 6.0 quake?
Solution 1: (a) The difference in magnitude is 2 whole units (8.0 - 6.0 = 2.0). Ground shaking amplitude increases by a factor of 10 for each unit: 10 to the second power (10 x 10) equals 100 times more ground shaking amplitude. (b) Total seismic energy release increases by a factor of approximately 32 for each whole magnitude unit. For a difference of 2 units, energy increases by 32 squared (32 x 32), which equals approximately 1,024 times more total energy released.
Exercise 2: Contrasting Magnitude and Intensity Ratings. In 1989, the Loma Prieta earthquake struck northern California with a Moment Magnitude of 6.9. In downtown Santa Cruz, built on loose river mud, the Mercalli intensity was rated as IX (Violent Shaking; buildings collapsed). In nearby Palo Alto, located the same distance from the fault but built on solid granite bedrock, the Mercalli intensity was rated as VI (Strong Shaking; superficial plaster cracks). (a) Explain why an earthquake has only one magnitude value but many different intensity ratings; (b) Explain why buildings on soft mud suffered greater intensity damage than buildings on solid bedrock.
Solution 2: (a) An earthquake has only one magnitude rating because magnitude measures the single, absolute total quantity of seismic energy released by the fault rupture at the focus. In contrast, Mercalli intensity measures the observed structural damage and human perception at specific geographic locations, which varies depending on distance from the epicenter, local building construction quality, and underlying soil geology. (b) Soft, unconsolidated water-saturated mud acts like jelly during seismic shaking. As seismic waves pass from dense bedrock into loose mud, they slow down; to conserve wave energy, their amplitude increases dramatically (seismic wave amplification). The mud can also undergo liquefaction, where shaking causes soil particles to lose contact and flow like liquid, causing foundations to sink and collapse. Bedrock vibrates with high frequency but low amplitude, transmitting seismic waves cleanly with minimal structural failure.