Earthquake Measurement Scales

Overview

This lecture explains how scientists measure earthquakes, covering both intensity and magnitude scales, and the difference between them.

Earthquake intensity measures the degree of damage or what people feel during an earthquake.
The Modified Mercalli Intensity (MMI) Scale is used, developed in 1902 by GSE Meri.
The MMI scale ranges from I (not felt) to XII (catastrophic destruction).
Intensity values decrease with distance from the epicenter.
Useful for studying historic earthquakes and assessing damage, regardless of magnitude.

Magnitude measures the total energy released by an earthquake.
The Richter Scale (introduced in 1935 by Charles Richter) uses seismograph data to calculate amplitude.
The Richter Scale is logarithmic; each step up is 10 times more ground motion and 32 times more energy released.
Example: A magnitude 6.0 earthquake releases 32 times more energy than a magnitude 5.0.
The calculation involves measuring the largest wave amplitude and the distance to the epicenter.

The Moment Magnitude Scale is more accurate and modern than the Richter Scale.
It takes into account the amount of slippage on the fault and the strength of the faulted rocks.
There is no upper limit to this scale; the largest recorded was 9.5 in Chile, 1960.
Moment Magnitude Scale is now the standard for large, global earthquakes.

Intensity — Measures the damage and human perception of earthquake shaking.
Magnitude — Measures the energy released during an earthquake.
Modified Mercalli Intensity (MMI) Scale — A scale from I to XII based on observed damage and felt reports.
Richter Scale — A logarithmic scale measuring earthquake magnitude based on seismic wave amplitude.
Moment Magnitude Scale — A scale measuring earthquake magnitude using fault slip and rock strength.

Review the chart showing magnitude, energy release, and ground motion in your notes.
Prepare for the next lecture on earthquake locations and tectonic plates.