Intensity and Magnitude

• What is magnitude:

A measure of the energy released by an earthquake and the size of the rupture that caused it. One single magnitude value, independent of where it is measured.

What does magnitude describe?

A single number that characterizes the relative energy release and the size of the quake, based on seismic wave energy.

How is magnitude measured?

• Instrumentally with seismometers, corrected for distance; seismologists primarily use Moment Magnitude (Mw) derived from seismic moment (fault area × slip).

• A logarithmic scale; each whole number increase represents about 33× more energy release.

What is intensity:

• A measure of how strong the shaking feels at a specific location and its effects on people, objects, and buildings.

• Many, varying across the affected region because shaking differs by location.

What does intensity describe?

The severity of shaking and observed effects at a particular site.

How is intensity measured?

• Using the Modified Mercalli Intensity (MMI) scale, from I (not felt) to XII (extreme).

• Originally based on human observations; today correlated with instrumental data like PGA/PGV and mapped with ShakeMap.

Explain the difference between magnitude and intensity

• Magnitude is a single, instrument-measured value that describes the energy released by an earthquake.

• Intensity describes the shaking and its effects at specific locations, varies across the region, and is measured using the Modified Mercalli Intensity (MMI) scale.

Explain what controls intensity at a particular location

• Earthquake magnitude (bigger = stronger shaking).

• Distance to the fault rupture (shaking decreases with distance).

• Local ground conditions (soft sediments amplify shaking; bedrock reduces it).

what is the Richter Magnitude, and how is it calculated?

• Created by Charles F. Richter to classify earthquakes in SoCal

• Logarithmic, each whole step equals x10 in wave amplitude and x32 energy

• Measured using standard Wood-Anderson seismographs and nomograph relating wave amplitude and distance

Explain why the Eastern US and the Western US experience wildly different intensity distributions for the same sized earthquake

• The Eastern US is built on old, hard, consolidated rocks that transmit seismic waves efficiently > so shaking travels far

• Western US has fractured, younger, softer rocks from active tectonics → waves are absorbed quickly and shaking is confined to smaller areas