lesson plan

EOS 170 – Natural Hazards Lecture Summary

Lecture 5 — Earthquake magnitude and intensity

Earthquake magnitude

In basic terms, an earthquake’s magnitude describes how energetic it is. Since larger earthquakes tend

to produce larger ground motions at a given distance, magnitudes were traditionally calculated by

measuring the peak seismic wave amplitude on a seismogram and correcting for the distance those

waves had travelled. This was first formalized for the California region by Charles Richter in the

Richter scale. Nowadays, computer algorithms are able to determine a more robust moment magnitude

(abbreviated Mw) using global seismic records and other available data such as displacements measured

with geodesy. The moment magnitude is related to the seismic moment, M0 through the equation

Mw = (log M0 − 9)/1.5.

The seismic moment (in units of Nm) is the truest measure of an earthquake’s energy, and is equal to

the product of the rupture area (in m2), the amount of slip (in m), and the shear modulus (‘stiffness’)

of the crust, which is typically ∼3 × 1010 Nm−2. Rearranging these equations, it takes a 101.5 or 32-fold increase in moment in order to raise Mw by one unit, and a 10

3 or 1000-fold increase to raise it

two units. Thus, a Mw 8 earthquake is 32 times larger than a Mw 7 earthquake and 1000 times larger

than a Mw 6 earthquake. Because the planet experiences (on average) about one Mw 8 earthquake for

each fifteen Mw 7 earthquakes and each one hundred and fifty Mw 6 earthquakes, a handful of the very

largest earthquakes dominate cumulative global seismic energy release recorded over the past century.

The equation for moment also explains why the largest earthquakes (those with Mw >8 are called

‘great earthquakes’) almost exclusively occur within subduction zones. Subduction megathrust faults

are long, smooth, and gently-dipping, offering a very large area over which an earthquake can grow,

with the potential for earthquakes as large as Mw ∼9.5 in some longer subduction zones. Continental plate boundary zones, on the other hand, are broken up into networks of disconnected fault segments,

each of which offers only a limited area for earthquake rupture. With a few exceptions, continental

earthquakes are restricted to Mw <8. Nevertheless, globally, more people are killed in Mw 6–8 crustal

earthquakes than in Mw 8–9 megathrust earthquakes. This is because of the crucial distinction between

magnitude and intensity.

Earthquake intensity

Intensity describes the effects of an earthquake as perceived by human observers. Intensities are

formalized in the Modified Mercalli Intensity (MMI) scale, named after Italian priest and volcanologist

Guiseppe Mercalli. MMI values range from I (not felt) and II (weak shaking) through XI (extreme)

and XII (cataclysmic). Reported intensities can be used to construct contours of ground shaking on

isoseismal maps. For historical earthquakes, the area of heavy shaking can be used to estimate the

earthquake location and magnitude, though human biases must be considered carefully. For modern

events, isoseismal maps are constructed using measurements of strong ground motions made with

accelerometer instruments, coupled with felt reports collected through websites such as the USGS’s

“Did You Feel It?”. These maps reveal some interesting patterns; clearly, larger earthquakes tend

to produce higher intensities at a given distance from the fault, but many other factors combine to

determine the level of ground shaking. Shaking is typically stronger in the direction in which the

earthquake rupture propagates. Earthquake depth is also important; shallower earthquakes generate

stronger surface shaking than deeper ones. Regional geology is also a factor: earthquakes in western

North America affect smaller areas than equally-sized events in the east, because the crust in the west

is broken up by faults and fractures that dampen (attenuate) passing seismic waves, whereas the crust

in the east is stronger. Local site effects are perhaps most important of all; ground shaking is amplified

by loose rocks with slower seismic wave velocities such as those in sedimentary basins. These differences

were illustrated in the 2018 Anchorage, 2017 Puebla and 2015 Gorkha earthquakes. Liquefaction of

water-saturated soils is another seismic hazard closely associated with sedimentary basins.

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