Lecture 6 - Earthquake Magnitude and Intensity Flashcards
What does the magnitude scale describe?
The size of an earthquake.
How did Charles Richter’s original scale work? (1935)
Magnitude was determined using the peak amplitude of seismic waves and correcting for the earthquake- seismogram distance.
Example: a M 5 earthquake generates 23 mm peak motion 220 km away, where a M 4 earthquake would generate only 2.3 mm. The same M 5 event would generate 100 mm peak motion 100 km away.
What is seismic moment?
It is a truer measure of an earthquake’s energy.
It is the product of the earthquake rupture area (or rupture length × width), the average slip that occurs, and the shear modulus of the surrounding rock (its resistance to deformation by shear stress).
The shear modulus of Earth’s crust can be approximated as 3 × 10¹⁰ N/m²
How do you calculate Moment (Nm) ?
Moment (Nm) = Rupture area (m²) × slip (m) × shear modulus (N/m²)
The shear modulus of Earth’s crust can be approximated as 3 × 10¹⁰ N/m²
What is Moment magnitude (Mw) and how do you calculate it?
The moment magnitude (abbreviated Mw) is the modern earthquake magnitude scale. It approximates Richter’s original scale, but is related directly to the moment through the equation:
Moment magnitude = (log10 Moment − 9)/1.5
How much bigger is a magnitude n earthquake than a magnitude n-1 earthquake?
e.g. n=6 so n-1=5
31.6 times bigger
Why is rupture area key to governing the moment and moment magnitude of an earthquake?
The rupture area and average slip correlate across a wide range of earthquake magnitudes.
The 1,000 km-long Cascadia megathrust is thought capable of hosting Mw 9 earthquakes, yet the 1,000 km-long San Andreas fault is thought limited to Mw 8 ruptures. Why?
The Cascadia megathrust is a subduction zone so it will have a larger rupture area than the San Andreas Fault.
Of the twenty largest earthquakes since 1900 (M 8.4–9.5), how many were megathrust earthquakes?
seventeen, including the top eight largest events (M > 8.8)
Why are megathrust earthquakes so large?
They rupture long, gently-dipping plate interface faults.
By how much does each unit increase in Mw correspond to decrease in frequency?
ten-fold
What is the largest magnitude of earthquake possible and where would the earthquake occur?
10.1
Peru-Chile trench
In an earthquake sequence (large earthquakes) what is the largest event called?
the mainshock
What percentage of earthquake sequences have a foreshock?
5%
Are foreshocks useful for earthquake prediction?
Unfortunately, foreshocks look identical to “normal” small earthquakes that happen all the time, so in practice they are not very helpful.
What does intensity describe?
The severity of shaking felt in an earthquake, and is formalized in the Modified Mercalli Intensity Scale (MMI). It is descriptive, with Roman numerals expressing perceived levels of shaking.
How can intensity be analysed?
Intensities can be plotted on a map, with lines of equal intensity termed isoseismals.
For pre-instrumental events, these may provide our best means for estimating the earthquake location and size.
What two sources of data are used to routinely generate isoseismal maps in the present day?
Accelerometers: a type of seismometer designed to record strong ground motions from nearby earthquakes (but insensitive to small ground motions from distant events).
Felt reports: nowadays, these are collected using citizen science websites like the USGS “Did You Feel It?”
Why did Wellington experience stronger shaking than Christchurch in the 2016 Mw 7.8 Kaikoura, NZ earthquake?
Wellington likely felt stronger shaking because of a directivity effect. The rupture migrated from SW to NE, amplifying surface waves in that direction.
Why is there a difference in attenuation between eastern and western North America?
Due to their tectonic histories.
Rock type.
What is basin amplification?
The increase in the size of seismic waves as they pass from hard bedrock into loose sediment.
What is liquefaction?
How water-saturated soils lose strength during shaking, a common site effect in sedimentary basins.
What are some examples of earthquakes where site effects was a factor in the damage?
2018 Mw 7.1 Anchorage intraslab earthquake
1964 Mw 9.4 Alaska megathrust earthquake
2015 Mw 7.8 Gorkha earthquake
2017 Mw 7.1 Puebla, Mexico intraslab earthquake
