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GEOL 106 Lecture Notes - Seismogram, Seismometer, Epicenter

Geological Sciences and Geological Engineering
Course Code
GEOL 106

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To locate the quake focus:
Need a minimum of 3 seismometer stations
Where do earthquakes occur?
At what depth?
o Less than 700 km
o 90% are less than 100 km deep
o Note: radius of earth is 6370 km
Why do earthquakes only occur in the outer 10% of the earth?
Only the outer part of the earth is rigid enough (ie.
Sufficiently ELASTIC) to experience BRITTLE FAILURE (fault
Deeper on the earth it is to PLASTIC (or liquid?)
How do we estimate energy released from a Quake?
1. “Measure” the Intensity
o based on observed damage (eg. The Mercalli Scale)
o Subjective
o Varies with distance from epicenter
2. Measure the Richter Magnitude
o a quantitative measure of energy released
determined from the maximum s-wave amplitude
on a seismogram (corrected for distance)
The lines on a skwigale is an open ended scale. (
we don’t know how high it could go)
o The largest earthquakes ever recorder are 9 on the
richter scale
Quakes <2.5 magnitude are not felt by humans
Each magnitude integer step corresponds to
approx. a 30-fold difference in energy release
How much more energy is released in a magnitude 7 earthquake
than in a magnitude 5 earthquake ? 30x30 = 900
How about a magnitude 8 earthquake? 30x30x30 = 27000
How about magnitude 9?
What do the Richter numbers mean in terms of ACTUAL ENERGY
How would you figure that out?
Compare to known explosions (eg nuclear blasts)
<3 OVER 1000000 /YEAR
>3 OVER 30000 / YEAR
>6 100 / YEAR
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