ATOC 185 Lecture Notes - Lecture 4: Krakatoa, Volcanism, Kauai
8 Jun 2018
School
Department
Course
Professor
Earthquakes
-
Week
4
Elastic
Rebound
Theory
- Explanation for how energy is released during earthquakes
-As rocks on opposite sides of a fault are subjected to force, the accumulate energy and slowly
deform until their internal strength is exceeded
-Sudden movement occurs along fault, releasing the accumulated energy and the rocks snap
back to their original, undeformed shape
-The deeper the rock is, the more ductile it is
- Warmer
-The rock that is deep enough to undergo plastic deformation will not generate
earthquakes
-Compressing, storing energy, deforming rocks, until elastic limit is reached
-When rock reaches elastic limit, it can’t handle the energy and it suddenly breaks
- Surface area - energy is liberated all at once, creates earthquakes
What
is
an
earthquake?
- Result of a sudden release of energy in the Earth’s crust that creates seismic waves
Violent
vs
Gentle
Earthquakes
-Why do violent earthquakes happen in subjective zones compared to gentle ones that occur at
divergent margins - why do we have this difference?
-80-90% of violent earthquakes occur along Ring of Fire
-When rocks resist for too long, elastic limit is very high
-The higher limit is, the more energy we store into the rocks
Earthquake
Generation
along
a
Fault
- Earthquake focus is its point of origin along a fault plane
-Epicenter is the vertical projection of focus to the surface (the focus, but on the surface)
Depth
of
Focus
- Earthquakes are classified by the depths of their foci below the surface, as follows:
- Shallow 0-70
km
-75-80% of all earthquakes happen at this depth because we have more fractures,
fault, and brittle deformation in the lithosphere
- Intermediate 70-300
km
-18-19% of earthquakes
-The warmer the rock gets, the less earthquakes we have
-Deep 300-700+
km
-3-5% of earthquakes
Types
of
Waves
1. Body Waves (travel through the earth) - ones first felt from focus of earthquake
a. Primary (P) Waves
i. Travel through solid, liquid, and gas
ii. Alternately compressional and expansive
iii. Speeds are about 5 km/s
iv. Vibrates parallel to direction of propagation of wave
b. Shear (S) Waves
i. Travel only through solids
ii. Push material at right angles to their travel path
iii. Speeds are 2-3 km/s
iv. Things move perpendicular to propagation of waves
v. Main difference from P Waves is amplitude
2. Surface Waves (travel along surface) - Don’t displace anything underground, are more
damaging
a. Rayleigh Waves
i. Produce rolling motions of the ground, instead of shaking like Love waves
b. Love Waves
i. Restricted to Earth’s surface
ii. Cause sideways shaking of the ground
iii. Speed slightly less than S waves
iv. Most destructive
Sizes
of
Earthquakes
- Size is main factor in earthquake destructiveness
-Two ways to estimate size
1. Richter Magnitudes
a. Measures maximum amplitude of ground shaking (vibrational energy)
b. Logarithmic scale
c. 1 Richter unit difference is x10 for ground motion and x33 for energy
d. Globally, small earthquakes are more frequent than large
i. About 800,000/year for 2.0 - 3.4 magnitude earthquakes
ii. Event of magnitude 8 occurs once every 5-10 years
2. Mercalli Index
a. Modified Mercalli Intensity Scale
i. Magnitudes do not necessarily describe the destructiveness of an
earthquake
ii. Earthquake may be close (more destructive) or distant (less destructive)
from a population center
iii. Event may be shallow (more) or deep (less)
b. Intensity vs Magnitude
i. Intensity measures degree of damage
ii. Magnitude measures degree of movement
c. Used to assign a measure of destructiveness to an earthquake
i. Degree of damage caused
d. Qualitative and based upon observation, effects on people, and damage to
buildings
e. Mercalli I: Very weak, not felt by people
f. Mercalli XII: Total destruction
Document Summary
As rocks on opposite sides of a fault are subjected to force, the accumulate energy and slowly deform until their internal strength is exceeded. Sudden movement occurs along fault, releasing the accumulated energy and the rocks snap back to their original, undeformed shape. The rock that is deep enough to undergo plastic deformation will not generate earthquakes. Surface area - energy is liberated all at once, creates earthquakes. Result of a sudden release of energy in the earth"s crust that creates seismic waves. 80-90% of violent earthquakes occur along ring of fire. When rocks resist for too long, elastic limit is very high. The higher limit is, the more energy we store into the rocks. Earthquakes are classified by the depths of their foci below the surface, as follows: 75-80% of all earthquakes happen at this depth because we have more fractures, fault, and brittle deformation in the lithosphere. Types of waves: body waves , primary .
