Earthquakes Flashcards
What are the possible results of brittle failure in the Earth’s crust?
earthquakes, joints, faults
What is an earthquake’s focus?
the place where energy is released (on the fracture, etc.)
What is an earthquake’s epicentre?
the place on the Earth’s surface directly above where the energy is released
What happens when the stress on the crust exceeds elastic reound?
stored energy is released suddenly, resulting in brittle failure - if it fractures, earthquake
What parts of an earthquake can be felt/seen?
ground motion, landslides, scarps, after shocks, tsunamis
What part of an earthquake causes the most damage?
indirect results of ground motion - falling debris, landslides…
Why are landslides so dangerous?
cut off area from outside world - cut utilities, dam rivers, break roads
How do the intensities of after shocks compare to the actual earthquake, and why do they cause damage?
less intense; hit things that were damaged by main earthquake
What is a scarp?
fault surface where it intersects the earth
How is earthquake intensity measured?
Mercalli Intensity Scale - by asking people what they felt
Why is measuring earthquakes with intensity bad?
intensity is independent of amount of energy released ;
varies based on the time and place;
hard to measure if no one was present;
can’t compare between earthquakes
What is used instead of intensity to measure earthquake strength?
magnitude - the energy released
What scale is used to measure magnitude?
Richter scale
What type of scale is the Richter scale?
exponential - each magnitude quake produces 10 times more ground motion than the magnitude - 1
What does measuring magnitude allow one to do that couldn’t be done by measuring intensity?
comparison of earthquake
What are the 3 notable magnitudes on the Richter scale?
M1: just able to record;
M5: damage begins
M8.5: maximum magnitude
What does the energy relassed (magnitude) of an earthquake really represeent?
the potential destructive power of an earthquake
What decides the actual destruction caused by an earthquake?
where the energy was released
What does a seismometer do?
generates a seismograph
How does a simple seismometer work?
earth moves while pen (fixed by spring) does not, so pen draws wiggly lines to represent ground motion
Where do seismometers work best?
in places without competing energy (traffic, people, etc.)
What does a typical seismograph look like?
small oscillations for P wave;
larger oscillations for S-wave;
huge oscillations for Surface-wave;
decreases except for spikes (aftershocks)
What effect does fault size (amount of sliding/fault surface area) have on earthquake size?
bigger faults make bigger earthquakes (based on magnitude) - pretty linear with magnitude
What effect does earthquake size (magnitude) have on the length of time the earthquake lasts?
bigger earthquakes last a longer time
Why do higher energy earthquakes cause so much damage?
double wammy - more ground shaking that lasts longer
How are earthquakes distributed in energy?
not evenly - many very small ones; not many high magnitude ones
How frequently do earthquakes that just start causing damage occur?
M5 - every 9 hours
What are two ways in which earthquake energy travels?
body waves and surface waves
What are body waves?
pass through interior of earth; travel in straight line from source
What are the types of body waves?
P-waves (compression waves) and S-waves (shear waves)
What are the characterisitcs of P-waves?
compressional and fast (4-7 km/sec)
What kind of particle oscillation do compressional waves cause?
particle oscillation in direction of wave oscillation
How fast do P-waves travel and why is it a range?
4-7 km/s; varies as a funciton of the material
In what sorts of materials do waves travel faster?
denser materials
What are the characteristics of S-waves?
shear, slow (2-5 km/sec), and do not travel through liquids
What kind of particle oscillation do shear waves cause?
particle oscillation perpendicular to wave propagation direction
How fast do S-waves travel and why is it a range?
2-5 km/sec; varies as a function of the material
How do we know the earth’s core is at least partly liquid?
S-waves do not travel through liquid, so earthquakes on one side of the core leave an “S-wave shadow” on the other side of the core where no S-waves arrive but P-waves do
How are earthquakes located?
measure difference in time between arrival of P-wave and S-wave to tell distance; do this at 3 stations and triangulate
What does the difference in arrival time between S and P waves tell you?
the distance of the earthquake (the farther it is, the longer the time difference)
What does the difference in arrival time between S and P waves NOT tell you?
the direction of the earthquake
Why are 3 stations required to locate earthquakes?
each station can only find distance of earthquake to it - must triangulate (see point where radius of earthquake distances from each sttion all meet)
What are surface waves?
waves that travel along earth’s surface
How are surface waves created?
when body waves intersect the surface of the earth - their energy disperses along it
What are some types of surface waves?
Love waves and Rayleigh waves
What are the general characteristics of surface waves?
disrupt surface; complex wave set; slowest
What kind of movements are generated by Love waves
side to side horizontal motion of surface
What surface waves are the fastest?
Love waves
What kind of movements are caused by Rayleigh waves?
rolling motion at surface; largest waves
What surface waves are the slowest?
Rayleigh waves
What surface waves cause most of the felt motion in an earthquake?
Rayleigh waves
How are earthquakes distributed geographically?
unevenly in depth, intensity, and geography
What characteristics of earthquakes are we good at predicting?
where and how big
What characteristic(s) of earthquakes are we bad at predicting?
when
Why can we predict where earthquakes happen?
earthquakes are unevenly distributed geographically and are most likely to happen where they’ve already occurred; (also some geological features)
Why can we predict how big earthquakes will be?
magnitudes often match those of previous earthquakes in a certain area
Why can’t we say when earthquakes will occur?
earthquakes are unevenly distributed in time
What often marks fracturing associated with magma movement?
earthquakes
What may accompany eruption/large movements of magma that accompany eruption?
“harmonic” earthquakes
How can earthquakes be used to map out distribution of magma under volcanos?
earthquakes often mark fracturing associated with magma; S-waves don’t travel through liquids - put seismometers around volcanos before eruption and observe earthquakes
