EQ L2: Earthquake Sources

Question 1

Name the 3 types of ground motion (deformation). How can they be observed in nature

Answer 1

permanent shifts in ground position, slow plastic movement, short oscillations after which ground returns to its origin. Observed thru elastic, plastic, and brittle deformation

Question 2

Describe elastic deformation

Answer 2

Forces acting on rock are relatively small. Resulting shape of rock is not permanently changed. Shape restored once force removed; no evidence of the rocks experiencing any force at all. This kind of deformation is experienced when energy is passed through the rocks as waves.

Question 3

How do we know elastic deformation exists in nature if they’re not permanently visible?

Answer 3

We know such deformation exists from moving pictures of wave motion. After the waves pass, if the ground returns to it’s original position, we know that elastic deformation has occurred.

Question 4

Elastic deformation isn’t a problem as long as….?

Answer 4

as long as motion does not cause damage before it’s over. However, because waves are propagated (transmitted) because of elastic behaviour, catastrophes even at great distances can result. The Mexico City earthquake disaster (1985) is a prime example of how elastic behaviour carrying enough energy caused major damage.

Question 5

Describe Plastic Deformation. Give examples.

Answer 5

occurs when applied force permanently changes the shape of a rock without breaking it. Ex.: bends or fold in layered rocks, either at a small scale in exposed rocks or at very large scales in the patterns visible in mountain ranges.

Question 6

Describe Brittle Deformation.

Answer 6

As forces applied to material increase, material stores the energy (i.e. stress accumulates). When stress exceeds strength of material. material breaks. Accumulated energy rapidly released as heat, motion, and sound. A catastrophic release of energy => motion is an earthquake. Energy dissipated in the form of waves (many types of seismic waves) that radiates away from location of breakage.

Question 7

List the types of Faults

Answer 7

dip-slip faults, strike-slip faults, oblique faults

Question 8

Describe dip-slip faults. How are the two types of dip-slip faults defined?

Answer 8

involve vertical motion along a slanting plane. There are two types of dip-slip faults defined in terms of the direction of motion of the side which leans over it’s neighbour. Reverse faults: the side leaning on its neighbour moves up. Normal faults: the side leaning on its neighbour drops down due to gravity.

Question 9

Describe strike-slip faults. How many types are there?

Answer 9

Involves horizontal motion. Two types, defined in terms of which direction the two sides move. If you stand with one foot on each side, either the left or the right side will appear to be coming towards you. In fact it does not matter which way you face; the sense of the motion is the same either way.

Question 10

Describe oblique faults.

Answer 10

Involves combination of vertical and horizontal motion.

Question 11

Which type of plate boundary would dip-slip and strike-slip faults occur?

Answer 11

Normal dip-slips: divergent boundaries (tensional forces)
Reverse faults: convergent boundaries (compressional forces)
Strike-slip faults: transform boundaries (where rocks on both sides are sheared)

Question 12

Do faults fail all at once?

Answer 12

No. An earthquake might be expected to enhance the likelihood of further earthquakes in regions near the original one where ground was not shifted to relieve accumulated stresses.

Question 13

What is the ideal fault to study from? Example?

Answer 13

One that has experienced earthquakes fairly frequently, both major and minor. Ex: a major fault in Turkey, below Turkish city of Izmit.

Question 14

What kind of scientific work is necessary to establish a good understanding of earthquakes and their effects?

Answer 14

1. careful field work involving deployment of many instruments to measure stresses all over the countryside around the fault.
2. sophisticated mathematical simulation of the physics of moving solids.

Question 15

When there is motion at a fault, is this motion simple?

Answer 15

No. Faults fail in complicated patterns. Motion along faults is NOT uniform and faults do not fail in a single jolt. There are foreshocks and after-shocks associated with all earthquakes, especially large ones.

Question 16

Amount of motion experienced during a real earthquake (based off of 1992 Landers, Calif. earthquake): In the area where motion started (directly underneath the epicentre), the slippage was ____ compared to the motion experienced 35 km to the north of the epicentre (indicated by the red zone centred at 5 - 10 km depth). This area experienced the ___ ground motion with slippage along the fault of 7 metres. Elsewhere there was much ____ ____, as indicated by the colours in the image.

Answer 16

small; most; much less;

Question 17

Describe the sequence of earthquakes.

Answer 17

Large earthquakes are usually part of a sequence of many earthquakes. The foreshocks occur first, although they do not always occur. Then comes the main earthquake. Lastly, the aftershocks occur, decreasing in frequency with time. The largest of the aftershocks is usually 1 magnitude smaller than the mainshock. The designation of whether a quake is a fore, main, or aftershock is usually not firmly established until the entire event is over.

Question 18

Largest of the aftershocks are usually how many magnitudes less than the mainshock?

Answer 18

1 magnitude less

Question 19

Large earthquakes are usually part of ___?

Answer 19

A swarm of earthquakes with many foreshocks and after-shocks.

Question 20

Most after-shocks occur within how long? What is the maximum time they can continue for?

Answer 20

Within two days. They may continue for four months.

Question 21

What factors affect the amount of energy released during an earthquake?

Answer 21

1. Area of zone broken
2. Strength of rocks being broken
3. Amount of motion

Question 22

Oldest method of estimating earthquake magnitude developed by who? What is the procedure called? Is it still useful?

Answer 22

Charles Richter. Richter Magnitude. Not useful: the procedure is accurate only if a particular type of seismometer is used and if the earthquake occurred in Southern California. Elsewhere, the differences in ground types would result in a magnitude that is poorly estimated.

Question 23

What alternative ways of estimating magnitude?

Answer 23

P-wave and S-wave amplitudes.

Question 24

Most reliable method of estimating magnitude?

Answer 24

careful records of ground motion at many locations. These records are then analyzed to determine how much the ground moved at the earthquake’s source location, and over what area this motion occurred.

Question 25

In addition to estimating the amount of motion and the area involved, the types of rocks involved must be known because tougher rocks require more energy to break than weaker ones. With these three parameters the so-called Seismic Moment M_0 can be determined from the following simple equation:?

Answer 25

M_0 = strength of rocks * area involved in the breakage * distance the fault moved.

Question 26

Moment Magnitude formula?

Answer 26

M_W = 2/3[(logM_0) - 6]

Question 27

Why is estimate using records of ground motions reliable?

Answer 27

This estimate is not based on any assumptions about how energy traveled through the ground, and hence is a more reliable estimate for earthquake magnitude. However it requires sophisticated observations in order to determine the parameters. The formula for M_W involves logarithms. It is an “empirical” formula, i.e. it was determined by considering a great many situations and finding the best mathematical relationship that explains them all.

Question 28

The amount of energy released by an earthquake can range from close to zero to an amount similar to all the energy involved in an average _______, all released at once in an earthquake

Answer 28

10-day hurricane. So we use a logarithmic scale for characterizing earthquake energy.

Question 29

Which magnitude earthquakes do you rarely feel and never cause much harm?

Answer 29

Magnitude <=4

Question 30

How much more energy does magnitude 7 quake release than magnitude 4?

Answer 30

Using formula, 33 000 times.

Question 31

How much more shaking does a magnitude 7 quake cause than a magnitude 4? (note: for SHAKING, each unit change in magnitude is equivalent to 10X change in shaking. This is the same as all the other disaster scales (except energy release))

Answer 31

1000 times. 10^7/10^4 = 10^3

Question 32

Which plate boundaries have largest earthquake?

Answer 32

Convergent boundary, as this type of plate boundaries collide and usually one tectonic plate subducts. It is know that the largest earthquakes occur in the subduction zone.

Question 33

How does gravity contribute to plate motion?

Answer 33

by pulling denser plates as they plunge at subduction zones

Question 34

3 types of forces that can affect plates?

Answer 34

compressional forces - those that cause squeezing

tensional forces - those that cause a pulling apart of material

shear forces- those that cause twisting or shearing

Question 35

Which tectonic boundaries will experience greater accumulation of stress before breaking: convergent, divergent, or strike-slip?

Answer 35

convergent (settings in which more force makes it harder to dissipate energy will accumulate more stress. Then, when the setting does finally break, the resulting release of energy will be larger)