EQ L3: Seismic Energy and Waves

Question 1

What is an accelerograph?

Answer 1

instrument that records acceleration of graph. (quantitative description of ground motion)

Question 2

Difference in appearance/graph between data on ground motion and seismograms?

Answer 2

they show larger ground motions that seismograms (otherwise looks the same)

Question 3

What does negative velocity signify?

Answer 3

Means the ground is moving ‘backwards’ or ‘downwards’

Question 4

What is the hypocentre or focus?

Answer 4

point where energy was released

Question 5

What’s the Felt Zone?

Answer 5

Region where ground motion was felt. These maps are built by collecting eye-witness accounts from people around the region.

Question 6

What leads to irregularities in a Felt Zone map?

Answer 6

type/characteristics of the ground

Question 7

When faults slip, where does the energy go?

Answer 7

The released energy is converted to wave energy. This can travel great distances, in a manner similar to the propagation of ripples after a pebble is tossed into water.

Question 8

The 2 main classes of seismic waves?

Answer 8

Body waves: those that travel through the interior of materials.

Surface waves: only travel along surfaces

Question 9

Describe the two forms of body waves.

Answer 9

The distinction is based on the type of particle motion associated with the wave.

Pressure or primary wave (P-wave): When particles move back and forth in line with the direction the waves are traveling, the wave is called a pressure or primary wave

Shear/ Secondary (S-wave): When particles move from side to side, perpendicular to the direction that waves are traveling, the wave is called a shear or secondary wave. These are referred to as S-waves. S-waves travel slower than P-waves, hence the terms “primary” and “secondary” to characterize rate of energy transfer.

Question 10

P-waves travel slower/faster than S-waves.

Answer 10

faster

Question 11

What are surface waves? What are two important types of surface waves?

Answer 11

Wave energy that travels along boundaries (rather than through materials). Rayleigh and Love waves

Question 12

How are Rayleigh and Love waves generated?

Answer 12

When P- and S-waves arrive at the Earth’s surface. The energy can not continue on into air, so some are reflected back down and the rest of the energy pushes and pulls the particles of the ground near the surface resulting in generation of Rayleigh and Love waves. They then travel along the surface, causing the damage we are all so worried about. Surface waves are much slower than body waves, and the energy carried by them can not depart from the surface along which they are traveling.

Question 13

Describe Rayleigh waves.

Answer 13

When a Rayleigh wave travels, particles experience a backward-rotating motion that is in line with the wave’s direction. These are the waves that cause the most damage because they are largest, and they are most clearly felt because they travel along the Earth’s surface. Rayleigh waves are those experienced by witnesses who say they felt as if they were in the ocean.

Question 14

Describe Love waves.

Answer 14

As a Love waves passes, particles experience a side-to-side motion that is perpendicular to the wave’s direction. This side-to-side motion is in a horizontal plane roughly parallel to the Earth’s surface.

Question 15

Particles experience a ______ motion in line with a Raleigh wave direction of travel. (bottom panel) Particles oscillate _____ and ______ to the direction of a passing Love wave.

Answer 15

backward-rotating; horizontally and perpendicular

Question 16

Which plane do Rayleigh waves cause motion?

Answer 16

x-z plane, perpendicular to the surface

Question 17

Which plane do Love waves cause motion in?

Answer 17

x-y plane; parallel to the surface

Question 18

Why does the whole pattern of energy resulting from an earthquake become very complicated very soon after the event?

Answer 18

All 4 wave types travel at their own velocity

Question 19

What parameters used to describe seismic waves?

Answer 19

The same parameters that describe any other type of wave: frequency, velocity, wavelength.

Question 20

How fast seismic signals travel in rock vs in air? What tone used to compare?

Answer 20

Travels 20 times faster in rock than in air. Compared with a Middle C tone.

Question 21

Frequency, velocity, wavelength of seismic waves in bedrock?

Answer 21

10Hz, 6500 m/s, 650 m

Question 22

Describe how the seismic energy travels?

Answer 22

When waves travel, its energy dissipates because rocks are not perfectly elastic. Seismic wave energy starts out as a very complicated mixture of all frequencies. The highest frequencies dissipate more quickly as waves travel through Earth’s materials.

This means that signals detected at greater distances will look different compared to signals recorded nearer the source. The signals detected there will consist primarily of the lower frequency components.

Question 23

What are amplifiers used for?

Answer 23

To boost traces of wave signals (frequencies)

Question 24

Compared to primary waves, secondary waves require (more)/(less) time to reach more distant seismograms.

Answer 24

more

Question 25

What happens to wave amplitudes with increasing distance from the epicentre?

Answer 25

They decrease with increasing distance from the epicentre. The greater the amplitude of the waves, the stronger the ground shaking.

Question 26

What happens to seismic waves at boundaries between differing materials?

Answer 26

Their velocity changes. Wave direction is bent when it travels across a boundary between materials with different elastic properties. (i.e. from slow to faster velocity, or from fast to slow) Refraction and reflection occur.

Question 27

What affects how seismic waves travel throughout the Earth?

Answer 27

Changes in wave direction imposed by changes in materials. Important: waves don’t travel straight through the planet.

Question 28

Examples of material boundaries in the Earth’s interior?

Answer 28

the boundary between the lithosphere and the mantle, the boundary between the mantle and the core, or boundaries (however smooth and subtle) between warmer upwelling mantle material (such as under a hotspot) and cooler downwelling or sinking, mantle material

Question 29

Seismic waves tend to travel along ____ pathways within the Earth. Why?

Answer 29

curved.

The pathways are curving because of the general increase in velocity with depth inside the Earth. Also, waves are bent and reflected at boundaries.

Question 30

What kind of seismic waves can’t travel through liquids?

Answer 30

Shear waves (S-waves)

Question 31

What happens before we can use seismic signals?

Answer 31

Once recorded, their pattern must be analyzed and important features to be identified.

Question 32

Main features that must be identified to locate an earthquake?

Answer 32

The relative times that P-waves and S-waves arrive. P-waves travel about 1.7 times faster than S-waves. Thus, the greater the difference in arrival times between these 2 wave types, the farther away the recording is from the earthquake origin. These arrival times do not represent the amount of time it took for signals to travel from the earthquake to the seismometer. At this stage of the process we don’t yet know where the earthquake occurred, so all we can do is notice how much time separates the P-wave and S-wave arrivals.

Question 33

What can we do once the relative times of arrival of P- and S-waves have been identified?

Answer 33

We can proceed to locate the earthquake’s epicentre.

If one distance is estimated, the earthquake location could be anywhere on a circle with radius equal to the distance from the seismometer (equivalent to the compass’s point in the figure). If three distances (epicentre to seismometer) can be estimated, the resulting three circles will intersect at the only possible location of the epicentre.

Question 34

Why do we need at least three widely separated locations of seismogram analysis to locate the source of an earthquake?

Answer 34

We need to determine difference in arrival times between P and S waves, convert the difference to distance, and plotting circles on a map, where the epicentre is the one point where all three circles intersect.

Question 35

How to convert seismic wave time into distance?

Answer 35

(ts-tp)(1/vs-1/vp); tp = d/vp, ts = d/vs

Question 36

How can earthquake magnitude be estimated?

Answer 36

observing the amount of ground motion

taking into account the materials through which waves traveled

the distance traveled

the physics of wave propagation

Question 37

Estimation of earthquake magnitude using nomogram?

Answer 37

M_L = 2.76log_d - 2.48 + log_10 A

Question 38

What’s an inverse question?

Answer 38

Question that involves using measurements to figure out structure or material properties.

Question 39

Inverse analysis works because ….

Answer 39

global signal paths for seismic waves are affected by changes in material properties. The patterns of travel time versus distance on the graph tell us about what’s hidden inside the Earth.

Question 40

What two parameters of seismic signals do we need to describe Earth’s structure or material properties?

Answer 40

The travel time for seismic signals (vertical axis) versus distance between source and measurement (horizontal axis). These can be measured. We need to derive how these two are related (i.e. velocity). This approach is more complicated than it looks because the seismic signals travel at different velocities through different materials. When seismic signals travel through several materials before being recorded, then the relationship between time and distance becomes even more complex.

Question 41

Thickness of crust?

Answer 41

0-100 km

Question 42

How deep outer core?

Answer 42

2900 km till 5100 km below surface (2200 km deep)

Question 43

Depth of inner core?

Answer 43

5100 km to 6378 km below surface

Question 44

Concentric layered model of our planet? Why do earthquake seismograms play a role in this?

Answer 44

It has a thin, hard outer crust floating on top of the lithosphere, which overlies the asthenosphere. The asthenosphere lies on top of the mantle, which overlie the two layers of core at the centre. This type of analysis, which yields a new understanding about the planet’s structure, could not be done without earthquake seismograms recorded all over the world.