Chapter 5: Earthquakes and Related Hazards

Question 1

How do earthquakes occur?

Answer 1

By definition, earthquake is the shaking or trembling of the surface of the Earth which is caused by the sudden release of energy stored in the rocks beneath the earth’s surface.

Question 2

How did the scientist develop the theory that explains how earthquakes occur?

Answer 2

based on the elastic properties of rocks

Question 3

this process is when rocks are subjected under a force (stress), they can become deformed and have a corresponding change in their shape (distortion) or volume (dilation).

Answer 3

Strain

change in volume or shape

Question 4

rocks are considered to be this type meaning that if the force (stress) is removed they will return to their original shape.

Answer 4

Elastic

Question 5

A property of all elastic materials which is the maximum amount of strain they can accumulate before either fracturing or undergoing plastic deformation.

Answer 5

Elastic Limit

Question 6

What happens when brittle materials reach their elastic limit?

Answer 6

they will undergo permanent deformation by fracturing

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane.

Question 7

What happens when ductile materials reach their elastic limit?

Answer 7

they will undergo permanent deformation by **flowing plastically **

Question 8

Two types of Earthquake

Answer 8

1. Volcanic
2. Tectonic

Question 9

type of earthquake due to volcanic activity (eruption or rising magma under a volcano)

Answer 9

Volcanic

Question 10

type of earthquake due to movement of rocks past one another along faults. when a rock breaks, waves of energy are sent out or produced, known as seismic waves, causing earthquakes.

Answer 10

Tectonic

Question 11

Will both iron and wooden rods will deform and return to their original shape? How?

Answer 11

Yes, as long as their elastic limit is not exceeded.

Question 12

What happens when an iron rod exceeds its elastic limit?

Answer 12

It deforms permanently by bending.

Question 13

What happens when a wooden rod exceeds its elastic limit?

Answer 13

it will suddenly break by fracturing, releasing energy in the form of vibrational waves.

Note: when fractured rods break, the separate pieces rebound and become straight again.

Question 14

This theory holds that earthquakes originate whena a force (stress) acts on a rock body, causing it to deform and accumulate strain. Eventually the rock reaches its elastic limit, at which point it ruptures or fails suddenly, releasing the strain it had accumulated.

Answer 14

Elastic Rebound Theory

Question 15

Where the release of energy generally begins at this point

Answer 15

Focus/Hypocenter

Question 16

point on the earth’s surface directly above the hypocenter

Answer 16

Epicenter

Question 17

What happens when rocks become more ductile (less brittle)?

Answer 17

they tend to accumulate less strain, and instead undergo plastic deformation.

this is the reason why earthquakes do not occur deeper than 435 miles (700 km) below the surface.

Question 18

Why are there no earthquakes occuring at deeper than 435 miles (700 km) below surface?

Answer 18

because higher temperatures cause the rocks to become so ductile that they deform only by plastic flow, hence they do not rupture.

Question 19

redistribution of strain commonly produces a series of smaller earthquakes

Answer 19

aftershocks

Question 20

occurs for days or weeks after the primary earthquake

Answer 20

main shock

Question 21

are earthquakes that precede larger earthquakes in the same location. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or “mainshock.”

Answer 21

Foreshock

An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.

Question 22

refer to vibrational waves that travel through solid earth materials which may be magmatic, tectonic, or aritifical in origin.

Answer 22

Seismic Waves

Question 23

Two classification of Seismic Waves

Answer 23

1. Body waves
2. Surface waves

Question 24

travel through the earth’s interior, spreading outward from the hypocenter in all directions (like sound in air). it is subdivided into primary and secondary waves.

Answer 24

Body waves

Question 25

compressional waves; parallel to direction the wave is travelling, causing rocks to alternately compress and decompress as successive waves pass through

Answer 25

Primary (P) waves

Question 26

transverse/perpendicular to direction of wave propagation

Answer 26

Secondary (s) waves

Question 27

waves that travel on earth's surface away from teh epicenter (like ripples on water); slowest wave (at speed of 10% slower than S-waves), can cause more property damage compared to body waves

Answer 27

Surface waves

Question 28

two types of surface waves

Answer 28

1. rayleigh 2. love

Question 29

known as ground roll, spread to the ground as ripples, similar to rolling waves on the ocean; move both vertically and horizontally in a vertical plane pointed in the direction in which the wave is travelling.

Answer 29

Rayleigh waves

Question 30

move the ground from side to side in a horizontal plane but at right angles to the direction of propagation.

Answer 30

Love waves

Question 31

Difference of **P-waves** to S-wave

Answer 31

- very fast at speeds of 4 to 7km/sec - first wave to arrive at a station - Can pass through solid and liquid

Question 32

Difference of **S-waves** to P-wave

Answer 32

- slow, at 2 to 5 km/sec - arrives at a later time than p-wave does - can pass through solid but not liquid

Question 33

particulary damaging to the foundations of structures

Answer 33

love waves

Question 34

propagate through the ground as ripples

Answer 34

rayleigh waves

Question 35

instrument used to detect seismic waves. determine the amount of ground motion, but does not record the motion.

Answer 35

Seismometer

Question 36

a seismometer with a recording device that produces a permanent record of earth motion, usually in the form of wiggly line drawn on a moving strip of paper.

Answer 36

Seismograph

Question 37

the paper record of earth vibration. different waves travel at different rates, so they arrive at seismograph stations in a definite order: first p waves, then s waves and finaly surface waves.

Answer 37

Seismograms

Question 38

the importance of the analysis of seismograms

Answer 38

can reveal the location and strength of earthquake

Question 39

Process of locating earthquakes

Answer 39

1. p and s waves that **start out from the hypocenter** 2. as they travel, they **gradually separate because of their different speeds**. 3. the interval of the **time of arrival between P and S waves increases with increasing distance of the seismic stations from the focus and epicenter**; **the longer the time, the greater the distance is.**

Question 40

True or False. A single station can record only the direction, not the distance to a quake.

Answer 40

False. A single station can record only the **distance**, not the **direction** to a quake.

Question 41

pinpoints the location of the earthquake

Answer 41

the intersection of the three circles

Question 42

classification of an earthquake with depth of focus at 0-70 km

Answer 42

Shallow

Question 43

classification of an earthquake with depth of focus at 70-350 km

Answer 43

Intermediate

Question 44

classification of an earthquake with depth of focus at 350-670 km

Answer 44

Deep

Question 45

whereby earthquakes are ranked based on a set of observations most humans could report objectively, particularly the type of damage sustained by buildings.

Answer 45

Mercalli intensity scale

Question 46

An italian seismologist who developed a means of comparing both modern and historical earthquakes through the use of firsthand human observations during earthquakes (1902).

Answer 46

Giuseppe Mercalli

Question 47

is a seismic scale used and developed by the Philipine Institute of Volcanology and Seismology **(PHIVOLCS)** to measure the intensity of an earthquake.

Answer 47

**PHIVOLCS Earthquake Intensity Scale** (PEIS)

Question 48

How was the **PEIS** developed?

Answer 48

developed as a response to the **1990 Luzon Earthquake (magnitude 7.7)** and was adtoped in the Philippines in 1996 replacing the **Rossi-Forel Intensity Scale**.

Question 49

one of the first seismic scales **developed by Michele Stefano de Rossi and Francois-Alphonse Forel** in the **late 19th century**, to reflect earthquake intensities and was used for about two decades by some countries until the introduction of the Mercalli Intensity Scale in **1902.**

Answer 49

**Rossi-Forel Intensity Scale**

Question 50

are useful because they quantify the amount of ground motion during an earthquake, and the energy that was released when the rocks ruptured.

Answer 50

Magnitude Scale

Question 51

rates earthquaks based on the size of their seismic waves, as measured by seismographs; governed by amplitude (wave height) and distance.

Answer 51

**Richter Magnitude Scale** (named after the seismologist, Charles F. Richter)

Question 52

is more acurate over a wide range of magnitudes and geologic conditions; based on the total amount of energy released and is determined by measuring the surface area of the ruptured fault and how far the land moved along the fault.

Answer 52

**Moment Magnitude Scale**