CHAP 8: Earthquakes + Earth's Interior *

Which of the following is a characteristic of an S wave?

"shaking" particle motion at right angles to the direction of wave travel

List and describe each of Earth's major layers.

Inner core = a solid, dense sphere Outer core = is liquid, based on the absence of S waves traveling through the core. Upper and Inner Mantle = More than 82 percent of Earth's volume is in the mantle. Continental crust - Heterogeneous structure and composition Oceanic crust - Forms at mid-ocean ridges. Composed of basalt and gabbro.

Distinguish between intensity scales and magnitude scales.

Intensity: a measure of the degree of earthquake shaking at a given locale based on the amount of damage Magnitude: estimates the amount of energy released at the source of the earthquake

How can the epicenter location of earthquakes at an oceanic-oceanic convergence zone be used to determine which plate is being subducted?

The epicenters will be located on the plate that is not being subducted. - As one plate is being subducted, the foci of the earthquakes will delineate the location of that plate as it descends, while the epicenters will be located above them on the plate that is not being subducted.

A smaller earthquake in Virginia was felt over a larger distance, as compared to a larger earthquake in California. What is a reason this occurred?

colder crust - Recall the intensity maps associated with both the 6.0 magnitude VA earthquake and the 5.8 magnitude CA earthquake.

Earthquakes occurring at depths up to 700 km (435 mi) are associated with which of the following plate boundaries?

convergent boundaries - Due to subduction of cold, brittle slabs of oceanic lithosphere at convergent boundaries, earthquakes can occur at depths up to 700 km (435 mi).

Which of the following locations on the Circum-Pacific Belt are likely to generate a tsunami?

west coast of South America - Recall the convergent plate settings that surround the Pacific Ocean.

Early in the study of earthquakes, seismologists learned that P waves arrive at seismograph stations all over Earth, but a shadow zone exists for S waves. Which of the following responses best explains this phenomenon?

The outer core of Earth is liquid. - Because the energy in S waves is transmitted at right angles to the direction of travel, they don't travel through liquid, producing a zone on Earth where S waves do not arrive. This shadow zone confirms the liquid composition of the outer core.

Which of the following responses best describes the epicenter of an earthquake?

The point on Earth's surface directly above the focus of an earthquake. - Seismologists define the epicenter of an earthquake as that point on Earth's surface immediately above the point where the energy of the earthquake is released (focus).

__________ is a seismic zone that has not produced a large earthquake for a considerable length of time.

A seismic gap

What are the major zones of Earth's interior and which types of waves pass through each?

Crust, mantle, outer core, inner core - P waves can pass through the Earth's core. - S waves and surface waves CANNOT pass through the Earth's core. - P waves slow down and refract at the boundary between the mantle and the outer core and S waves do not travel through the outer core. - A shadow zone exists where S waves do not arrive in the side of Earth opposite the focus of an earthquake, as S waves do not travel through liquid, the shadow zone that exists on Earth indicates that the outer core is liquid.

Most earthquakes are the result of movement along which of the following features?

Faults - Earthquakes occur when there is movement along faults.

Which of the three types of seismic waves travels through rock with the greatest velocity?

P waves - As elastic energy is released in an earthquake, P waves are the first seismic waves to arrive at a seismograph.

As the distance between the epicenter of an earthquake and a seismograph station increases, so does the difference in the arrival times of the P and S waves. What causes this?

P waves are faster than S waves. - P wave velocity is greater than S wave velocity, so as the distance between the epicenter of the earthquake and the seismograph station increases, so does the difference between P and S wave arrival times.

A tsunami is caused by __________.

anything that displaces water

The hanging wall moves __________ relative to the footwall in a __________ fault.

down; normal - Think about the two types of faults shown in the video and how the rock blocks move relative to one another.

As stress is applied to rocks and deformation occurs, which of the following terms best characterizes the energy that is stored in the process?

elastic energy - As stress is exerted on rocks, they become deformed and store elastic energy. It is released when earthquakes occur and the rocks undergo elastic rebound. - This stored energy is released during an earthquake.

When a fault is expressed at the surface, it is called a __________.

fault scarp - This feature is formed when the Earth's surface breaks along a fault.

Amongst all seismic waves, surface waves __________.

have the slowest velocity

The name of the site where slippage begins and earthquake waves radiate outward is called the __________.

hypocenter - Think back to the diagram in the video. This feature is the location at depth where initial movement occurs.

A __________ is located at a plate boundary between a subducting slab of oceanic crust and an overlying plate.

megathrust fault - What is the name of the feature produced at convergent boundaries?

Which of the following might be seen where strike-slip movement has occurred?

offset fences

Earthquakes with a deep focus are most often associated with which of the following tectonic settings?

trenches - As a slab of oceanic lithosphere is subducted at trenches, this process can result in deep-focus earthquakes.

The names primary and secondary refer to ___________.

wave speed

What are the main factors that affect the amount of destruction caused by seismic shaking?

Amount of destruction attributable to an earthquake varies based on: Magnitude of the earthquake Proximity to the epicenter

List and describe the major destructive forces that earthquake vibrations can trigger.

Amplification of seismic waves - Soft sediments amplify seismic waves more than solid bedrock Liquefaction of the ground - Unconsolidated materials saturated with water turn into a mobile fluid Landslides and ground subsidence - Ground shaking causes loose sediments on a slope to slump Fire - Can start when gas and electrical lines are destroyed by an earthquake. Broken water lines make fire control problematic. Tsunamis, or seismic sea waves

What is an earthquake?

An earthquake is the vibration of Earth produced by the rapid release of energy. - Energy released radiates in all directions from its source — the focus - Energy is in the form of waves - Sensitive instruments around the world record the event

If the stress applied to the rock is greater than rock strength, what happens?

Blocks of rock will slide relative to one another along a fault, causing an earthquake. - Think about how deformation of rocks relate to the deformation of a stick.

Compare and contrast the types of seismic waves and describe the principle of the seismograph.

Body waves - Travel through Earth's interior Two types based on mode of travel: 1) Primary (P) waves 2) Push-pull (compress and expand) motion, changing the volume of the intervening material Travel through solids, liquids, and gases Generally, in any solid material, P waves travel about 1.7 times faster than S waves 3) Secondary (S) waves - Shaking motion at right angles to their direction of travel. Travel only through solids. Slower velocity than P waves. Slightly greater amplitude than P waves Surface waves - Travel along outer part of Earth. Complex motion. Cause greatest destruction. Waves exhibit greatest amplitude and slowest velocity. Waves have the greatest periods (time interval between crests). Seismographs are instruments that record seismic waves and records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

What are the types of earthquake waves? How do they differ?

Earthquakes generate three types of seismic waves: P (primary) waves, S (secondary) waves and surface waves, which arrive at seismic recording stations one after another. Both P and S waves penetrate the interior of the Earth while surface waves do not. Due to this, P and S waves are known as "body waves".

Compare and contrast the goals of short range earthquake predictions and long-range forecasts.

Short-range predictions - The goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame. Currently, no reliable method exists for making short-range earthquake predictions. Long-range forecasts - Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years or more

What do we call seismic waves that are transmitted along the outside of Earth?

Surface Waves - As elastic energy is released during an earthquake, some of the energy travels around the outside of Earth as surface waves.

Which of the following describes the curved path of seismic waves through the mantle?

Waves are concave up because rock density increases with depth. - Seismic waves travel faster through denser material.

How do waves refract in rocks?

Waves refract toward the boundary when moving from less to more dense material. AND Waves refract away from the boundary when moving from more to less dense material. - Waves will refract toward the boundary when moving into denser material and away from the boundary when moving into less dense material.

What best describes the movement of P waves?

compression and expansion

Visualize the hypocenter "focus" and epicenter of an earthquake.

look at picture

How is earthquake strength expressed? (two scales)

1) Intensity: a measure of the degree of earthquake shaking at a given locale based on the amount of damage 2) Magnitude: estimates the amount of energy released at the source of the earthquake The strength of an earthquake is measured using a Richter scale. The Richter scale accounts for the decrease in in wave amplitude with increasing distance.

On average, how many damaging earthquakes occur each year?

1,000 - Whereas few are considered large enough to be newsworthy, Earth experiences approximately 1000 damaging earthquakes annually.

Define earthquake intensity.

A qualitative measure of the amount of ground shaking at a certain location. - Earthquake intensity is not derived from a seismograph.

Which of the following responses provides the best reason for why the asthenosphere is not capable of storing elastic energy?

Elastic energy can only be stored in cold, brittle rocks. - Because the asthenosphere is composed of mantle material that acts like a plastic rather than a rigid solid, it cannot store elastic energy.

The point within Earth from which earthquake wave energy radiates is known by which of the following terms?

Focus - Two of these terms are often confused in the media, but one is a point at the surface and the other exists at depth. - The point from which wave energy associated with an earthquake radiates is known as the focus of the earthquake.

Which of the following events allows rocks on either side of a fault to rebound elastically, causing an earthquake?

Friction along the fault plane is overcome. - An earthquake occurs when the friction preventing or slowing movement along a fault is overcome, energy is released, and the rocks respond elastically.

How is the epicenter of an earthquake determined?

Location on the surface directly above the focus. Epicenter is located using the difference in velocities of P and S waves. Three station recordings are needed to locate an epicenter.

Why is moment magnitude (MW) preferred over Richter magnitude (ML)?

Moment magnitude measures the total energy released during an earthquake and can adequately measure the energy of large earthquakes.

Sketch and describe the mechanism that generates most earthquakes.

Movement along faults

List the major differences between P, S, and surface waves.

P-Waves... Travel the fastest, arrive first to a seismic station and are a push-pull wave where the motion of the material is back and forth (or parallel) to the direction of the wave as it travels forward. S-Waves... Travel 60% as fast as P-Waves, arrive second to a seismic station and are a shear wave where the motion of the material is up and down (perpendicular) to the direction of the wave as it travels forward. Surface Waves... Travel the slowest, arrive last to a seismic station, have the largest amplitude of the three types and cause the most destruction.

Which statement provides the best explanation of why most earthquakes occur at plate boundaries?

Plate boundaries are locations on Earth where portions of the lithosphere interact as they move past each other. - As portions of lithosphere move past each other at plate boundaries, earthquakes occur due to the buildup and release of stress.

In calculating the location of the epicenter of an earthquake, which of the following factors is most useful?

The difference in arrival time of P and S waves - Because the difference in arrival times of P and S waves at a seismograph station is related to the distance of the seismograph station from the earthquake, this calculation can be used to determine the location of the epicenter of an earthquake.

Richter magnitude (ML) is derived from measuring __________ and __________, and then plotting them on a Richter diagram.

S minus P wave time; maximum S wave height - Recall from the video the two measurements obtained from the seismograph reading.

Which of the following types of seismic body waves travels only through solids?

S waves - Due to the shaking motion of S waves perpendicular to the direction of travel, they travel only through solids. Liquids do not transmit shearing forces.

Which of the following motions best describes the movement of S waves as they travel through rocks following the release of energy during an earthquake?

Shaking particles at right angles to the direction of travel - As S waves move through rock, the particles are shaken at right angles to the direction of travel, much as a rope goes up and down when snapped. -S waves are sometimes referred to as "shear waves."

During the 1906 San Francisco earthquake, the Pacific Plate moved 4.7 meters (15 feet) north relative to the North American Plate. Which of the following types of stress was exerted on the rocks during this earthquake?

Shear Stress - As the Pacific Plate moved north relative to the North American Plate, shear stress was exerted on the rocks.

Which type of seismic waves generally cause the most structural damage as they travel through Earth?

Surface waves - Because surface waves have high amplitude and can have a component of horizontal movement as well as vertical movement, they tend to cause the most damage to structures as they move through Earth.

Explain how Earth acquired its layered structure and briefly describe how seismic waves are used to probe Earth's interior.

The heavy elements sank to the bottom and made the outer and inner cores. Lighter elements didn't sink as far and formed the mantle and crust. Differentiation of the material when our planet was entirely molten. The less dense material "floats" toward the surface and the denser material "sinks" towards the core. Seismic waves are reflected, refracted, and diffracted as they pass through the planet. They reflect off boundaries between different layers, they refract when passing from one layer to another layer, and they diffract around any obstacles they encounter. This gives us a "picture" of what the interior is made of.

Where are the principal earthquake zones on Earth?

The principal earthquake epicenter zones are along the outer margin of the Pacific Ocean and the oceanic ridge system.

In addition to a travel-time graph, at least how many seismograph stations are needed to determine the location of the epicenter of an earthquake?

Three - You need a minimum of three seismograph stations to locate the epicenter of an earthquake using a travel-time graph. - Determining the location of the epicenter is sometimes referred to as "triangulation."

What happens to tsunami waves as they move closer to shore?

Velocity decreases and height increases. - Think about how tsunami wave changes are influenced by the depth of the ocean.

What is a tsunami?

aka seismic sea waves. Destructive waves that are often inappropriately called "tidal waves". Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake. In the open ocean height is usually less than 1 meter. In shallower coastal waters the water piles up to heights that occasionally exceed 30 meters. Can be very destructive!