ISNS 2329 Earthquakes and Volcanos: Unit 3

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Acceleration during an earthquake is relative to

Acceleration due to gravity

Footwall

Block below the fault plane

Reverse faults form in response to

Compression stress

Seismic waves travel

Faster through hard rocks than they do soft rocks or sediment

Strike-slip fault

Fault where the slip is predominantly horizontal and parallel to the fault plane

Most accurate measure of the size of large earthquakes

Moment magnitude (Mw)

Magnitude 7 earthquake has an amplitude

1,000 times greater than a magnitude 4 earthquake

Types of buildings most vulnerable to high-frequency P and S waves close to the epicenter of an earthquake

1. Buildings constructed of stone or brick 2. Short buildings

Fastest to slowest P wave velocity

1. Granite rock 2. Packed sand 3. Loose sand 4. Water 5. Air

Four types of seismic waves in order of decreasing velocity with fastest at the top

1. P waves 2. S waves 3. Love waves 4. Rayleigh waves

Arrival times of these two waves are used to calculate the location of an earthquake's epicenter

1. S wave 2. P wave

2 types of body waves

1. S waves 2. P waves

Earthquake with the largest moment magnitude calculated was

1960 Chile

Minimum number of seismographs needed to determine the location of an epicenter of an earthquake

3

Best time for an earthquake in California is

3 a.m.

Magnitude 5 earthquake is

48 times greater than a magnitude 4 earthquake

A fault rupture of 10 km produces

A magnitude 6 earthquake

Earthquake intensity

A measure of the effect of an earthquake on people and buildings; Greatest in areas underlain by sediment

Earthquake magnitude

A measure of the energy released during an earthquake

Principle of Intertia

A stationary object will remain stationary unless acted upon by an external force

Methods to seismically retrofit a building

A. Add buttresses B. Infill walls to create shear walls C. Isolate the building from the ground D. Add bracing

Richter magnitude scale is only useful for assessing earthquakes that

A. Are moderate in size B. Occur nearby

Ways a building can be designed to eliminate the occurrence of resonance

A. Change the height of the building B. Change the degree of attachment to the foundation C. Change the type of building materials

Describe an S wave

A. Has a shearing motion B. Cannot travel through fluids C. The second to reach a recording station; Can do severe damage to buildings

Intensity of an earthquake varies with

A. Magnitude B. Distance from epicenter C. Bedrock type D. Building style

Describe a primary P wave

A. Moves in a push-pull fashion B. The first to reach a recording station C. Can travel through any material

S waves can travel through

A. Mudstone B. Sandstone C. Granite rock

Nature of Surface Waves

A. Slow moving B. Low frequency

Dip-fault slips generally form in response to

A. Tensions forces B. Compressional forces

Acceleration during an earthquake is related to

Acceleration due to gravity; 9.8 m/ sec^2 aka 1.0g

Period

Amount of time it takes for a building to sway back and forth one time

How magnitude is measured

Analyzing seismic wave traces on a seismogram

Seism

Another term for earthquake

Long (L) waves aka surface waves

Are referred to as such because they take more time to complete one cycle of motion and are the slowest moving

The decrease in velocity in both P and S waves at about 100 km depth marks the top of the

Asthenosphere

Faults can be recognized in the field, even if the fault plane itself is not exposed

Because faults offset rock units

Why it often takes weeks to determine magnitude for great earthquakes

Because the area of the rupture zone is defined by aftershocks, which continue for many weeks following a great earthquake

Inertia

Causes heavy weight to not move with the framework of a seismograph

Where high frequency seismic waves cause the most damage

Close to the epicenter

P wave velocity

Depends on the material through which the wave passes

Normal faults are most likely found at a

Divergent plate boundary

Decreases with the distance from the epicenter

Earthquake intensity

Aftershocks

Earthquakes that follow the mainshock; Common because the movement along the fault increases stress on adjacent sections of that fault.

Seismic waves

Energy released when a fault ruptures radiates outward from the hypocenter

Transform Faults

First recognized by Wilson in 1965; special horizontal-movement fault that connects the ends of two offset segments of plate edges; Allow spreading centers to wrap around the curved surface of the earth

Veins of valuable ores are common along faults because

Fluids containing dissolved metals can more easily flow through fault zones

Concrete

Great compressional strength, vulnerable to brittle failure

Left-lateral strike-slip fault

If a feature truncated by a strike-slip fault is displaced to the left on the other side of the fault

Right-lateral strike-slip fault

If a feature truncated by a strike-slip fault is displaced to the right on the other side of the fault

Base isolation

In the form of ball bearings or shock absorbers protects a building from the worst of the ground shaking

Seismigraph

Instrument used to record seismic waves

Seismic Moment (M0)

Is equal to the shear strength of rocks times the rupture area of the fault times the average displacement of the fault

Mainshock

Largest earthquake in a series that occurs on the same portion of a fault

Wood

Lightweight, flexible

Types of surface waves

Love Waves and Rayleigh waves

Magnitude

Measure of the amount of energy released during an earthquake

Why it's of greater concern to design buildings to withstand horizontal ground movement rather than vertical

Most buildings are already designed to handle the vertical forces associated with their own weight

Frequency

Number of waves passing a given point during one second

Body wave (mb) scale for earthquake magnitude is based on amplitudes of

P waves with 1 to 10 second periods

Fracture

Planar feature where no movement occurs

Fault

Planar feature where two sides move past each other; A fracture in bedrock along which sliding has occured

Epicenter

Point on earth's surface where a fault first ruptures

Hypocenter

Point where a fault first ruptures

Mercalli Intensity Scale

Ranges from 1 (not felt) to 12 (damage nearly total)

Acceleration

Rate of change in velocity of the ground as it is moved by seismic waves

Surface wave (Ms) scale for earthquake magnitude is based on the amplitudes of

Rayleigh waves with 18 to 22 second periods

When the period of seismic waves is equal to the period of a building

Resonance can occur, which causes catastrophic failure

San Andreas fault in CA is the most famous example of a

Right-lateral strike-slip fault

Body waves

Seismic waves that pass through the interior of the planet

Surface waves

Seismic waves that travel across the surface

Foreshocks

Smaller earthquakes that precede the mainshock

P waves can travel through air, where they take the form of

Sound waves; sonic boom

When bracing is incorporated into a building's design, it's usually constructed of steel because

Steel is flexible

Law of Original Continuity

Tells us that when a sedimentary rock layer abruptly terminates, something must have happened to it after it formed; Sedimentary layers extend laterally until they thin out at their edges.

Charles Richter based his scale on the idea that

The bigger the earthquake, the greater the shaking of the earth

Using S-P time from a single seismograph, what can be determined about the epicenter

The distance from

Normal Dip-slip faults occur when

The hanging wall moves down relative to the football.

Reverse Dip-slip faults occur when

The hanging wall moves up relative to the footwall

The greater the length of the fault rupture

The lower the frequency of the seismic waves it produces.

Why S waves disappear at the core-mantle boundary

The outer core is liquid, and S waves can't travel through a liquid

Seismology

The study of earthquakes

Seismologists must record the movement of earthquake waves in

Three directions

Most critical factor when determining loss of life from an earthquake

Time and day of event

Why geologists map the location of active faults

To assess earthquake hazards

Retrofit

To reinforce a building to increase its resistance to seismic shaking

Law of original horizontality

Used to determine that rock layers were deformed into folds after they were deposited

Earthquake

Vibrations we feel when sudden movement occurs along a fault

Steel

Vulnerable to compressive stress

Shear walls

Walks designed to take horizontal forces from floors and roofs and transmit to the ground

An uplifted mound or hill

When a left-lateral strike-slip fault has a right-stepping bend

How to recognize a left-lateral strike-slip fault

When you straddle the fault, the left-hand side has moved toward from you

Law of Superposition

Within a sequence of undisturbed sedimentary, the oldest layer is at the bottom

If the S-P Time is 4.5 minutes

You are 3,000 km away from the epicenter


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