ISNS 2329 Earthquakes and Volcanos: Unit 3
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
