Midterm Study Guide 3

Question 1

How does a seismograph work?

Answer 1

Ones designed to detect vertical movement: a contraption with a spring and a ball of mass
attached to it that has a recording pen attached to it. When the earth moves up and down, so does the spring and lines are written on a scroll of paper.
Ones designed to detect horizontal movement: there is a contraption with this pole on a hinge that can move side to side with the ball of mass that has the recording pen. When the earth moves right and left, it records on a scroll of paper lying on its side.

Question 2

How does one locate an earthquake?

Answer 2

You take the arrival times of P and S waves from three different seismographs from 3 different locations in order to triangulate the point the location of the earthquake

Question 3

What’s magnitude?

Answer 3

● The amount of energy released as the rock breaks.
● Log base 10 Richter scale. For every change in mag, ground amplitude increase by a factor
of 10 and energy by a factor of 30.

A logarithmic scale based on the maximum amplitude of ground motion, recorded on a standard seismograph, correcting for the distance to the source
ML = log10 (amplitude of seismograph) + distance correction
Total energy and rupture area are better estimates of earthquake size than the maximum amplitude of seismic waves recorded on a seismograph.

Question 4

What’s intensity?

Answer 4

Intensity I, or MI
Based on extent of damage and human perception (modified Mercalli scale)

● The measure of damage and deaths an earthquake caused. Mercalli intensity scale quantifies the perceived intensity. It’s possible to have a high magnitude earthquake with a low intensity.

Question 5

What’s moment? no need to know how to calculate

Answer 5

Moment M0
Based on physical quantities relating to the source.

● M_0= GuA
○ G = shear modulus
○ A = LW = size of area displaced

○ u = surface displacement, offset, or fault
slip

Question 6

Name four hazards associated with earthquakes.

Answer 6

● Mass Movements
● Tsunamis
● Fires
● Epidemics­­caused by destruction of infrastructure

Question 7

How does one determine the ‘probability’ of an earthquake?

Answer 7

● if you take how often earthquakes usually occur along a fault line and then see how long it has been since the last earthquake, you can get a rough estimate for the probability that an earthquake will occur

● fault zone geometry

Question 8

Why does the same size earthquake in two different regions kill vastly different numbers of people?

Answer 8

● One region may have a larger Earthquake Risk than the other.
○ Earthquake Risk is the Earthquake Hazard
(the proximity to faults, the maximum
earthquake magnitudes on those faults, and
the local geology) multiplied by the
Construction (whether the structures of the
buildings there are strong or weak). This
equation estimates the Earthquake Risk, or
the likelihood of the loss of life and
property.

Question 9

Why is the degree of shaking around an earthquake epicenter not evenly distributed in all directions?

Answer 9

● The uneven distribution of shaking is due to the different composition of earth around the epicenter. Areas that composed of more densely packed material such as clay, will experience less shaking, because energy will be absorbed. However, areas with porous material such a gravel will experience heavier shaking due to greater shock of energy, because there is nothing to absorb the energy.

Question 10

Where will the next ‘big’ earthquake occur?

Answer 10

● Southern San Andreas

● Bay area

Question 11

stress and force

Answer 11

Stress = force/area (unit: N/m^2)

Question 12

travel time

Answer 12

time lapse between P and S wave

Question 13

Seismic gap

Answer 13

A segment of an active fault known to produce significant earthquakes, that has not slipped in an unusually long time when compared with other segments along the same structure.

Question 14

magnitude

Answer 14

A logarithmic scale based on the maximum amplitude of ground motion,
recorded on a standard seismograph, correcting for the distance of the source.

Question 15

earthquake engineering

Answer 15

The application of civil engineering to reduce life and economic losses due to earthquakes. From an engineering perspective, seismic risk is: the probability of losses occurring due to earthquakes within the lifetime of a structure.

Question 16

moment

Answer 16

A quantity used by earthquake seismologists to measure the size of an earthquake. The scalar seismic moment Mo is defined by the equation Mo=\mu AD, where:

• \mu is the shear modulus of the rocks involved in the earthquake (in dyn/cm2)
• A is the area of the rupture along the geologic fault where the earthquake occurred (in cm2), and
• D is the average displacement on A (in cm).

Mo thus has dimensions of energy, measured in dyne centimeters.
The seismic moment of an earthquake is typically estimated using whatever information is available to constrain its factors.

Question 17

fault­

Answer 17

a fracture in earth’s crust that is the locus of relative ground movement or rupture.
Earthquakes happen at faults, and they’re identical to plate boundaries. The larger the fault,
the larger the earthquake.

Question 18

Gutenberg­-Richter relationship

Answer 18

for every increase in magnitude on the richter scale, the number of earthquakes per year decreases by a factor of ten.

Question 19

three types of faults/earthquakes

Answer 19

Transform Fault Normal Fault, and Reverse Faults

Question 20

seismic cycle

Answer 20

Reid’s (1910) Elastic Rebound Theory:

Rocks deform elastically, then rebound during an earthquake rupture.

Question 21

Rayleigh wave

Answer 21

a surface wave that moves like an ocean wave, except that the particle in motion is retrograde elliptical.

Question 22

Tsunami

Answer 22

Giant wave caused by earthquakes or volcanic activity under the sea
● In the sea, tsunami waves do not dramatically increase in height because their wavelengths
are long
● Once they reach shallow water, they slow down and grow in energy and height
● A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. when
it does, it produce a vacuum effect that sucks coastal water seaward and exposes sea floors(
warning sign of tsunami)

Question 23

epicenter

Answer 23

a. the point on the Earth’s surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates.

Question 24

precursor (for earthquakes)

Answer 24

animal behavior, water levels, increase/decrease in seismicity before the main event, electromagnetic signals, changes in seismic wave speeds

Question 25

S wave

Answer 25

also called shear waves because they don’t change the volume of the material they propagate, they shear it. Also known as secondary waves

Question 26

period and frequency:

Answer 26

Period is the time required for one complete cycle of vibration to pass given point. Frequency of a wave is the number of wave crests that pass a point in one second. As the frequency of a wave increases, the period of the wave decreases.

Question 27

Strain

Answer 27

a description of deformation in terms of relative displacement of particles in the body that excludes rigid-body motions.

Question 28

Body Wave

Answer 28

consists of Primary waves (P-waves) and secondary waves (S-waves)

    a. Body waves travel through the interior of         
        the Earth.

Question 29

Intensity

Answer 29

A measure of earthquake impact. Found by taking the energy density at a point in space and multiplying it by the velocity at which the energy is moving.

Question 30

Omori’s law (empirical law)

Answer 30

the rate of aftershocks is inversely proportional to the time since the main shock. Ex) the probability of an aftershock at day two is half that of the probability on day one (the day after the earthquake). At day ten, it’s a tenth of day one, etc.

Question 31

rupture

Answer 31

During seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically the focus lies towards one end of the slip surface and much of the propagation is unidirectional

Question 32

Early Warning

Answer 32

Systems set up to warn of earthquake - usually only seconds notice.

Question 33

attenuation

Answer 33

The decrease in amplitude of a wave as it radiates outward. Shaking decays with distance from the eq/fault, loses energy as more and more is transformed to heat.

Question 34

paleo-seismology

Answer 34

looking & old sediments & rocks to find signs of ancient EQs

fundamental period

Question 35

seismic wave

Answer 35

Surface waves (arrive last on the seismograph,do most damage during earthquake because they travel along the surface): Rayleigh (perpendicular to surface) and Love waves(parallel to surface)

Body waves: P-waves arrive first on the seismograph and S-waves arrive second on the seismograph

Question 36

liquefaction

Answer 36

When non-densely packed material is shifted, typically an earthquake, and the material beings to act like liquid. Meaning objects will “sink” into soil and gravel that is not densely compact and heavily porous leaving room for shifting.

Question 37

seismograph

Answer 37

a graph that records ground motion

Question 38

P wave

Answer 38

The P-wave is a type of body wave that is capable of traveling through the earth at a velocity of around 5 to 8 km/s, and is the first wave from an earthquake to reach a seismometer.

Question 39

surface wave

Answer 39

travel along the earth surface, visibly like a ocean waves, slower velocity than body waves ( P and S waves)

Question 40

risk equation

Answer 40

Risk = hazard x vulnerability (vulnerability = [exposure x fragility]/resiliency)

Question 41

Bath’s law

Answer 41

(empirical law)- The largest aftershocks are about 1.2 M smaller than the main shock.

Question 42

aftershock

Answer 42

Happen after major earthquakes, and occur on and around the edge of the rupture area.

Question 43

normal mode

Answer 43

A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. Each building has its own normal mode, so a mismatch of normal modes can cause buildings to collide into each other.

Question 44

Love wave

Answer 44

a surface wave that is trapped inside the crust of the Earth - these are the only waves you can trap.

Question 45

site amplification

Answer 45

shear stress = shear modulus x strain; sites with a lower shear modulus (sediments) will have more strain than sites with a higher shear modulus (bed rock) and will therefore amplify the propagation of the earthquake.

Question 46

seismic hazard

Answer 46

the likelihood and intensity of strong ground motion and other damaging effects at any location.