Lesson 2: Earthquakes Flashcards
For rocks to be deformed, they must be acted upon by stress which can be classified into three (3) basic types. what are they?
compression, tension, shear
stress that pushes on rocks from opposite directions which causes rocks to be shortened parallel to the stress applied
compression
stress that pulls rocks from opposite directions, resulting it to become stretched/lengthened
tension
stress that occurs when rocks are being pushed in an uneven manner, causing the rocks to be skewed such that different sides of a rock body slide or move in opposite directions
shear
Rocks near the surface of the earth are _______
elastic
when a force (stress) that is acting on them is removed, the rocks will return to their original shape
elastic
the point in which rocks no longer behave elastically and deformation becomes permanent.
elastic limit
the specific fracture along a fracture plane where rocks slide past one another
fault
do all fractures involve slippage/movement? do all faults?
all faults do, not all fractures, some are just joints
What type of fault is the North Bohol Fault (NBF)?
Reverse Fault with minor right- and left lateral displacement
what type of fault is the PFZ
left lateral fault
thrust vs reverse fault
thrust -gentler slope (less than 30 degrees
reverse -steeper
fault scarp vs fault-line scarp
fault scarp is fresh, fault-line scarp if underwent significant weathering
location of north bohol fault
uh several towns, but famously a scarp is seen in Anonang Inabanga, Bohol
He is a Scottish geologist who authored the The Dynamics of Faulting and Dyke Formation with Application to Britain (Edinburgh, 1942, 1951)
Ernest Masson Anderson
He systemized our knowledge of the geometry and stress fields of various faults.
Ernest Masson Anderson
According to a specific famous Scottish geologist, the direction of the maximum principal stress along normal faults is ?
vertical
refer to vibrational waves that travel through solid earth materials
seismic waves
Types of origins of seismic waves
magmatic, tectonic, or artificial
2 classifications of seismic waves
body waves
surface waves
waves that - travel trough the earth’s interior, spreading outward from the hypocenter in all directions (like sound in air)
body waves
subdivisions of body waves
primary waves
secondary waves
compressional waves; parallel to direction the wave is travelling, causing rocks to alternately compress and decompress as successive waves pass through.
primary (P) waves
Body waves that travel transverse/perpendicular to direction of wave propagation
secondary (S) waves
travel on the earth’s surface away from the epicenter (like ripples on water); slowest wave (typically at a speed that is 10% slower than S-waves), can cause more property damage compared to body waves.
surface waves
2 basic types of surface waves
Rayleigh waves
Love waves
waves also 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;
Rayleigh waves
Surface waves that move the ground from side to side in a horizontal plane but at right angles to the direction of propagation.
Love waves
speed of waves P and S?
P →4 to 7 km/s
S →2-5 km/s
describe the order of arrival of the 2 types of body waves
P is first to arrive, S arrives at a later time than P
describe the mediums of transport of the two types of body waves
P →can pass through solid and liquid
S →cannot pass through liquid, only solid
the instrument used to detect seismic waves.
seismometer
describe how a seismometer works
➮A heavy suspended mass is held as motionless as possible
➮suspended by springs or hanging it as a pendulum.
➮When the ground moves, the frame of the instrument moves with it.
➮The inertia of the heavy mass keeps it from moving and act as a point of reference in determining the amount of ground motion, but does not record the motion
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.
seismograph
the paper record of earth vibration.
seismogram
explain how a seismogram works
➮The different waves travel at different rates, so they arrive at seismograph stations in a definite order: first P waves, then S waves, and finally, the surface waves.
➮Analysis of seismograms can reveal the location and strength of the earthquake.
describe how earthquakes can be located
- P and S waves start out from the hypocenter.
- As they travel, they gradually separate because of their different speeds.
- 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.
➮The interval of arrival between S and P waves is used to calculate the distance of the seismograph station from the earthquake source.
➮The increase in P-S interval increases with distance so a travel-time curve can be constructed from earthquake records.
describe how one station vs many can record an earthquake
➮A single station can record only the distance, not the direction to a quake.
➮The location of an earthquake is determined by drawing circles on a map (or globe) with the seismograph stations distributed in different parts of the globe as the centers and the corresponding
distances from the earthquake as the radii.
➮The intersection of the three circles pinpoints the location of the earthquake.
classification of earthquakes according to the depth of focus
1) Shallow – 0-70 km
2) Intermediate – 70-350 km
3) Deep – 350-670 km
How were the major layers of the earth inferred?
basically seismic surveys.
changes in seismic wave velocities/behavior that depend on the change of density/characteristic of earth layers
the layer in which almost all rocks that are exposed on the Earth’s surface are formed
lithosphere
defined as a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the earth’s surface.
earthquake
Prior to modern science, many people believed what about earthquakes?
many people believed earthquakes were random events; some even thought they were punishment by gods for evil or immoral behavior.
when rocks are subjected under a force, also called ________, they can become deformed and have a corresponding change in their shape (_______) or volume (________), a process known as __________;
stress;
distortion;
dilation;
strain
rocks are also considered to be _______, meaning that if the force is removed, they will ________.
elastic;
return to their original shape
maximum amount of strain they can accumulate before either fracturing or undergoing plastic deformation
elastic limit
When brittle materials reach their elastic limit they undergo __________deformation by________, whereas ductile materials deform by _________.
permanent;
fracturing;
flowing plastically
2 types of earthquakes
volcanic;
tectonic
describe volcanic earthquakes
earthquakes due to volcanic activity (eruption or rising magma under a volcano)
earthquakes due to movement of rocks past one another along faults.
tectonic earthquakes
tectonic earthquakes occur when a rock breaks, ________ are sent out or produced, known as ________, causing earthquakes
waves of energy;
seismic waves
Based on the relationship between stress and strain and the deformation of rocks, earth scientists have developed the _________ that explains the occurrence of earthquakes
elastic rebound theory
explain the elastic rebound theory
➮This theory holds that earthquakes originate when 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.
➮This sudden release of strain, lasting anywhere from several seconds to a few minutes, is transformed into vibrational wave energy that radiates outward and causes the ground to shake
The release of energy generally begins at a point called ?
focus/hypoceneter
the point on the earth’s surface directly above the hypocenter is termed as ?
epicenter
When rocks become more ductile (less _____) they tend to _______, and instead undergo _________.
brittle;
accumulate less strain;
plastic deformation
why do earthquakes not occur deeper than a specific depth below the earth’s surface?
because of plastic deformation. below this depth, the higher temperatures cause the rocks to become so ductile that they deform only by plastic flow, hence do not rupture
depth past where earthquakes no longer occur
700 km
a series of smaller earthquakes which may continue to occur for days or weeks after the primary earthquake
aftershocks
how do aftershocks happen?
Rocks in tectonically active areas usually contain numerous faults and the sudden release of strain along one fault can alter the distribution of strain on the other faults. This redistribution of strain commonly produces the aftershocks
aka the primary earthquake
main shock
what type of force are earthquakes much stronger? implication?
under compressional force, thus convergent boundaries where compressive forces dominate, rocks are able to accumulate much more strain before rupturing than at divergent boundaries where tensional forces are dominant.
also under shear forces in transform fault boundaries
aside from the type of force, what other factor is key to the ability of a rock body to store strain? explain.
frictional resistance.
➮In areas where tensional forces dominate the friction along faults is naturally low, allowing them to slip in an almost continuous process.
➮When a rock body experiences this, it obviously cannot build up much strain, which helps explain why large magnitude earthquakes generally do not occur at divergent boundaries.
the process of an almost continuous slipping of faults in low friction areas
fault creep
type of fault: San Andreas Fault
transform fault, right-lateral (dextral) transform fault that separates the Pacific and North American Plates.
why is the San Andreas Fault often referred to as a fault zone?
due to the network of interlocking faults located on either side.
why can strain relieved along one fault disrupt the delicate balance of relationships within the fault zone, triggering additional earthquakes.
because of the interlocking nature of fault zones
In northern California where the San Andreas fault moves offshore, the boundary of the North American plate changes from _______ setting to one of ________.
from a transform (shear) setting to convergence (compression)
a series of relatively small oceanic plates overridden by the north American plate
Cascadia subduction zone
what is produced/generated by the Cascadia subduction zone
- Cascade Mountain Range
- subduction zone earthquakes
4 key factors as to why subduction zone earthquakes are capable of releasing unusually large amounts of energy
(1) the way the overriding plate buckles and becomes locked.
(2) the surface area over which the slippage or rupture occurs can be quite large compared to that in other plate settings.
(3) the descending oceanic plate is relatively cool, which makes the rocks more brittle and capable
of accumulating more strain before rupturing.
in addition to the intense ground shaking,
(4) some of this energy can be transferred to the ocean, creating tsunamis that reach heights of 100 feet (30 m) as they crash into coastal areas.
is a planar zone of seismicity corresponding with the down-going slab in a subduction zone.
Wadati-Benioff zone
what is particularly worrisome about the Cascadia subduction zone and what is the implication?
worrisome: the last major earthquake to occur there was in 1700
implication: this means that over the past 300 years strain may have accumulated to dangerously high levels.
fault in the PH that hasnt moved in a long time
Marikina Valley fault
earthquakes that occur far from a plate
boundary or active mountain belt and are generally believed to be related to tectonic forces that are being transmitted through the rigid plates.
intraplate earthquakes
explain how stress and strain works for intraplate earthquakes
➮tectonic forces cause crustal rocks to slowly accumulate strain,
➮which is then released along buried fault systems,
➮producing earthquakes in the interior of continents.
areas in the US with considerable history of producing powerful intraplate earthquakes
➮New Madrid seismic zone;
➮Charleston seismic zone
This strong intraplate earthquake (Mm 7.5) occurred in a heavily populated area, which was totally unprepared largely because the people had no memory of a large earthquake ever occurring in the
region.
The 250,000 to 650,000 people that perished provide a sober lesson for cities located in areas with large but infrequent earthquakes.
1976 Tangshan disaster in China
what is the common saying among seismologists
“EARTHQUAKES DON’T KILL PEOPLE, BUILDINGS DO.”
the leading cause of death and property damage in most earthquakes.
Structural failure
the forces engineers need to take into account when designing structures, and which is the most important? and why?
➮gravity;
➮horizontal (lateral) forces: wind
➮gravity is the most important because all
structures have mass, at a bare minimum they must be strong enough to support their own weight against the force of gravity.
in what direction are structures usually the strongest? and why?
➮vertical direction
➮because of gravity being the most important force that engineers have to take account of when designing structures
is wind a minor consideration when designing buildings?
yes, compared to compared to the vertical load or weight.
is the lack of structural strength in the lateral direction a concern?
➮in most places of the world, not really
➮but it becomes one of critical importance in areas where strong earthquakes occur, like the Philippines :”)
the most destructive kinds of earthquake waves, and why?
➮surface waves
➮due to the fact they cause the ground to vibrate in a lateral direction and at the same time, roll up and down like an ocean wave.
The most dangerous types of homes, and why
➮those constructed of unreinforced masonry
➮because they offer very little resistance to lateral shearing motion.
describe homes constructed of unreinforced masonry
➮walls are usually constructed of brick
or stone bound together with mortar
➮as opposed to reinforced walls with internal supports of wood or steel
describe the normal setting of building structures
➮for multistory buildings, many of
which are nonresidential, construction usually involves an interior skeleton made of steel or steel-reinforced concrete.
➮Under normal conditions the entire weight of the building is easily supported by its vertical columns
describe the setting of building structures during an earthquake
➮the strong lateral forces will cause the structure to sway.
➮In some cases this swaying motion may become so great that some of the floors within the building become detached from the columns, leaving the floors to fall freely.
a process when a floor of a building becomes free, it naturally falls onto the one below, which can cause additional floors to fail in a cascading manner that
engineers
pancaking
example of structural failures during earthquakes
- pancaking
- rupture of steel reinforced concrete columns
explain how rupture of steel reinforced concrete columns happen
➮columns are widely used for supporting highways, bridges, and buildings.
➮However, they can fail when the swaying motion of a structure becomes so great that the concrete columns, which are quite brittle, reach their elastic limit and literally explode.
➮Once the concrete shatters, the entire structure can collapse since the steel reinforcing rods alone are not capable of supporting the weight of the structure.
3 factors that affect ground shaking
- Period, Natural Vibration Frequency, and Resonance
- Focal Depth and Wave Attenuation
- Ground Amplification
the time (in seconds) it takes for a building to naturally vibrate back and forth
Period
describe how the size of structures relate to the period of natural vibration
the smaller the structure, the shorter the time it takes to vibrate back and forth
T or F: The ground, where structures are built, follows the building’s natural period during motion
The ground, where structures are built, also HAS ITS OWN natural period during motion
Describe how the type of ocean wave can affect different sized ships
➮small frequent waves will shake small boats but wont affect large ships
➮large infrequent waves shake large ships but wont affect small boats
describe the relationship on the lithology and period
➮harder lithologies like hard bedrocks have shorter periods
➮while softer sediments have longer periods
refers to the vibration of a structure/building at a fixed frequency;
natural vibration frequency
the number of times the motion is repeated in a set
amount of time.
frequency
relationship between building height to the natural vibration frequency
as building height increases, the natural vibration frequency decreases
what is then the danger of earthquakes and the natural vibration frequency of buildings?
➮The problem occurs during an earthquake when the natural vibration frequency of a given building matches that of the seismic waves
➮whereby the amplitudes of the individual waves combine
phenomenon when the amplitudes of the individual waves combine causing a building to sway more violently
resonance
how many stories of buildings are the most susceptible to resonance
Buildings around 10–20 stories high
T or F: tall skyscrapers are unlikely to experience resonance as their vibration frequency is beyond the frequency range of most seismic waves
True
energy of the resulting seismic waves steadily decreases as they travel away from the focus
wave attenuation
explains why the most dangerous earthquakes tend to be those with a combination of large magnitude and shallow focal depth
wave attenuation
T or F: it is quite possible for a relatively shallow, low-magnitude quake to generate greater ground motion than a deep, high-magnitude quake.
True
explain how Seismic waves experience different amounts of wave attenuation, depending on the types of geologic materials the waves pass through
➮Loose materials and rocks of lower density will absorb more energy from passing seismic waves compared to rocks that are more rigid and dense
➮On areas of rigid rocks, seismic waves are able to retain more of their energy as they travel farther. Because the waves undergo less attenuation, they therefore have the potential to cause damage farther from the focus.
When seismic waves travel through weaker materials, they slow down and lose energy at a faster rate. This, in turn, causes wave amplitude to increase, creating a phenomenon known as ?
ground amplification
how does Ground amplification occur in sedimentary basins
➮when seismic waves enter a basin and begin to amplify since they are forced to slow down in the sedimentary material.
➮Such areas may have an extended duration of ground shaking and subsequent increase in the likelihood of structural failure as seismic waves can become trapped within a basin and undergo internal reflection creating a reverberating effect.
➮Also, the convex shape of a basin can cause waves to refract and merge, focusing their energy into localized areas which then experience more
intense shaking.
secondary earthquake hazards
intense ground shaking often
produces secondary hazards such as:
- Liquefaction
- ground displacement
- ground fissures
- earthquake-induced mass wasting
- fires
- tsunamis
process when compacted sand-rich layers of sediment that are normally in contact with one another behave as fluid
liquefaction
explains how liquefaction happens
➮this happens as a result of the shearing motion of S-waves that increase the water pressure within the pore space of the sediment
➮thereby preventing the vibrating sand grains from making contact with one another.
➮As soon as the shaking stops, the sand-rich material will again behave as a solid as the individual sand grains are able to make contact with each other.
structure evidence for historical earthquakes esp for trenching
sand blows
rocks on either side of a fault move way from each other horizontally and/or vertically. Because of the potential for displacement, critical structures like dams, nuclear power plants, underground pipelines, hospitals, and schools should not be built across known faults
ground displacement
are large open cracks that typically develop close to the surface in loose sediment where there is little resistance to the rolling and stretching motion associated with surface waves.
ground fissures
When earthquakes provide one of the basic triggering mechanisms for the downslope movement of earth materials due to gravity such as landslide (debris slump), rock falls, and mudflows.
earthquake-induced mass wasting
secondary hazard when underground gas lines are easily broken when surface waves roll through a city and are likely to be ignited by sparks from countless electrical shorts in damaged buildings and downed power lines.
fires
a series of ocean waves that form when energy is suddenly transferred to the water by an earthquake, volcanic eruption, landslide, or asteroid impact.
tsunamis
when do most of tsunamis form
The majority of tsunamis, however, form during subduction zone earthquakes when crustal plates abruptly move and displace large volumes of seawater.
In 1902, an Italian seismologist developed a means of comparing both modern and historical earthquakes through the use of firsthand human observations during earthquakes. who was he?
Giuseppe Mercalli
what was the scale created by the famous Italian seismologist
Mercalli intensity scale
scale whereby earthquakes are ranked based on a set of observations most humans could report objectively, particularly the type of damage sustained by buildings.
Mercalli intensity scale
is a seismic scale used and developed by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) to measure the intensity of an earthquake.
Philippine Earthquake Intensity Scale (PEIS)
how was the PEIS developed?
It was developed as a response to the 1990 Luzon Earthquake (magnitude 7.7) and was adopted in the Philippines in 1996
what did PEIS replace?
Rossi-Forel Intensity Scale
who developed the RFIS?
Michele Stefano de Rossi
and
Francois-Alphonse Forel
range of the PEIS
Intensity scale ranges from I to X, with X classified as completely devastating.
these types of scales are useful because they quantify the amount of ground motion during an earthquake, and the energy that was released when the rocks ruptured.
magnitude scales
scale that - rates earthquakes based on the size of their seismic waves, as measured by seismographs; governed by amplitude (wave height) and distance
Richter Magnitude Scale
who developed the Richter Magnitude Scale
Charles F. Richter
the problem with Richter Magnitude Scale
➮as the number of seismograph stations around the world steadily increased, scientists eventually realized that results obtained using the Richter magnitude scale were not always consistent with one another,
➮particularly for large-magnitude earthquakes.
based on similar types of seismogram measurements as Richter’s, but is more accurate 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
Moment Magnitude Scale
describe the magnitude of the values in the Richter magnitude scale
➮each increase represents a 10-fold increase in ground shaking
➮ this corresponds to about a 30-fold increase in energy released at the focus
7 classes of earthquakes and their corresponding magnitudes
Microearthquake — 1.0-1.9
Minor — 2.0-3.9
Light — 4.0-4.9
Moderate — 5.0-5.9
Strong — 6.0-6.9
Major — 7.0-7.9
Great — 8.0-+
Mw 8.1 earthquake that happened in Davao City 1979
>intensity V
>59 depth
Moro Gulf Earthquake
3 methods of assessing earthquakes
- long-term forecasting
- short-term prediction
- other methods
Forecasting based mainly on the knowledge of when and where earthquakes occurred in the past.
Long-term forecasting
T or F: In seismically active areas, small earthquakes are more likely to occur as
the amount of time increases since the last major event.
False, LARGE earthquakes are more likely to occur as
the amount of time increases since the last major event
two important aspects in long-term forecasting
- paleoseismology
- seismic gaps
study of prehistoric earthquakes
paleoseismology
what does paleoseismology involve
the study of offsets in sedimentary layers near fault zones to determine recurrence intervals of major earthquakes prior to historical records
a zone along a tectonically active area where no earthquakes have occurred recently, but it is known that elastic strain is building in the rocks.
seismic gap
what happens if a seismic gap can be identified?
then it might be an area expected to
have a large earthquake in the near future
involves monitoring of processes that occur in the vicinity of earthquake
prone faults for activity that signify a coming earthquake.
short-term prediction
Anomalous events or processes that may precede an earthquake are called ?
precursor events
what do precursor earthquakes signal?
a coming earthquake
why has short-term earthquake prediction been difficult to successfully obtain
the processes that cause earthquakes occur deep beneath the surface
and are difficult to monitor.
* earthquakes in different regions or along different faults all behave
differently, thus no consistent patterns have so far been recognized
Delete
Delete
what are the 6 earthquake precursors
- increase in foreshocks
- slight swelling/uplift or tilting of the ground surface
- decreased electrical resistance
- fluctuating water levels in wells
- increased concentration of radon gas in groundwater
- generation of radio signals
Causes microcracks to form prior to complete rupture, or main shock.
increase in foreshocks
microcracks
increasing the rock volume
Slight swelling/uplift or tilting of the ground surface
water entering new void spaces that is more conductive than surrounding minerals.
Decreased electrical resistance
water entering new cracks causes water
levels to lower; levels rise when voids close again.
Fluctuating water levels in wells
new cracks allowing the gas, a radioactive decay product of uranium, to escape from rocks and enter wells.
Increased concentration of radon gas in groundwater
caused by changes in rock strain or movement of saline groundwater.
Generation of radio signals
3 examples of non-conventional/non-intrusive methods
- microtremor survey method
- refraction microtremor survey
- horizontal-to-vertical ratio method
This method uses seven (7) portable seismometers that will record microtremors for a few minutes
microtremor survey method
what are instruments for microtremor survey method equipped with?
GPS for time synchronization and
location coordinates
full GPS
Global Positioning System
describe how refraction microtremor method works
- A series of geophones planted on the ground connected to
a seismograph - A hammer striking a steel plate is used as the seismic
source - Propagating waves are measured and analyzed
Uses the same instrument as
the ones used in microtremor
array method
horizontal-to-vertical spectral ratio method
do you need to set up horizontal-to-vertical spectral ratio method
no, only single station
describe the recording of horizontal-to-vertical spectral ratio method
Records fundamental ground period of an area
Only requires a recording time of 20 minutes at most
4 ways to reduce earthquake risks
- seismic engineering
- early warning systems
- planning and education
- earthquake control?
specific example of seismic engineering
Addition of cross-bracing and shear walls, base isolation, wrapping of columns with a steel jacket, and spiral wrapping technique on vertical reinforcing rods
benefits of seismic engineering
➮provide greater structural strength with respect to the shear forces generated by lateral ground motion and a structure’s own inertia; and
➮reduce the actual amount of shear force that can develop on the structure.
instead of demolishing old buildings with outdated designs or without any seismic controls, what can be done?
A somewhat expensive, but viable option is to retrofit existing buildings with seismic controls
The basic idea behind this is to take advantage of this time lag and the fact that P-waves do very little damage. The first P-wave then is simply used as an alert that the highly destructive S-waves and surface waves will soon follow.
Early Warning Systems
3 specific examples of early warning systems
- Only seconds are needed for preprogrammed systems to close valves on gas lines, thereby reducing the risk of uncontrolled fires.
- Trains can be programmed to automatically stop.
- Electric utilities can also shut down critical control systems on electrical grids and at power plants.
the first step in planning and education as a way to reduce earthquake risks
➮The first step is to determine the level of severity of risk in a given area
➮by conducting hazard assessment
➮and subsequent construction of hazard maps.
based on hazard assessments, what do government agencies do to mitigate earthquake hazards?
➮government agencies will develop
building codes
➮that require appropriate levels of seismic engineering in buildings and other structures
in terms of education what can be done to reduce earthquake risk?
➮Raising of awareness on what to do before, during, and after an earthquake on all levels of society, from school-aged children up to emergency management
officials.
➮Regular earthquake drills
examples of how humans can induce earthquakes
- fluids can change affect the energy distribution of rocks (dam construction, injecting toxic wastes into wells)
- nuclear testing (explosions causing earthquakes)
