Earthquake and Faulting, Tectonic Plate
Plate Tectonic Theory
Earthquake
- manifestation of ground shaking caused by sudden
release of energy
• dislocations of crust
• volcanic eruptions
• man-made explosions
• collapse of underground cavities
(mine/karsts)
- natural disturbances
• fault rupture-induced
• volcanic
• mining-induced
• large reservoir-induced
• ⁃ occurrence explain by the Plate Tectonic
Theory (large-scale tectonic processes)
- Tectonic Plate Theory - derives from the theory of
continental drift and sea floor spreading
- 19th Century (Udias, 1999) - understanding
between geophysics and geology began
- Earthquake is recognized as a symptoms of active
tectonic movement (Scholz, 1990)
THE STRUCTURE OF THE EARTH
1. Crust
- outer layer
- thickness: 25-60km under continents and 4-6
under oceans
2. Mantle
- interior below the crust
- depth of 30 km to about 2900 km
- consists of silicate rocks
3. Outer Core
- layer surrounding the inner core
- liquid layer
- made up of iron and nickel
- temperature similar to inner core
4. Inner Core
- center and hottest part
- solid and made up of iron and nickel
- temperature of up to 5500 C
- engine room of the earth
PLATE BOUNDARIES
Plates
- large and stable rock slabs (thk 100km)
- forming in crust or lithosphere and part of upper
mantle
- Lithosphere moves differentially on the
underlying Asthenosphere (softer warmer layer,
400km thk, depth 50km in upper mantle)
Continental Drift Theory
- lithosphere divided into 15 plates (incl.
Continental and Oceanic Crusts)
Seismic Belts
point where two plates meet
TYPES OF PLATE BOUNDARIES
1. Divergent or Rift Zones
- two plates separate themselves from one another
- effusion of magma or lithosphere diverges from
the interior
- Mid-Atlantic Ridge and Pacific Ring of Fire
2. Convergent or Subduction Zones
- adjacent plates converge and collide
- forming a mountain range and a deep seafloor
trench
- Under- Thrusting Plate - subduction process
carries slab like plate into dipping zone (WadiBenioff Zone) (650-700km)
- Mariana Trench - North Pacific Boundaries
(deep seafloor trench)
- when oceanic crust tops one of the colliding
plates, it sinks down the mantle
TWO TYPES:
Oceanic - two plates consisting of oceanic
lithosphere collide. Oceanic Rock is mafic and
heavy. It sinks easily and destroyed in
subduction zones
Continental - grinding plates consists of
continental lithosphere. Lightweight rock and do
not subduct
3. Transform Zones or Transcurrent Horizontal
Slip
- two plates glide past one another
- without creating new lithosphere or subducting
old
- can be found on oceanic or continental
lithosphere
- earthquakes are common along these faults
- San Andreas Fault Zone - connecting two
spreading ridges (North America - Gulf of
California to the South and Pacific Plates- Gorda
Ridge in the North)
FAULTING
- Elastic Rebound - rock on one side of the fault
suddenly slip with respect to the other (resulting
to fracture - FAULT)
- Earthquake- ground motion affected in the slip
mechanism of active faults
- Azimuth(∅) - angle between trace of fault (0<
theta < 360)
- Dip (𝛿) - between fault and horizontal plane
(0<theta<90)
- Slip or rake (𝜆)- between direction of relative
displacement and horizontal direction (-180 <
lambda < 180)
- Relative Displacement (𝛥𝜇) - distance traveled
by a point. Varies along fault plane
- Area (S) - surface area
Azimuth, Dip, and Slip or Rakes angles and its
dimensions are given by its area (S) and the fault slip is
measured by the relative displacement.
COMMON MECHANISM OF EARTHQUAKE
1. Dip-slip Fault
- move along the direction of the dip plane
1. Normal Fault - Footwall upward, hanging
wall downward. Tensile forces cause shearing
failure
2. Reverse Fault - Hanging wall upward,
footwall downward. Compressive forces causes failure
2. Strike-slip Fault
- move horizontally past one another
- right-lateral or left-lateral
- widespread, and many found at the boundary
between obliquely converging oceanic and
continental
- San Andreas Fault - San Francisco earthquake
of 1906 (max movement of 6m)
- Anatolian Fault - Izmit earthquake of 1999
(moved more than 2.5m)
- Blind Trust fault aka reverse fault - worrisome,
hidden. They are folded over the tip of the fault.
3. Oblique-slip fault
- normal or reverse or left or right lateral
- has a component of dip slip and strike slip
Earthquakes will be concentrated in the vicinity of
faults. Faults that are moving more rapidly will tend to
have higher rates of seismicity. Larger faults = larger
events. Earthquakes continue to occur on unknown or
inactive faults.
PPT NI SIR
Common Natural Hazards in the Ph
1.
Earthquake
2.
Tsunami
3.
Volcanic Eruptions
4.
Typhoon
5.
Storm Surge
6.
Flood
EXAMPLES:
1. Flood in Metro Manila, Rizal and Laguna due to
Tropical Cyclone Ondoy (2009)
2. Storm Surge in Eastern Visayas due to Tropical
Cyclone Yolanda (2013)
3. 1991 Pinatubo Eruption and Subsequent Lahars
4. M7.8 Luzon Earthquake (July 16, 1990)
- Magsaysay Bridge, Dagupan
- Dagupan City
- Saint Mary’s Academy, Agoo
- Carmen Brudge, Rosales
Earthquake
- Vibratory ground motions
- Totally unexpected and nearly
instantaneous devastation
- not designing them to resist such forces can
bring serious economic consequences
-
-
threat that earthquakes pose to life and
property is not the only concern to modern
societies
strong earthquakes at any given location
occur, on average, only once every 50–100
years
Outermost Part of Earth’s interior
- Asthenosphere – lies below lithosphere, low
viscosity, shear strength, can flow like a
liquid
- Lithosphere – consists of crust and rigid
uppermost part of mantle. Broke up into
tectonic plates
The Pacific Plate
- boundary of the Pacific plate is the source of
nearly half of the world’s great earthquakes
- Stretching 40,000km (24,000miles) around
the circumference of the Pacific Ocean
(Japan, West Coast of North America, and
highly populated areas – Ring of Fire)
Ring of Fire
- the plates that make up the Pacific basin are
generally subducting beneath continental
plates, causing subduction-zone volcanism at
the surface
- Nearly 80% of the earth’s volcanoes are
found near the tectonic plate boundaries of
the Pacific Ocean.
- Tectonic plates move relatively slowly (5cm
per year is relatively fast) and irregularly
Theory of Tectonic Plates
- explains the relative movement of the plates
present in the Earth’s outer layer
- one giant super continent – Pangaea
- Pangaea - Due to the heat built up
underneath the continent, it began to rift
and split apart
Types of Plate Boundaries
1. DIVERGENT BOUNDARIES - Plates are moving
apart or away from each other.
2. CONVERGENT BOUNDARIES - Plates are colliding.
3. TRANSFORM BOUNDARIES - Plates slide past each
other
The lateral movement of the plates is mainly at the
speeds of 50-100 mm annually. Earthquakes, volcanoes,
mountain and oceanic trench formation normally occur
along these boundaries.
Fault
-
longer a fault the larger the earthquake it
can generate
Platelets – smaller sub plates
Jostling – simple blocks of crust that
occasionally move and shift
2.) the process where an oceanic plate slides beneath a continental plate is known as
SUBDUCTION
3.) They travel more slowly and reach the surface after the P-waves. They transmit more energy, thus causing the
bulk of damage to structures.
S-WAVES
4.) They travel with a velocity that depends on the elastic properties of the rock through which they travel
P-WAVES
5.) The lateral movement of the plate is mainly at the speeds of _____ mm annually
50-100
6.) A fault in which the upper block, above the fault plane, moves up and over the lower back.
REVERSE FAULT
8.) The occurrences of earthquake may be explained by the theory of large-scale tectonic processes, referred to as
__________
PLATE TECTONICS
9.) The portion of the Earth’s interior below the crust, extending from a depth of about 30km to about 2,900km ; it
consists of dense silicate rocks.
MANTLE
10.) It is characterized by rising hot mantle material and sinking colder material, forming convection currents. It is
the primary driver of Plate tectonic movements.
MANTLE CONVECTION
11.) The San Andreas fault of California is which type of fault?
STRIKE SLIP FAULT
12.) These are large and stable rock slabs with a thickness of about 100km forming the crust or lithosphere and
part of the upper mantle of the earth.
PLATES
13.) What is the angle between the fault and the horizontal plane?
DIP
14.) It occurs when the hanging wall moves upward in relation to the footwall, compressive force causes the
failure.
REVERSE FAULT
15.) Obtained when the block underlying the fault plane, or “footwall”, moves up the dip and away from the block
overhanging the fault plane, or “hanging wall”
NORMAL
16.) The motion of this wave is a combination of longitudinal, compressional, dilation. As result, the particles move
elliptically in a vertical plane.
RAYLEIGH WAVES
17.) The energy of __________ spreads from the source in two directions rather than in three.
LOVE WAVES
18.) It emanates from the focus and travel in all directions through the body of the Earth.
BODY WAVES
19.) The point on the Earth’s surface directly above the focus is the _________.
EPICENTER
20.) It causes alternate push (compression) and pull (tension) in the rock.
P-WAVES
•
Faulting and Seismic Zones of the
Philippines
WIKIPEDIA
• Philippine Archipelago – geogically part of
Philippine Mobile Belt (PMB) located at
Philippine Sea Plate, the South China Sea Basin
of the Eurasian Plate, and the Sunda Plate.
• Philippine Trench (Mindanao Trench) –
submarine trench (found directly east of the
PMB), result of a collision
• Philippine Sea Plate – subducting under PMB at
the rate of about 16 cm (6.3 in) per year.
• Philippine Fault System – consists of seismic
faults
• Galathea Depth in the Philippine Trench (at
Philippine Sea) – deepest point in the Ph, 3rd
deepest in the world
• Philippines
- Located on the western fringes of the Pacific
Ring of Fire
- Experiences frequent seismic and volcanic
activity
- Benham Plateau – east in the Philippines, an
undersea region active in tectonic subduction
- 20 earthquakes registered daily
- 1990 Luzon Earthquake – last major
earthquake
- Active Volcanoes: Mayon Volcano, Mount
Pinatubo, and Taal Volcano
- Eruption of Mt. Pinatubo – second largest
terrestrial eruption of the 20th century
- World’s second-biggest geothermal energy
producer behind US
- 18% of the country’s energy needs
ACCORDING TO NATURAL HAZARDS AND EARTH
SYSTEM SCIENCES OPEN ACCESS DISCUSSIONS
• Philippine Archipelago
- mature island arc, accreted to the Eastern
Margin of the Eurasian Plate
-
composed of a complex mixture of 20 terranes
formed through plate interaction of the
Philippine Sea Plate, Eurasian Plate, and Indo-
Australian Plate
-
characterized by a system of subduction zones,
•
Philippine Mobile Belt (PMB) – actively
deforming region of the Ph and bound on both 25
sides
West-Dipping Subduction Zones
East Luzon Trough
Philippine Trench
East-Dipping Subduction Zones
Negros Trench
Sulu Trench
Cotabato Trench
•
•
-
collision zones, and wrench faults
•
•
-
Left-lateral Philippine Fault – between west and
east dipping subduction zones
Straddles the entire length PMB
Active Faults in Bohol
Cebu lineaments
Central Negros Fault
Panay Fault
West Mindanao Fault
Major Sites of Seismicity
Trenches in the east and west of the archipelago
➢ South China Sea subducts along the eastdipping Manila trenches
➢ Sulu Sea subducts along the Negros trenches
➢ Celebes Sea subducts along the Cotabato
trenches
➢ Sulu Trench is the locus where the Sulus
Sea is consumed
➢ Philippine Trench is where the Philippine
Sea Plate subducts
PHILIPPINE FAULT ZONE (PFZ)
• Extends 1200 km across the Philippine
Archipelago
• Left-lateral strike-slip fault extends NW-SE
(N20 – 40W)
• The fault experiences a slip rate of
approximately 2-2.5 cm/yr
According to the seismologists, the deepest earthquakes
happen on fault lines.
PHILIPPINES 5 ACTIVE FAULT LINES
1. Marikina Valley Fault
- Most dangerous fault
- Fault cuts through almost all the progressive
and crowded portions of Manila
- Two major segments: West Valley Fault and
East Valley Fault
- West Valley Fault – capable of producing large
scale earthquakes (magnitude of 7 above)
o Locations that the Marikina Valley runs
through:
▪ Marikina, Montalban, San Mateo,
Pasig, Taguig, Muntinlupa, San
Pedro, Binan, Santa Rosa, Carmona,
Tagaytay, Calamba, Oriental Mindoro
2. Western Philippine Fault
- Runs in the waters of the western part of the PH
o Places where the fault line passes:
▪ Mindoro Strait, Luzon Sea, Sulu Sea,
Panay Gulf
3. East Philippine Fault
- Found beneath the ground
- Found at the Philippine Sea
4. Southern of Mindanao Fault
- Runs through the Celebes Sea as well as the
Moro Gulf
5. Central Philippine Fault
- Runs through several provinces in cities from
northern part of archipelago to southern part of
Davao
o Places where the fault line passes:
▪ The entire Ilocos Norte, Aurora,
Quezon, Masbate, Eastern Leyte,
Southern Leyte, Agusan Del Norte,
Agusan del Sur, Davao del Norte
- Root of the earthquake which destroyed the
Guinsaugon, Baguio and the Central Luzon part
in 1990
4. The 1994 Mindoro Earthquake
Magnitude: 7.1
Description
- November 15
- Maximum intensity of VII (Very Strong)
- 35 km long ground rupture (Aglubang
River Fault)
Results
- Total of 1530 houses in the coastal areas
of Bacoand Calapan, Oriental Mindoro
were swept away by a tsunami
generated by the earthquake
- Killed 78 people
LARGEST EARTHQUAKE IN PH SINCE 1960s
1. The 1968 Casiguran Earthquake
Magnitude: 7.3
Description
- August 2
- Moment magnitude 7.6
- Max intensity of IX (9)
- Epicenter was in Casiguran, Quezon
(now Aurora Province)
Results
- Killed 270 people
- Caused massive landslide and tsunami
- Almost 300 people killed when the Ruby
Tower in Binondo collapse
5. The 2013 Bohol Earthquake
Magnitude: 7.0
Description
- October 15
- Epicenter 6 km S24W of Sagbayan
- Depth of focus was 12 km
- Affected the whole central Visayas
Region (Bohol and Cebu)
Results
- NDRRMC – 222 dead, 8 missing, 976
injured
- More than 73,000 structures damaged
- More than 14,500 totally destroyed
- Deadliest earthquake in the PH after 23
years since 1990 Luzon Earthquake
2. The 1976 Moro Gulf Earthquake
Magnitude: 8.0
Description
- A tsunami also took place on August 17
near the islands of Mindanao and Sulu
- Highest magnitude calculated
Results
- 3000 casualties
- Officially the strongest and deadliest
earthquake in Ph history
- Almost 40,000 people in Regions 9 and
12 were left homeless
3. The 1990 Luzon Earthquake
Magnitude: 7.8
Description
- Maximum intensity of IX (Violent)
- Produced a 125 km-long ground rupture
(Dingalan, Aurora to Cuyapo, Nueva
Ecija)
- Strike-slip movements
Results
- Total of 1,621 deaths
- Serious damage of properties
CHAPTER 2:
Seismic Waves
Surface waves – propagate across the outer layers of the
Earth’s crust.
Seismic waves - energy waves that are generated by an
earthquake or explosion and propagate within the Earth or
on its surface.
– Similar to a transverse wave but travel along the
boundary between the Earth’s surface and air, i.e.,
through the crust.
Seismograph – used by seismologists to record the
frequency and amplitude of the waves to study Earth’s
subsurface activities.
– Have a lower frequency than body waves. They
responsible for the damage and destruction caused
by earthquakes.
Body waves – travels through the Earth’s interior layers.
Also called pressure waves, they can propagate in solid
and liquid materials.
P-waves – longitudinal or primary waves,
– causes alternate push (compression) and pull
(tension) in the rock. It exhibits similar properties
to sound wave and can cause little damage.
– have the highest velocity of all seismic waves
and thus will reach all seismographs first.
S-waves – Transverse or secondary waves that propagate
vertical and horizontal side to side motion.
– Such wave produces shear stress in the rock
along the path. Also called shear waves.
– They can only travel in a rigid, solid material
through the vibration of particles in a direction
perpendicular to the propagation.
Rigidity - the resistance to shearing of a material.
Epicentral Distance - This technique is based on the
principle that it takes longer for S-waves that for P-waves
to arrive to an observation point.
– considers the fact that there is always a time lag
between the arrival of P and S waves
– The particles in a surface wave move in a circular
or elliptical motion. The intensity of surface waves
diminishes as they go deeper from the surface.
Rayleigh Waves - “R- or LR-waves”. Named after British
physicists Lord Rayleigh.
– its motion is a combination of longitudinal,
compressional, and dilation.
– As a result, the particles move elliptically in a
vertical plane.
– These waves are dispersive, and the amplitudes
generally decrease exponentially with depth in
the Earth. LR-waves are caused by constructive
interference of body waves, such as P and SV.
Love Waves – “L- or LQ-waves”. Named after British
mathematician A. E. H. Love.
– the particles of Love waves jerk back and forth
perpendicular to the direction of wave propagation,
similar to S-waves.
– The motion of particles of Love wave makes a
horizontal line perpendicular to the direction of
propagation.
– The energy of Love waves spreads from the source in
two directions rather than in three. The amplitude often
decreases rapidly with depth.
– Love waves travel faster than Rayleigh waves.
– LQ-waves have large amplitudes and long periods (60300 seconds) are also called “G-waves”, Gutenberg
(Richter, 1958). For these periods, the waves travel with a
velocity of about 4.0 km/sec and are pulse-like.
CHAPTER 4:
Measurement of Earthquake;
Magnitude vs Intensity
Magnitude - quantitative measurement of earthquake
size and fault dimensions. It is based on the maximum
amplitudes of body or surface seismic wave.
Measurement of Earthquakes
*The “size” of earthquakes generally refers to the amount
of energy release by the event.
Two approaches by which Earthquake size is categorized
are:
1. Intensity
2. Magnitude
Intensity - based on a qualitative description of the
damage caused by an earthquake to the natural and built
environment at a particular location and the associated
human reaction.
Common Intensity Scales
i. Mercalli-Cancani-Seiberg (MCS)
– 12-level scale
– Used in southern Europe
ii. Modified Mercalli (MM)
– 12-level scale
– Proposed in 1931 by Wood and Neuman
iii. Medvedev-Sponheuer-Kamik (MSK)
– 12-level scale
– Developed in Central and Eastern Europe
iv. European Macroseismic Scale (EMS)
– 12-level scale
– adopted since 1998 in Europe
ii. Japanese Meteorological Agency (JMA)
– 7-level scale
– Used in Japan
Intensity scales are used to plot contour lines of equal
intensity or “isoseismals”
Intensity maps – provide approximate distributions of
damage and extent of ground shaking, maps of local site
intensity include reports of all observation sites and
whether or not the strong motion was felt.
Common Magnitude Scales
i. Local (Richter) magnitude (𝑀𝐿)
– measures the maximum seismic wave
amplitude A (in microns) recorded on standard
Wood-Anderson seismographs located at a distance
of 100 km from the earthquake epicenter.
– has a natural period of 0.8 seconds, a critical
damping ration of 0.8 and an amplification factor
of 2,800. It amplifies waves with periods between
approximately 0.5 and 1.5 seconds, wavelengths of
500 m to 2 km.
ii. Body wave magnitude (𝑀𝑏)
– measures the amplitude of P-wave with a
period of about 1.0 second, i.e., less than 10-km
wavelengths.
– It can measure distant event not less than 600 km.
P-wave are not affected by the depth of energy
source
iii. Surface wave magnitude (𝑀𝑆)
– measure the amplitude of LR-waves with a
period of 20 sec. of about 600 km wavelength.
– It used for large earthquakes but it cannot be used
to characterize deep or relatively small regional
earthquakes.
v. Moment magnitude (𝑀𝑤)
– a function of the seismic moment 𝑀0.
– It is not related to any wavelength as a result it
can be used to measure the whole spectrum of
ground motion and the extent of deformation at the
earthquake source and be estimated from the fault
displacement as follows.
Properties of major magnitude scales:
INTENSITY SCALES
Japanese Seismic Intensity Scale
0
No sensation
I
Slight
II
Weak
III
Rather strong
IV
Strong
V
Very strong
VI
Disastrous
VII
Ruinous
Modified Mercalli Intensity (MMI) Scale
I
II
III
IV
Not felt except by a very few under
especially favorable circumstances.
Felt only by a few persons at rest,
especially on upper floors of building.
Felt quite noticeably indoors, especially
on upper floors of buildings, but many
people do not recognize it as an
earthquake.
During the day felt indoors by many,
outdoors by few.
Medvedev-Sponheuer-Kamik (MSK)
I
Not noticeable
II
Scarcely noticeable
III
Weak, partially observed only
IV
Largely observed
V
Awakening
VI
Frightening
Damage to buildings
V
Felt by nearly everyone, many awakened.
VII
VI
Felt by all, many frightened and run
outdoors.
VIII Destruction of buildings
VII
Everybody runs outdoors.
Damage slight in specially designed
structures; considerable in ordinary
VIII
substantial buildings, with partial
collapse; great in poorly built structures.
Damage considerable in specially
designed structures; well-designed frame
IX structures thrown out of plumb; great in
substantial buildings, with partial
collapse.
Some well-built wooden structures
destroyed; most masonry and frame
X
structures destroyed with foundations;
ground badly cracked.
Few, if any, (masonry) structures remain
XI standing. Bridges destroyed. Broad
fissures in ground.
Damage total. Waves seen on ground
XII surface. Lines of sight and level are
distorted. Objects thrown into the air.
IX
General damage to buildings
X
General destruction of buildings
XI
Catastrophe
XII
Landscape changes