Download Earthquakes - provigeolowersix

Lower six
Ms Shalto
Figure showing the tectonic setting of earthquakes
 Movement and slipping along plate boundaries can
form an earthquake.
• Depending on the type of movement, the earthquakes
occur in either a shallow or deep level in the crust.
• The majority of tectonic earthquakes originate at
depths not exceeding tens of kilometers.
• Where old and cold oceanic crust descends beneath
another tectonic plate, “Deep Focus Earthquakes” may
occur at much greater depths (up to seven hundred
kilometers!).
• These earthquakes occur at a depth at which the
subducted crust should no longer be brittle, due to the
high temperature and pressure. A possible mechanism
for the generation of deep focus earthquakes is faulting.
•
Earthquakes may also occur in volcanic regions and are
caused there both by tectonic faults and by the
movement of magma (hot molten rock) within the
volcano. Such earthquakes can be an early warning of
volcanic eruptions.
Seismic Waves: Body Waves -
Primary (P)
The fastest wave, and
therefore the first to arrive at
a given location.
Also known as
compressional waves, the P
wave alternately compresses
and expands material in the
same direction it is traveling.
Can travel through all layers
of the Earth.
Seismic Waves: Body Waves -
Secondary Waves (S)
The S wave is slower
than the P wave and
arrives next, shaking
the ground up and
down and back and
forth perpendicular to
the direction it is
traveling.
Seismic Waves:
Surface Waves
They follow the P and S waves.
These waves travel along the surface
of the earth
Also known as:
 Rayleigh waves , also called
ground roll, travel like ocean waves
over the surface of the Earth, moving
the ground surface up and down.
They cause most of the shaking at the
ground surface during an earthquake.
 Love waves are fast and move the
ground from side to side.
 Intensity scales measure the
amount of shaking at a
particular location.
 The intensity of an earthquake
will vary depending on where
you are.
 Magnitude scales, like the
Richter magnitude and moment
magnitude, measure the size of
the earthquake at its source.
 Magnitude does not depend on
where the measurement of the
earthquake is made.
 On the Richter scale, an increase
of one unit of magnitude (for
example, from 4.6 to 5.6)
represents a 10-fold increase in
wave amplitude on a
seismogram or approximately a
30-fold increase in the energy
released.

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an
earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed;
walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks
may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures;
considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial
collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage
great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails
bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
 The vibrations produced by
earthquakes are detected, recorded,
and measured by instruments call
seismographs.
 The zig-zag line made by a
seismograph, called a "seismogram,"
reflects the changing intensity of the
vibrations by responding to the
motion of the ground surface
beneath the instrument.
 From the data expressed in
seismograms, scientists can
determine the time, the epicenter,
the focal depth, and the type of
faulting of an earthquake and can
estimate how much energy was
released.
 An oceanic spreading
ridge is the fracture zone
along the ocean bottom
where molten mantle
material comes to the
surface, thus creating
new crust.
 This fracture can be seen
beneath the ocean as a
line of ridges that form
as molten rock reaches
the ocean bottom and
solidifies.
 Major earthquakes may
occur along subduction
zones.
 The most recent subduction zone type earthquake occurred in 1700.
 Scientists believe, on
average, one subduction
zone earthquake occurs
every 300-600 years.
 A transform fault is a
special variety of strikeslip fault that accommodates relative
horizontal slip between
other tectonic elements,
such as oceanic crustal
plates.
 Intraplate seismic
activity occurs in the
interior of a tectonic
plate.
 Intraplate earthquakes
are rare compared to
those located at plate
boundaries.
 Very large intraplate
earthquakes can inflict
very heavy damage.
Distribution of seismicity associated with
the New Madrid Seismic Zone since
1974.
Structural Damage
 Buckled roads and rail
tracks
Landslides
Avalanches
Alterations to Water Courses
Fire resulting from an earthquake
Earthquake activity beneath a volcano almost always increases before an eruption because
magma and volcanic gas must first force their way up through shallow underground
fractures and passageways. When magma and volcanic gases or fluids move, they will either
cause rocks to break or cracks to vibrate. When rocks break, high-frequency earthquakes
are triggered. However, when cracks vibrate either low-frequency earthquakes or a
continuous shaking called volcanic tremor is triggered.
 Satellites can record infrared radiation where more
heat or less heat shows up as different colors on a
screen. When a volcano becomes hotter, an eruption
may be coming soon.
New Madrid, Tennessee
San Andreas Faultline
 Scientists consider seismic
activity as it is registered on a
seismometer.
 A volcano will usually register
some small earthquakes as
the magma pushes its way up
through cracks and vents in
rocks as it makes its way to
the surface of the volcano.
 As a volcano gets closer to
erupting, the pressure builds
up in the earth under the
volcano and the earthquake
activity becomes more and
more frequent.
This is an image of an analog recording
of an earthquake. The relatively flat
lines are periods of quiescence and the
large and squiggly line is an
earthquake.
Below is a digital seismogram. The data
is stored electronically, easy to access
and manipulate, and much more
accurate and detailed than the analog
recordings.
 Tiltmeters attached to
the sides of a volcano
detect small changes in
the slope of a volcano.
 When a volcano is about
to erupt, the earth may
bulge or swell up a bit.
Installing a tiltmeter
 Hydrogeologic responses to large distant earthquakes
have important scientific implications with regard to
our earth’s intricate plumbing system.
 The exact mechanism linking hydrogeologic changes
and earthquakes is not fully understood, but
monitoring these changes improves our insights into
the responsible mechanisms, and may improve our
frustratingly imprecise ability to forecast the timing,
magnitude, and impact of earthquakes.
 The cause of unusual animal behavior seconds before
humans feel an earthquake can be easily explain-ed. Very
few humans notice the smaller P wave that travels the
fastest from the earthquake source and arrives before the
larger S wave. But many animals with more keen senses are
able to feel the P wave seconds before the S wave arrives.
 If in fact there are precursors to a significant earthquake
that we have yet to learn about (such as ground tilting,
groundwater changes, electrical or magnetic field
variations), indeed it’s possible that some animals could
sense these signals and connect the perception with an
impending earthquake.
Tsunamis can be generated by:
 Large Earthquakes (megathrust events such as Sumatra,




Dec. 26, 2004)
Underwater or near-surface volcanic eruptions (Krakatoa,
1883)
Comet or asteroid impacts (evidence for tsunami deposits
from the Chicxulub impact 65 mya)
Large landslides that extend into water (Lituya Bay, AK,
1958)
Large undersea landslides (evidence for prehistoric
undersea landslides in Hawaii and off the east coast of
North America)
Tsunami wave propagation characteristics – note that as water depth
becomes smaller, waves slow down, become shorter wavelength, and have
larger amplitude.
 A tsunami warning system
is a system to detect
tsunamis and issue
warnings to prevent loss of
life and property.
 It consists of two equally
important components: (1)
a network of sensors to
detect tsunamis and
 (2) a communications
infrastructure to issue
timely alarms to permit
evacuation of coastal areas.
Tsunami Monitoring Buoy:
Reports rises in the water
column and tsunami events
 Near the source of sub-
marine earthquakes, the
seafloor is "permanently"
uplifted and downdropped, pushing the
entire water column up
and down.
 The potential energy that
results from pushing water
above mean sea level is
then transferred to
horizontal propagation of
the tsunami wave (kinetic
energy).
 Within several minutes
of the earthquake, the
initial tsunami is split
into a tsunami that
travels out to the deep
ocean (distant tsunami)
and another tsunami
that travels towards the
nearby coast (local
tsunami).
 Several things happen as
the local tsunami travels
over the continental
slope. Most obvious is
that the amplitude
increases. In addition,
the wavelength
decreases. This results in
steepening of the
leading wave--an
important control of
wave runup at the coast.
 Tsunami runup occurs
when a peak in the
tsunami wave travels
from the near-shore
region onto shore.
 Runup is a measurement of the height of the
water onshore observed
above a reference sea
level.