CE-312

1
(No Transcript)
2
Lecture12
CE-312 Engineering Geology and Seismology Instr
uctor Dr Amjad Naseer
Department of Civil Engineering N-W.F.P
University of Engineering and Technology, Peshawar
3
Outlines of the Presentation

• Earthquakes
• Causes and effects

4
Some facts about the Earthquake
How Many Earthquakes Happen Each Year? There
are over a million earthquakes annually,
including those too small to be felt. How Many
Earthquakes Happen Every Month? Day? Minute?
Per month Approximately 80,000 Per
day Approximately 2,600 Per minute Approximatel
y 2 And one earthquake is felt approximately
every 30 seconds. Of these only a relative few
are capable of causing damage. Earthquakes are
common natural events. How Deep Do Earthquakes
Occur in the World? Earthquakes occur in the
crust or upper mantle which ranges from the
surface to about 800 kilometers deep (about 500
miles).
5
Some facts about the Earthquake
How Many Earthquakes Happen Each Year? There
are over a million earthquakes annually,
including those too small to be felt.
Description Magnitude Frequency per year
Great 8.0 1
Major 7.0-7.9 18
Large 6.0-6.9 120
Moderate 5.0-5.9 1,000
Minor 4.0-4.9 6,000
Generally felt 3.0-3.9 49,000
Potentially perceptible 2.0-2.9 300,000
6
Some facts about the Earthquake
Where Do Most Earthquakes Occur in the World?
The surface of the earth is divided like a
jigsaw puzzle into giant pieces called tectonic
or crustal plates. These giant pieces move slowly
over partially melted rock known as the mantle.
As they move, they slide along each other move
into each other, move away from each other, or
one slips under another. On these active plate
boundaries about 95 of all the world's
earthquakes occur. California, Alaska, Japan,
South America, and the Philippines are all on
plate boundaries. Only 5 are in areas of the
plates far away from the boundaries. These are
called mid-plate or intra-plate earthquakes and
are, as yet, poorly understood.
7
Some facts about the Earthquake
Can We Predict Earthquakes? Scientists
estimate earthquake probabilities in two ways by
studying the history of large earthquakes in a
specific area and the rate at which strain
accumulates in the rock. Scientists study the
past frequency of large earthquakes in order to
determine the future likelihood of similar large
shocks. For example, if a region has experienced
four magnitude 7 or larger earthquakes during 200
years of recorded history, and if these shocks
occurred randomly in time, then scientists would
assign a 50 percent probability (that is, just as
likely to happen as not to happen) to the
occurrence of another magnitude 7 or larger quake
in the region during the next 50 years.
8
Some facts about the Earthquake
Can We Predict Earthquakes? But in many
places, the assumption of random occurrence with
time may not be true, because when strain is
released along one part of the fault system, it
may actually increase on another part. Four
magnitude 6.8 or larger earthquakes and many
magnitude 6 - 6.5 shocks occurred in the San
Francisco Bay region during the 75 years between
1836 and 1911. For the next 68 years (until
1979), no earthquakes of magnitude 6 or larger
occurred in the region. Beginning with magnitude
6.0 shocks in 1979, the earthquake activity in
the region increased dramatically between 1979
and 1989, there were four magnitudes 6 or greater
earthquakes, including the magnitude 7.1 Loma
Prieta earthquakes. This clustering of
earthquakes leads scientists to estimate that the
probability of a magnitude 6.8 or larger
earthquake occurring during the next 30 years in
the San Francisco Bay region is about 67 percent
(twice as likely as not).
9
Some facts about the Earthquake
Can We Predict Earthquakes? Another way to
estimate the likelihood of future earthquakes is
to study how fast strain accumulates. When plate
movements build the strain in rocks to a critical
level, like pulling a rubber band too tight, the
rocks will suddenly break and slip to a new
position. Scientists measure how much strain
accumulates along a fault segment each year, how
much time has passed since the last earthquake
along the segment, and how much strain was
released in the last earthquake. This information
is then used to calculate the time required for
the accumulating strain to build to the levels
that result in an earthquake. This simple model
is complicated by the fact that such detailed
information about faults is rare. In the United
States, only the San Andreas Fault system has
adequate records for using this prediction
method.
10
Some facts about the Earthquake
Can We Predict Earthquakes? Both of these
methods, and a wide array of monitoring
techniques, are being tested along part of the
San Andreas Fault. For the past 150 years,
earthquakes of about magnitude 6 have occurred an
average of every 22 years on the San Andreas
Fault near Park field, California. The last shock
was in 1966. Because of the consistency and
similarity of these earthquakes, scientists have
started an experiment to "capture" the next Park
field earthquake. A dense web of monitoring
instruments was deployed in the region during the
late 1980s. The main goals of the ongoing Park
field Earthquake Prediction Experiment are to
record the geophysical signals before and after
the expected earthquake to issue a short-term
prediction and to develop effective methods of
communication between earthquake scientists and
community officials responsible for disaster
response and mitigation. This project has already
made important contributions to both earth
science and public policy.
11
Effects of Earthquake
Strong Ground Motion or Ground Shaking The most
destructive of all earthquake hazards is caused
by seismic waves reaching the ground surface at
places where human-built structures, such as
buildings and bridges, are located. When seismic
waves reach the surface of the earth at such
places, they give rise to what is known as strong
ground motion. Strong ground motion causes
buildings and other structures to move and shake
in a variety of complex ways. Many buildings
cannot withstand this movement and suffer damages
of various kinds and degrees. Most deaths,
injuries, damages and economic losses caused by
earthquakes result from strong ground motion
acting upon buildings and other man-made
structures not capable of withstanding such
motion. It is for this reason that it is often
said, "Earthquakes don't kill people, buildings
do.
12
Effects of Earthquake
Surface Rupture Surface rupture occurs when
movement on a fault deep within the earth breaks
through to the surface. NOT ALL earthquakes
result in surface rupture. Ground failure,
rather than ground shaking, is the principal
cause of damage to water and sewer lines. The
brittle sewer pipes tended to fail under much
lower strains than water lines, so damage to
sewer lines is considerably more extensive.
Identifying where and to what degree subgrade
utilities are at risk from earthquakes can be
accomplished by accurately delineating regions at
risk of ground failure during earthquake shaking.
13
Effects of Earthquake
Landslides Buildings aren't the only thing to
fail under the stresses of seismic waves. Often
unstable regions of hillsides or mountains fail.
In addition to the obvious hazard posed by large
landslides, even non lethal slides can cause
problems when they block highways they can be
inconvenient or cause problems for emergency and
rescue operations.
14
Effects of Earthquake
Liquefaction One of the most important types of
ground failure which can occur during an
earthquake is known as liquefaction. What is
Liquefaction? Liquefaction refers to a process
resulting in a soils loss of shear strength, due
to a transient excess of pore water pressure.
Soil with a high water table being strongly
shaken during an earthquake that is, cyclically
sheared. The soil particles initially have large
voids between them. Due to shaking, the particles
are displaced.
15
Effects of Earthquake
Liquefaction
16
Effects of Earthquake
Tsunamis Another important class of earthquake
effects are tsunamis, which are generated by
earthquakes which have occurred beneath the ocean
floor. Tsunamis are immense sea waves. "Tsunami"
is actually a Japanese word meaning "huge wave".
Japan is one of the most seismically active
countries in the world and has experienced many
earthquakes and tsunamis. These waves travel
across the ocean at speeds as great as 597 miles
per hour and may be 15 meters (49 feet) high or
higher by the time they reach the shore.
17
Effects of Earthquake
Effects of Earthquake on Buildings
Fig 2.5 How the building damages during an
Earthquake (Courtesy of National Disaster
management Division India)
18
Effects of Earthquake
Effects of Earthquake on Buildings
19
Effects of Earthquake
Effects of Earthquake on Buildings
20
Effects of Earthquake
Effects of Earthquake on Buildings
21
Locating an Earthquakes Epicenter

• The source of an earthquake within the earth is
  the actual place of rock slippage along a fault.
• The hypocenter or Focus, the point where the
  fault starts to move, can be located by using P
  and S waves.
• The point at the earth's surface directly above
  the hypocenter is called the epicenter.

22
Locating an Earthquakes Epicenter

• Around the world, abrupt motions of the earth are
  continuously monitored by Seismographs.

23
Locating an Earthquakes Epicenter
).
Seismogram
Seismograph
24
Locating an Earthquakes Epicenter

• Seismic wave behavior
• P waves arrive first, then S waves, then L and R
• After an earthquake, the difference in arrival
  times at a seismograph station can be used to
  calculate the distance from the seismograph to
  the epicenter (D).

25
Locating an Earthquakes Epicenter
26
Locating an Earthquakes Epicenter
S-P (S minus P) time formula The correlation
between distance and the difference between
arrival times is given by
Where D is the distance to the source, vs is the
velocity of the secondary wave and vp is the
velocity of the primary wave. These velocities
range from 3 to 8 km/sec (Primary) and 2 to 5
km/sec (Secondary).
27
If the speeds of the seismic waves are not known,
use Travel-Time curve for that region to get the
distance
Seismic Travel-time Curve
1. Measure time between P and S wave on
seismogram 2. Use travel-time graph to get
distance to epicenter
Ideally
28
3-circle steps
3-circle method
1) Read S-P time from 3 seismograms. 2) Compute
distance for each event/recording station pair
(D1, D2, D3) using S-P time formula. 3) Draw each
circle of radius Di on map. 4) Overlapping point
is the event location. Assumption Source is
relatively shallow epicenter is relatively close
to hypocenter.