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Lecture#12
CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar
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Outlines of the Presentation
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Earthquakes
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Causes and effects
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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 Approximately 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).
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
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 midplate or intra-plate earthquakes and are, as yet, poorly
understood.
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.
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
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.
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
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”.
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.
Fig: 2.4.2 Hector Mines E/q, USA
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.
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 soil’s
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.
Effects of Earthquake
Liquefaction
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.
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)
Effects of Earthquake
Effects of Earthquake on Buildings
Effects of Earthquake
Effects of Earthquake on Buildings
Effects of Earthquake
Effects of Earthquake on Buildings
Locating an Earthquake’s 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.
Locating an Earthquake’s Epicenter
Around the world, abrupt motions of the earth are
continuously monitored by Seismographs.
Locating an Earthquake’s Epicenter
).
Seismogram
Seismograph
Locating an Earthquake’s 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).
Locating an Earthquake’s Epicenter
Locating an Earthquake’s Epicenter
S-P (S minus P) time formula:
The correlation between distance and the difference between
arrival times is given by
 1

1

t  D 



p
 s

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).
Seismic Travel-time Curve
If the speeds of the seismic waves are not known, use Travel-Time
curve for that region to get the distance
1. Measure time
between P and S wave
on seismogram
2. Use travel-time
graph to get distance
to epicenter
Ideally
v p  3 vs
3-circle steps:
3-circle method:
1) Read S-P time from 3
seismograms.
north
D1
D2
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.
D3
4) Overlapping point is
the event location.
Assumption: Source is
relatively shallow;
epicenter is relatively
close to hypocenter.