EARTHQUAKES

 What are Earth Quakes?
 Where do Earth Quakes occur?
 How do Earth Quakes occur?
 What are effects of Earth Quake?
 How are Earth Quakes measured?
Objectives

What are Earthquakes?
 The shaking or trembling caused by the sudden
release of energy
 Usually associated with faulting or breaking of
rocks
 Continuing adjustment of position results in
aftershocks

 Most earthquakes occur along the edge of the
oceanic and
 Along faults: normal, reverse, transform
Where do earthquakes occur:

Fractures, faults
Energy released and propagates
in all directions as seismic waves
causing earthquakes
How do earthquakes occur?
focus
epicenter

Earthquake = Vibration of the Earth produced by the rapid
release of energy
Seismic waves = Energy moving outward from the focus of an
earthquake
Focus= location of initial slip on the fault; where the
earthquake origins
Epicenter= spot on Earth’s surface directly above the focus
Definitions

- Earthquakes and Seismic Waves

Types of SeismicWaves
P-waves:
• called compressional, or push-pull waves
• Propagate parralel to the direction in which the wave is moving
• Move through solids, liquids
S-waves:
• Called shear waves
• Propagate the movement perpendicular
to the direction in which the wave is
Moving
Surface waves (L-waves or long waves).
• Complex motion
• Up-and-down and side-to-side
• Slowest
• Most damage to structures, buildings

Locating the shaking
Measure time between P and S waves on a
seismogram
Need at least 3 seismographs

What is the Elastic Rebound Theory?
Explains how energy is
stored in rocks
• Rocks bend until the
strength of the rock is
exceeded
• Rupture occurs and the
rocks quickly rebound to
an undeformed shape
• Energy is released in
waves that radiate
outward from the fault

How is an Earthquake’s Epicenter Located?
• P waves arrive first, then
S waves, then L
• Average speeds for all
these waves is known
• 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.

 Three seismograph stations
are needed to locate the
epicenter of an earthquake
 A circle where the radius
equals the distance to the
epicenter is drawn
 The intersection of the
circles locates the epicenter

 Earthquakes are measured based on three different
scales.
 The first measures the actual size or magnitude of the
quake.
 The second measures the effects or damage of the
quake.
 The third measures the total energy of the quake.
How are Earthquakes Measured?

 A seismograph is the the tool used to measure the strength
of an earthquakes
 The seismograph prints out a seismogram that scientists
read to determine the strength of the quake.
 The data from the seismogram is translated into a 1-10 rating
on the Richter Scale. Each step in the scale represents a
tenfold increase in the size of the quake!
How is the Magnitude of Earthquakes Measured?

Seismic waves cause the seismograph’s drum to vibrate. But
the suspended weight with the pen attached moves very
little. Therefore, the pen stays in place and records the drum’s
vibrations.
What is a Seismograph

1) Magnitude: Richter Scale
• Measures the energy released by fault movement
• related to the maximum amplitude of the S wave
measured from the seismogram
• Logarithmic-scale; quantitative measure
• For each whole number there is a 31.5 times increase in
energy
eg. an increase from 5 to 7 on the Richter scale = an
increase in energy of 992 times!!
Earthquake Effects : two ways to measure

2) Intensity: Mercalli Scale:
•Assigns an intensity or rating to measure an
earthquake at a particular location (qualitative)
•I (not felt) to XII (buildings nearly destroyed)
•Measures the destructive effect
•It is a linear scale
Intensity is a function of:
• Energy released by fault
• Geology of the location
• Surface substrate: can magnify shock waves e.g. Mexico City
(1985) and San Francisco (1989)

Largest earthquake in the world
Chile : 1960 May 22
Magnitude 9.5
More than 2,000 killed, 3,000 injured, 2,000,000
homeless, and $550 million damage in southern Chile
tsunami caused 61 deaths
$75 million damage in Hawaii;
138 deaths and $50 million damage in Japan;
32 dead and missing in the Philippines;
and $500,000 damage to the west coast of the United
States.

Measuring Earthquakes
The Mercalli scale was developed to rate earthquakes according
to the amount of damage at a given place.

Preferred by scientists and seismologists to the Richter scale
because moment magnitude is more precise.
not based on instrumental recordings of a quake, but is based
on the area of the fault that ruptured in the quake.
• calculated by multiplying the area of the fault's rupture surface
by the distance the earth moves along the fault….this
calculation is more precise than a tool or scientist opinion!
Moment Magnitude Scale

Comparison between the Richter and Moment Magnitude Scales
Earthquake Richter Scale Moment
Magnitude
Scale
New Madrid,
MO, 1812
8.7 8.1
San Francisco,
CA, 1906
8.3 7.7
Prince William,
AK, 1964
8.4 9.2
Northridge, CA,
1994
6.4 6.7

Shaking and ground rupture
 Shaking and ground rupture are the main effects created
by earthquakes, principally resulting in more or less
severe damage to buildings and other rigid structures.
 The severity of the local effects depends on the complex
combination of the earthquake magnitude, the distance
from the epicenter, and the local geological and
geomorphological conditions, which may amplify or
reduce wave propagation.
How Earthquakes Cause Damage

Landslides and avalanches
 A landslide is a geological phenomenon that includes
a wide range of ground movements.
 Rockfalls, deep failure of slopes and
shallow debris flows were common.
 Although the action of gravity is the primary driving
force for a landslide to occur, there are other
contributing factors affecting the original slope stability.
 Typically, pre-conditional factors build up specific sub-
surface conditions that make the area/slope prone to
failure, whereas the actual landslide often requires a
trigger before being released.

Earthquakes can cause fires by damaging
electrical power or gas lines.
In the event of water mains rupturing and a loss
of pressure, it may also become difficult to stop the
spread of a fire once it has started.
Fires of the 1906 San Francisco earthquake
Fires

Soil liquefaction occurs when, because of the shaking,
water-saturated granular material (such as sand) temporarily
loses its strength and transforms from a solid to a liquid. Soil
liquefaction may cause rigid structures, like buildings and
bridges, to tilt or sink into the liquefied deposits. For example,
in the 1964 Alaska earthquake, soil liquefaction caused many
buildings to sink into the ground, eventually collapsing upon
themselves.
Soil liquefaction

Tsunami
 Tsunamis are long-wavelength, long-period sea waves produced
by the sudden or abrupt movement of large volumes of water.
 Large waves produced by an earthquake or a submarine landslide
can overrun nearby coastal areas in a matter of minutes.
 Tsunamis can also travel thousands of kilometers across open
ocean and wreak destruction on far shores hours after the
earthquake that generated them.
 Ordinarily, subduction earthquakes under magnitude 7.5 on the
Richter scale do not cause tsunamis, although some instances of
this have been recorded.
 Most destructive tsunamis are caused by earthquakes of
magnitude 7.5 or more.

A flood is an overflow of any amount of
water that reaches land.
Floods occur usually when the volume of
water within a body of water, such as a river or
lake, exceeds the total capacity of the formation,
and as a result some of the water flows or sits
outside of the normal perimeter of the body.
However, floods may be secondary effects of
earthquakes, if dams are damaged. Earthquakes
may cause landslips to dam rivers, which collapse
and cause floods.
Floods

Base-isolated buildings can sway back and
forth.
Fixed-base building absorb the energy of
seismic waves
Construction Methods

Gas and water pipes should be flexible so they don’t break open.
Construction Methods

Drop, cover, and hold
Cover your head and
things that could break
or fall over.
Protecting Yourself

When outdoors, move to open areas without trees and sit down.
Put together an earthquake kit with food and first aid supplies.
Protecting Yourself