Download The Focus and Epicenter of an Earthquake

The Focus and Epicenter of
an Earthquake
• The point within
Earth where rock
under stress breaks
is called the focus
• The point directly
above the focus on
the surface is the
epicenter
WHAT ARE SEISMIC WAVES?



Vibrations that travel through
Earth carrying the energy
released during an earthquake
Energy starts at the focus and
moves outward
Two types:

Body waves


P and S
Surface waves
BODY WAVES: P AND S WAVES

Undisturbed Material
P Waves
S Waves
Body waves
 P or primary/pressure
waves
 fastest waves
 travel through
solids, liquids, or
gases
 Waves compress
and expand like an
accordian
 S or secondary/shear
waves
 slower than P
waves
 travel through
solids only
 Move up and down
as well as side to
side
SURFACE WAVES:
R waves
R waves
L waves
L waves

Surface Waves
 Travel just below or along the ground’s surface
 Slower than body waves
 Surface waves are the most damaging and are
especially damaging to buildings.
Seismographs
record earthquake
events
HOW IS AN EARTHQUAKE’S EPICENTER
LOCATED?
Seismic wave behavior



P waves arrive first, then S waves, then surface waves
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.
WHERE DO EARTHQUAKES OCCUR AND HOW
OFTEN?
~80% of all earthquakes occur in the circum-Pacific belt
~15% occur in the Mediterranean-Asiatic belt
~remaining 5% occur in the interiors of plates and on
spreading ridge centers
 more than 150,000 quakes strong enough to be felt are
recorded each year
HOW DO YOU LOCATE AN EPICENTER?
The Focus and Epicenter of an Earthquake
•The point within Earth where rock under stress breaks is called the
Focus
•The point directly above the focus on the surface is the Epicenter.
Surface
A
focus
epicenter
B
Which seismic
Station will
Receive the
Wave first?
A or B
HOW DO YOU LOCATE AN EPICENTER?
You must have three different seismographs at three different
Locations - then you can pinpoint the different times each picked
up the earthquake wave. A scientist will do this in a lab.
Surface
A
focus
epicenter
B
HOW IS AN EARTHQUAKE’S EPICENTER
LOCATED?



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
HOW IS AN EARTHQUAKE’S EPICENTER
LOCATED?
Time-distance graph
shows the average
travel times for Pand S-waves. The
farther away a
seismograph is
from the focus of
an earthquake, the
longer the interval
between the
arrivals of the Pand S- waves
HOW ARE THE SIZE AND STRENGTH OF AN
EARTHQUAKE MEASURED?

Magnitude: the
measurement of
earthquake
strength based on
seismic waves and
movement along
faults
THREE WAYS OF MEASURING
EARTHQUAKES:

Mercalli Scale measures the intensity of an
earthquake

Richter Scale is a rating of the size of seismic
waves

Moment Magnitude Scale estimates the total
energy released by an earthquake
MERCALLI
SCALE
RICHTER SCALE
THE ECONOMICS AND SOCIETAL IMPACTS OF EQS
•
•
•
•
Building collapse
Fire
Tsunami
Ground failure
Damage in Oakland, CA, 1989
WHAT ARE THE DESTRUCTIVE EFFECTS OF
EARTHQUAKES?

Ground Shaking
 The earthquake’s amplitude, how long it lasts, and
damage increases in poorly consolidated rocks
EARTHQUAKE HAZARDS




Local soil conditions: loose soil shakes more violently
than the surrounding rock (house built on solid rock
will shake less than a house build on sandy soil)
Liquefaction: earthquakes violent shaking suddenly
turns loose soil into liquid mud
Aftershocks: buildings weakened by an earthquake
may collapse during aftershocks
Tsunamis: earthquakes on the ocean floor cause large
waves that can grow to the height of a six story
building
CAN EARTHQUAKES BE PREDICTED?
Forecasting when and where earthquakes
will occur and their strength is difficult.
 One way seismologist forecast earthquakes
in a place is to observe their past strength
and frequency.

EARTHQUAKE HAZARD

Earthquake hazard is
a measurement of
how likely an area is
to have damaging
earthquakes in the
future. An area’s
earthquake-hazard
level is determined
by past and present
seismic activity.
FORECASTING EARTHQUAKES

A seismic gap is an area along a fault where
relatively few earthquakes have occurred
recently but where strong earthquakes are
known to have occurred in the past. Ex:
California
The gap hypothesis states that sections of active
faults that have had relatively few earthquakes
are likely to be the sites of strong earthquakes in
the future.