People and Earthquakes
Seismologists (people who study
earthquakes) are studying ways to reduce the
impact of an earthquake on people and
property
They specialize in different areas like:
A) Predicting when and where a quake will
hit in order to give people time to evacuate
B) Improving construction designs and sites
to minimize damage
Magnitude vs. Intensity
Seismologists measure earthquake strength
in 2 ways:
A) Magnitude – this measures the amplitude
of the largest wave on a seismograph
(basically a measure of how much the
ground shakes)
B) Intensity – this measures how much the
earthquake affects people
Richter Scale of Magnitude
* determined by measuring the amplitude of
the largest wave recorded on the seismograph
* It is a logarithmic scale that goes from 0 to
~9
* each unit of magnitude corresponds to 10x
greater wave amplitude and 30 x greater
energy release
* so a 5.0 quake is 10 x stronger than a 4.0
quake
* largest quake ever recorded = ~9.5 on
Richter scale (1960 in Chile)
(the equivalent of one billion tons of TNT)
Richter Magnitudes & Effects
< 2.0
not felt by people (over 600,000
per yr)
6.0 – 6.9 destructive in populated areas
( ~ 266 / year)
> 8.0 destroy communities near
epicenter (~ 1.4 /yr)
Earthquake Intensity and magnitude
Intensity of shaking & damage at
a specific location
Depends on distance to earthquake
● Mercalli intensity scale
& strength of earthquake
● Magnitude
A measure of the energy released in an earthquake
Depends on size of fault that breaks
~2000/day
~800/day
16/day
1 every 3 days
This is a seismograph
and a seismogram record
of an earthquake
Mercalli Intensity Scale
Mercalli Intensity Scale – measures the
damage or effect of the earthquake on people
and property. It is calculated based on
eyewitness accounts.
It assesses damage at a specific location &
depends on:
- quake strength
- epicenter distance
- surface material
- building design
Earthquake Intensity and magnitude
Intensity of shaking & damage at
a specific location
Depends on distance to earthquake
● Mercalli intensity scale
& strength of earthquake
● Magnitude
A measure of the energy released in an earthquake
Depends on size of fault that breaks
Examples:
2 = felt only by people lying down
6 = felt by all, heavy furniture moves,
slight structure damage
11 = few buildings standing, bridges
destroyed, cracks in ground
Problems caused by earthquakes
Liquefaction
- this is when a sandy or
silty soil acts like a liquid
when it is shaken by an
earthquake
- the quake distorts the
soil shape and the pores
collapse which causes
buildings on top to
collapse
Problems caused by earthquakes
Tsunamis* Earthquakes at sea can cause seismic sea
waves or Tsunamis to form
* Tsunamis are most devastating around the
Pacific Ocean Ring of Fire because of the
converging oceanic/contintal plate
movements
Indonesian Earthquake Dec. 26, 2004
Fast facts:
Magnitude 9.0
Epicenter: 150 miles off of Northern Coast of
Sumatra
Cause: The devastating megathrust
earthquake of December 26, 2004, occurred
on the interface of the India & Burma plates
& was caused by the release of stresses that
develop as the India plate subducts beneath
the overriding Burma plate.
Tsunami formation
Up & down motion of quake
moves water up and down
Original wave splits,
1 heads toward open ocean,
1 towards local coast
both ½ of original wave height
As waves move, the
wavelength decreases
& they get taller
(amplification)
Runup – when wave approaches
land, the wave builds up (but does
not cause a giant wave),
instead it is a very powerful wave that hits
shoreline
Map of travel times for P waves (in minutes)
Sad reminders to be wary of nature:
over 140,000 people died in this after
Christmas Tsunami
Quake-Resistant Structures
The # of deaths associated with quakes has
gone down a lot because cities build (or
retrofit buildings) to withstand a tremor
How do they do this?
- They use steel rods to reinforce walls in
older buildings
- They install flexible, circular moorings in
newer buildings to help them ride out the
wave & cushion the building