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spyfleaUrban and Civil

Nov 25, 2013 (3 years and 9 months ago)

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1
Natural Disasters

Earthquakes & Their Damages

2
San Francisco
-

1906


M = 7.8; 3,000 killed

3
What is an Earthquake?


Ground shaking usually accompanied by sudden
movement on a fault

4
What Causes Earthquakes?


Elastic Rebound Theory



movement on 2 sides of a fault leads to bending of rock
until it snaps to release the bending strain

5
Stress vs. Strain


Stress



forces imposed on a rock


Strain



change in shape of a rock in response
to the imposed tress

6
Types of Strain



Elastic



Rocks revert to their
original shape when force is
relieved



Plastic



Rocks permanently change
shape





Brittle



Rocks fracture or break

7
Stress vs. Strain


With increasing stress, a rock deforms elastically, then
plastically, before ultimately fracturing

8
Where Do Earthquakes Occur?


Faults



fractures in Earth’s crust where rocks
on one side move past those on the other side

9
Types of Faults

10
Fault Terminology


Hanging wall


Fault block above the fault


Fault block below the fault


Footwall

11
Normal Fault


Hanging wall moves
down

relative to footwall


Exhibits lengthening or extension of the crust

12
Reverse Fault


Hanging wall moves
up

relative to footwall


Exhibits shortening or compression of crust


Have dips > 45
°



If dip’s < 45
°
, then thrust fault

13
Strike
-
Slip Fault


Horizontal displacement


Right lateral


As you face the fault, opposite side of
plate moves right


Types of Strike Slip:


Left lateral


opposite side of plate moves left

14
San Andreas Fault

15

What Type of Fault is This?

Reverse Fault

16
What Type of Fault?




Normal Fault

17
Type of Fault?

Thrust Fault

18
Measuring Earthquakes


Seismograph


An instrument that
measures Earth’s
movement



Seismogram


Records seismic waves

19
Locating Earthquakes


Focus

(hypocenter)


Location within the Earth where movement originates


Epicenter


Point on Earth’s surface directly above the focus


Energy radiates in all directions from the focus


Energy is in the form of “seismic waves”

20
Seismic Waves


Types of Seismic Waves


P
-
waves (primary)


Push
-
pull motion


Travel fastest


Travel through solids and liquids


S
-
waves (secondary)


Side
-
to
-
side motion


Slower than p
-
waves


Travel through solids only


Surface waves (Love)


Travel on Earth’s surface


Slowest


Causes the most damage

21
Step 1


Locating Epicenter


Measure the distance between P
-

and S
-
waves


This is the time difference in arrival times

22
Step 2
-

Locating Epicenter


Plot the time difference on y
-
axis (time interval)


Trace plot to
blue line

to determine distance to
epicenter


The greater the difference, the farther the epicenter

23
Step 3
-

Locating Epicenter


Measure the distance
around each
seismic
station



Epicenter may be located
anywhere on that line



Minimum of 3 seismic
stations needed to
determine epicenter


Triangulation

200 km

24
Measuring Earthquakes


Magnitude

-

measure of the strength of an
earthquake or the amount of energy released


25
Determining Magnitude


Measure amplitude of the
strongest wave



Amplitude is the height on
paper



Plot distance between p
-

and s
-
wave



Plot amplitude



Connect plots to determine
magnitude

26
Mercalli Intensity Scale

Intensity



measures two things:

1.
how strongly people feel the shaking

2.
The severity of the damage

Higher Roman #’s depict greater intensities

27
Intensity Map


Typically, intensity decreases
as you move away from
epicenter



Distribution not perfectly
circular from epicenter


Elongate parallel to fault


Intensities increase over
loose wet muds


i.e. near epicenter


intensity = 8


Oakland


intensity = 9

28
Loma Prieta, 1989



October 17
th
, 5:04 p.m.



San Francisco Giants vs. Oakland A’s



Also called “Earthquake Series”

29
Loma Prieta, 1989


7.1 M; 63 dead; >13,000 injured,


>1,000 homes destroyed, >23,000
damaged


$5.9 billion damage

30
Liquefaction


Occurs when water
-
saturated ground is reorganized due to
violent shaking


Sediment collapses, expels water, and causes ground to
subside

31
Northridge, 1994


January 17
th
, 4:30 a.m.


6.7 M, 67 deaths


Blind thrust


11 miles
deep

32
Damage Control


Collapsed structures


Reinforcement

Northridge Meadows

33
Damage Control


Collapsed Structures


Reinforce garages &
windows

34
Damage Control


Bridges collapse

35
Steel Reinforced Concrete

Steel sheathing to reinforce columns

36
Damage Control


Houses slip off foundations


Anchor foundations

37
Damage Control


Taller Buildings


Anchor together


Build far apart

38
Damage Control



Safeguarding your home