Download Hazards Chapter 3

CHAPTER 3
Plate Tectonics and
Earthquakes
Earthquakes
A major Earth occurrence throughout
geologic time
… interesting: if we look at in terms of their
association with plate tectonics, the Earth
cannot do without earthquakes
Great variability both in frequency and
intensity across this time
… interesting: it is possible that, on a worldwide
basis, about a million tremors occur each year;
however, only about 10% are felt by
populations
--- Looking at death tolls I guess we should
be glad that most of the annual
earthquakes go unnoticed
1976 Tangshan (China) – 300,000
2001 northern India – 20,100
2003 Bam (Iran) – 41,000
Seismology (study of earthquakes) has been
able to do a lot in recent history to help
understand the mechanism of earthquakes
--- we would very much like to do the thing
that we cannot do… consistently predict quakes
Geographic Occurrence of Earthquakes
There are clearly defined regions of
earthquake concentration
Circum-Pacific Ring
Mid-Atlantic Ridge Belt
Alpine-Mediterranean Belt
… but, quakes can potentially occur anywhere
Causes of Earthquakes
Most earthquakes begin in the Earth’s
lithosphere where, through geologic forces,
sufficient strain builds up to cause
fracturing of rock formations
--- these are geologic faults … along fault
lines rock strata may be displaced
vertically and/or laterally
This motion is neither “smooth” or constant
and most frequently occurs in sharp
grinding shocks that in a few seconds
displaces millions of tons of rock and soil
--- fault systems exhibit varying degrees of
active-ness, and in general, the more
active a fault system is, the more
frequent and minor the earthquakes
that the system spawns
-
Active faulting is associated with zones of
plate tectonics and continental drifting
… patterns of earth surface movement
collision/convergence,
divergence,
transverse movement
Development Of The Plate Tectonics
Concept
*With apologies to any of my physical students in the
audience*
Man has historically wondered about / tried
to explain what they saw and experienced
on the surface of the Earth
… the problem has always been that we are
so small and short-lived, and what we
wanted the answer for was so large and
slowly evolving
To get an answer took centuries
-
catastrophism probably dominated
explanation until the Age of Exploration
Exploration and greater attempts at
accurate mapping brought recognition of
the shape of continental margins
… noted by individuals such as Francis
Bacon [latter 1500s] (Novum Organum)
… Eduard Suess [1800s] argued for a
southern seas pro-continent
Gondwanaland
Alfred Wegener – German meteorologist
argued a theory of continental drift
The Origins of Continents and Oceans
(1915)
based on known fossil, climate and geology
on opposite sides of the Atlantic Ocean
argued the existence of a super
pro-continent Pangaea
Wegener was not widely accepted - he was
missing some important answers and had
to make some unsupported assumptions
Qs: where was it? Why the split? What
moves continents?
it was half a century before the supporting
data was found… Wegener doesn’t live
to see it
Harry Hess’s (Princeton, 1963) study of
ocean bottom and seafloor spreading is
credited with legitimizing Wegener
- paleomagnetism of iron-bearing oceanic
rock
- temperature of oceanic rock
-
so, in addition to earlier evidence
(1) ocean floor mapping revealed presence of
mid-ocean ridges whose configuration paralleled
the edges of continents
(2) parallel bands of magnetism is oceanic ridges on
both sides of the active ridge zones in both
Atlantic and Pacific Oceans
(3) discovery that continental rock approaches 4 bill
yrs age; oceanic rock is young at 200 mill yrs
(4) oldest oceanic rock associated with continent
margins; youngest with mid-ocean ridges
(5) rock temperature is highest at ridges, lowest at
continental margins
Aside
-
-
-
Greek philosophers attributed earthquakes
to subterranean winds or fires deep in the
Earth
130 AD the Chinese scholar Cheng Heng
reasoned that earthquakes were waves in
the Earth spreading outward from a single
source. He constructed a bronze vessel of
eight dragons each balancing a small metal
ball – the passing earthquake would cause
one or more of the balls to fall
(1st seismograph?)
Aside, cont
-
1859 an Irish engineer, Robert Mallet. Drew
on his knowledge of construction materials
strength and behavior when exposed to
stress to conclude, “either by sudden
flexure and constraint of the elastic
materials forming a portion of the eath’s
crust or by their giving way and becoming
fractured”, earthquakes were created
Aside, cont
- latter 1870s, English geologist John Milne
devised a forerunner of the seismograph
with a needle suspended over a smokedglass plate. It was the first instrument to
distinguish between p- and s-waves
- early 20th C. Russian seismologist Prince
Boris Golitzyn invented the seismograph.
He used a magnetic pendulum suspended
between the poles and an electromagnet
Earthquake Energy and Waves
- Rock strata along fault zones build high
levels of stress
--- as long as stress forces do not exceed
friction the rock mass will remain static
--- where stress exceeds friction,
slippage/movement occurs
- The higher the stress level before friction
fails, the greater and more violent will be
the subsequent movement
- Because of stress vs friction, movement
most frequently occurs where frictional
forces are weakest (focus)
… we designate epicenter as the point on
the Earth surface directly above the
focus
- Stored potential energy is released as
kinetic energy as earthquake waves
spreading outward from the eqicenter
Body waves – travel through/within the Earth
(1) primary wave (p-wave): compression
waves through the rock particles
[fastest because little distortion of rock]
(2) secondary wave (shear wave):
“s”–waves push rock particles
perpendicular to wave direction
[only about one-half speed of p-wave]
Surface waves – longest and slowest waves;
extremely destructive to surface structures
(produce Earth surface heaving)
(1) Rayleigh waves (R-wave)
(2) Love waves (L-wave)
- Difference in wave speed [seconds]
between P-waves and S-waves makes
possible determination of origin
- Presence or absence of S-waves give clues
to Earth composition
Four Zones of Seismic Activity
(1) Where plates slide past each other
- tranform or transverse faulting
- predominately horizontal plate movement
… may give rise to an opposing motion that
your text likens as escape tectonics where a
plate boundary is forced into a opposite
countering motion
- frequency and intensity of earthquake activity
is a function of plate speed and hardness of
opposing plate boundaries
- San Andreas Fault, CA
Four Zones of Seismic Activity, cont
(2) Collision zones between continental and
oceanic plates
- these are subduction zones where
lighter continental rock forces denser
oceanic rock into the Earth
- sites of enormous stress, and thus of
great earthquake activity both in
number and intensity
- Japan and the Philippines are classics
Four Zones of Seismic Activity, cont
(3) Boundary zones between active continental plates
- easily the best example is the continuing
India and Eur-Asia collision [Fig. 3.25]
following the breakup of Gondwanaland
… since initial contact India has moved 1,250 mi
(2,000 km) further north at a rate of 2 in
(5 cm) per yr, and has resulted in:
(1) single highest concentration of mts
(2) greatest concentration of mt systems
(3) extensive regional patterns of earthquakes
Four Zones of Seismic Activity, cont
(4) Spreading Centers and Earthquakes
- few earthquakes for the % of plate
boundaries that spreading centers
represent [volcanoes can be “wow”]
--- a lack of rigidity and reduces the
potential for building stress and
tension… therefore, less
earthquake potential
--- Iceland is the classic example
180 mill yrs on a spreading center
[see Fig. 3.19, Thingvellier]
To a lesser degree the activity of the younger
Red Sea-Gulf of Aden-East Africa Rift Valley
plate zone(s) [three spreading plates here]
--- have the characteristic long-narrow
spread center [Fig. 3.20]
rock material pulled both upward
(doming) and outward (rifting)
eventually the rifted area fills to become
a rift lake or will admit the ocean
Intensity and Magnitude
- We can describe an earthquake in several ways –
subjectively and quantitatively “assessing” its
damage. But, we need a way to standardize the
assessment of strength
- Two aspects are commonly utilized:
(1) intensity – a more descriptive term –
represented by the Mercalli Scale [for
Giuseppe Mercalli, 20th C. Italian
seismologist]; a largely subjective
description of damage at the point of
measurement
Intensity and Magnitude, cont
(2) magnitude – logarithmic quantification
of earthquake impact destruction;
the Richter Scale [for Charles Francis
Richter, American seismologist] is
common to all of us and uses a 10x,
0-to-? open ended scale, scale for
deriving the amplitude of various quake
waves
Interesting: Until 1979 it was thought that 8.5 was
the practical limit; now thought to be 9.5
Adjustment to ‘Quakes
Experts have assembled a Theoretical Range
of Adjustment to Earthquake Hazard
(in G. White, Natural Hazard: Local, National and Global)
(1)
(2)
(3)
(4)
Affect the Cause
Modify the Hazard
Modify the Loss Potential
Adjust to the Damages