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) – 300,0002001 northern India – 20,1002003 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 BeltAlpine-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 evolvingTo 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 driftThe Origins of Continents and Oceans (1915) based on known fossil, climate and geology

on opposite sides of the Atlantic Oceanargued the existence of a super

pro-continent PangaeaWegener 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 smoked-glass 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

(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

Intensity and Magnitude, cont

Adjustment to ‘Quakes

Experts have assembled a Theoretical Range of Adjustment to Earthquake Hazard(in G. White, Natural Hazard: Local, National and Global) (1) Affect the Cause(2) Modify the Hazard(3) Modify the Loss Potential(4) Adjust to the Damages