Earthquakes

Vibration of Earth produced by the rapid release of energy

The movement of the ground, is caused by waves of energy released as rocks move along faults which means that earthquakes are associated with movements along faults

Fault a large fracture in rocks, from several m to Kms long, where rocks move along which displacement has occurred

Explained by the plate tectonics theory

Elastic rebound theory

earthquakes occur as rock elastically returns to its original shape

Mechanism for earthquakes was first explained by H. Reid

Elastic Rebound Theory Rocks spring back into its original shape as stress removed

foreshocks & aftershocks

Earthquakes are often preceded by foreshocks & followed by aftershocks

Foreshocks indication, weak earthquakes, releas some of energy

Aftershocks followed the earthquakes, releas some of energy as crust return to its original shape (crust unstable)

Earthquake waves

Seismology Study of earthquake waves
Seismograph Earthquake recording instrument, Records movement of Earth
Seismogram Record made by seismograph

Types of earthquake waves
1. Body waves: Primary (P) & Secondary (S)
2. Surface waves

Types of earthquake waves

Body waves: Primary waves (P)
– Push-pull (compressional) motion
– compression & expansion
– First to be recorded at a seismograph
– Travel through solids, liquids, & gases
– Greatest velocity of all earthquake waves
– longitudinal wave: Causes rock to vibrate in the same direction the wave is traveling

Body waves: Secondary waves (S)
– Shake or transverse (shearing) waves
– causes the rock to vibrate at right angles to the direction of travel (to the direction of propagation)
– Travel only through solids
– Slower velocity than P waves

Surface waves
– Complex motion
– Slowest velocity of all waves
– The largest & most destructive
– like ocean waves

Locating an earthquake

Earthquake zones are closely correlated with plate boundaries
1. Circum-Pacific belt
2. Oceanic ridge system

When analysing an earthquake the first task seismologists undertake is determining its epicenter

Principle for study Earth’s interior

Most of our knowledge of Earth’s interior comes from the study of P & S waves

Travel times of P & S waves via Earth depend on properties of materials

Velocity of waves increases with depth.

Seismic waves are refracted as they pass through the earth

Waves follow strongly curved paths

S waves travel only through solids 

Earthquake intensity and magnitude 

Intensity A measure of the degree of earthquake shaking at a given locale based on the amount of damage
– measured by the Mercalli Scale

Magnitude amount of energy released
– Concept introduced by C.Richter 
– Measured using the Richter scale or Moment Magnitude scale

Magnitude Scales

Richter scale
– Based on amplitude of largest wave
– logarithmic scale: 1°→1, 2°→10, 3°→100
– Each unit = to 32-fold energy increase
– Doesn’t useful for large earthquake (>7)

Moment magnitude scale 
– Measures very large earthquakes
– Derived from the amount of displacement that occurs along a fault zone

< 2 → Micro
2 – 3.9 → Minor
4 – 4.9 → Light
5 – 5.9 →Moderate
6 – 6.9 → Strong
7 – 7.9 → Major
8 – 9 → Great

Earthquake destruction

Factors that determine structural damage
1. Intensity of the earthquake
2. Duration of the vibrations
3. The design of the structure
4. Nature of the material upon which the structure rests

Destruction results from
1. Ground shaking
2. Liquefaction of the ground
3. Tsunami, or seismic sea waves
4. Landslides & ground subsidence
5. Fires

Liquefaction caused by
1. Saturated material turns fluid
2. Underground objects float to surface
– during Liquefaction water-saturated soil behaves as a fluid. It becomes incapable of supporting much weight
– such as sandy soil

Earthquake prediction

Short-range prediction
– no reliable method yet devised

Long-range forecasts
– Premise is that earthquakes repetitive
– Region is given a probability of a quake

Earth’s Layers defined by composition

Crust Thin, rocky, & outer layer

Continental crust
– Upper crust composed of granitic rocks
– Lower crust is more akin to basalt
– Average density is about 2.7 g/cm³
– older than oceanic (Up to 4Ga)
– thickness 35-40 km
– thickness at some mountainous 70 km

Oceanic Crust 
– Basaltic composition 
– Density about 3.0 g/cm3
– Younger (<180Ma) than the continental
– thickness 7km

Mantle : Below crust to 2900Km depth
– Composition : of the uppermost is igneous peridotite & changes at greater depths

Core : A sphere having a radius of 3486 km
Location Below mantle
– Composition : iron-nickel alloy
– density ≈ 11 g/cm3

Earth’s Layers defined by physical properties

Lithosphere Crust & Uppermost mantle
– 100km thick
– Cool, rigid, & solid

Asthenosphere Beneath the lithosphere
– Upper mantle
– To a depth of about 660Km
– Soft, weak layer that is easily deformed

Mesosphere lower mantle
– 660-2900 km
– More rigid layer
– Rocks are hot & capable of gradual flow

Outer core Liquid layer
– 2270 km thick
– Convective flow of metallic iron within generates Earth’s magnetic field

Inner Core Sphere with a radius of 1216km
– Behaves like a solid

Rock Deformation

Deformation general term refers to changes in the original shape & size of a rock body

Most crustal deformation occurs along plate margins

Rocks deform permanently in 2 ways:
1. brittle deformation
2. ductile deformation

Brittle deformation is the fracturing of an object once its strength is exceeded

Ductile deformation type of solid state flow that produces a change in the size & shape of an object without fracturing the object

Stress

Stress is the force/unit area acting on a solid

Strain is the change in shape or volume of a body of rock as a result of stress

Types of Stress
1. Tensional stress
2. Compressional stress
3. Shear stress

Geologic Structures

Anticlines Fold upfolding or arching of strata

Synclines Fold downfolds in strata
– found in association with anticlines

Domes circular or elongated structure, produces by upwarping of the crust
– Older rocks are exposed in the center
– the youngest rocks exposed in the flanks

Basin circular or elongated structure, produces by upwarping of the crust
– youngest rocks are exposed near the center
– the oldest rocks exposed near the flanks

Faults: are fractures in the crust along which displacement has taken place

Faults

Dip- slip Faults: movement parallel to dip
1. Normal fault: occur if the hanging wall block moves down relative to the footwall
2. Reverse fault: hanging wall block moves up relative to the footwall block
3. Thrust fault: reverse with dips < 40°

Strike-slip faults are faults in which the movement is horizontal & parallel to the trend, or strike, of the fault surface

fault-block mountains

formed as large blocks of crust are uplifted & tilted along normal faults

Grabens formed by downward displacement of fault-bounded blocks

Horsts are elongated, uplifted blocks of crust bounded by faults

Mountain Building

Folded Mountains formed by compressional forces

Orogenesis result in forming of mountains

Young mountain belts (less than 100Ma):
1. American Cordillera
2. Alpine-Himalaya chain
3. volcanic island arcs (Philippines, Japan)

Older mountain belts
1. Appalachians in N-America
2. Ural Between Europe & Asia

Occurrence

Most mountain building occurs at convergent plate boundaries

Colliding plates provide the compressional forces that fold, fault, & metamorphose the thick layers of sediments deposited at the edges of landmasses

Ocean-Ocean Convergence produces volcanic mountains (volcanic islands arc)

Ocean-Continental Convergence volcanic mountains & folded mountains


Accretion process that occurs when crustal fragments collide with & stay connected to a continental plate

Accretionary wedge is the accumulation of different sedimentary & metamorphic rocks with some scraps of ocean crust

Terranes any fragments that have a geologic history distinct from that of the adjoining fragments
– Terranes occur along the Pacific Coast

The End