SeismicitySeismicity

Seismic Image of Venezuela BasinSeismic Image of Venezuela Basin

Seismic wavesSeismic waves

Seismic wave travel in a Seismic wave travel in a homogeneous Earthhomogeneous Earth

Seismic wave travel in which density Seismic wave travel in which density increases monotonouslyincreases monotonously

Seismic wave travel in a layered Seismic wave travel in a layered EarthEarth

The P-wave shadow zoneThe P-wave shadow zone

The S-wave shadow zoneThe S-wave shadow zone

P and S-wave velocitiesP and S-wave velocities

The Earth InteriorThe Earth Interior

The Earth InteriorThe Earth Interior

The distribution of landThe distribution of land

Hypsographic CurveHypsographic Curve

Extreme elevations are rare

Two common elevations Ocean basins

(abyssal plains)

Continental interiors

Why??

The weight of an Object is determined The weight of an Object is determined by its mass and the force of gravityby its mass and the force of gravity

F = the force of gravity (weight)F = the force of gravity (weight)

g = the gravitational constant (6.672 x 10g = the gravitational constant (6.672 x 10-11-11 N N · m· m22 · kg · kg-2-2))

mm11 = the mass of the object = the mass of the object

mm22 = the mass of the Earth = the mass of the Earth

r = the distance between the object and the center of the Earthr = the distance between the object and the center of the Earth

1 22

m mF g

r

Weight varies due to three factorsWeight varies due to three factors

1. Altitude1. Altitude

As altitude increases r increases, so F decreasesAs altitude increases r increases, so F decreases

1 22

m mF g

r

Weight varies due to three factorsWeight varies due to three factors

2. Latitude2. Latitude

Weight varies due to three factorsWeight varies due to three factors

3. Local variations in rock density3. Local variations in rock density

Geographic Survey of IndiaGeographic Survey of India

Mid-19Mid-19thth century century

Sir George EverestSir George Everest

Triangulation systematically Triangulation systematically disagreed with astronomical disagreed with astronomical near the Himalayasnear the Himalayas

Why? Why?

Archdeacon Pratt’s theory of IsostasyArchdeacon Pratt’s theory of Isostasy

Water density 1.0

0.5 0.50.3 0.30.2 0.2

density= 0.1

Each block weighs 2 g

Astronomer Royal Airy’s theory of IsostasyAstronomer Royal Airy’s theory of Isostasy

Water density 1.0

0.5 0.50.3 0.30.2 0.2

density= 0.1

Each block weighs 2 g

Water density 1.0

Wood density 0.5

2 g 2 g3 g 3 g5 g 5 g

8 g

Uncorrected gravity profile at 20,000 ft.Uncorrected gravity profile at 20,000 ft.

Free-Air correction to 0 ft.Free-Air correction to 0 ft.

Bouguer Corrected profileBouguer Corrected profile

1 22

m mF g

r

Free-air and Bouguer gravity Free-air and Bouguer gravity anomaly map of the Alpsanomaly map of the Alps

Isostasy Isostasy and and

ErosionErosion