2. Internal Structure of the Earth

Earth ’ s Internal Structure: Compositional Layers Crust:  Continental crust (20-70 km)  Oceanic crust (~6 km) Mantle  Upper mantle  Lower mantle (660km km) Core  Outer core (liquid)  Inner core

Earth ’ s Internal Structure: Mechanical Layers Lithosphere Asthenosphere (weak layer) Mesosphere (mantle) Outer Core (liquid) Inner Core

Earthquake Seismology I. Earthquake descriptors II. Seismic waves III. Earthquake location

What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy released radiates in all directions from its source, the focus (or hypocenter) Energy propagates in the form of seismic (elastic) waves Sensitive instruments (seismometers) around the world record the event

What causes an earthquake? Earthquakes are usually caused by sudden movement on faults

(hypocenter) Basic terminology

Earthquake Descriptors Epicentral angle

How are earthquakes generated ? Elastic rebound Earthquake mechanism  Stick: stress builds up on rough surfaces that is locked  Slip: sudden slip on the locked surface (focus) when stress becomes too high  Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape (elastic rebound) Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome

Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years Foreshocks and aftershocks

Seismology The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese Seismographs, instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

Basics of waves frequency(1/T), period(T), amplitude (A), velocity (V) and wavelength (V/f)

Seismic waves: Body wave P wave S wave Surface wave Reyleigh wave Love wave

 Travel through Earth’s interior  Two types based on mode of travel  Primary (P) waves  Push-pull (compress and expand) motion, changing the volume of the intervening material  Travel through solids, liquids, and gases  Generally, in any solid material, P waves travel about 1.7 times faster than S waves  Secondary (S) waves  shear motion at right angles to their direction of travel  Travel only through solids Body waves

Body Waves P and S waves Particle Motion

Surface Waves

Surface waves are BAD !!

Wave Velocity

One dimensional wave equation: x+dxx When the hammer hit the bar, a stress imbalance occurs between x and x+dx: Using Taylor expansion,

Recall that: But: Where dA is the cross-section area of the bar.

We have: For elastic media, stress is related to strain, which is measured by the gradient of displacement:

We have: Wave equation: