Chapter 11

Where do earthquakes begin?

rock slippage originates in the ground at the focus or hypocenter

What is the epicenter?

It is the point on the ground surface directly above the focus

Reid’s Elastic Rebound Theory

The movement of tectonic plates causes the rocks to bend and store elastic energy. Once the strength of the rocks is exceeded, slippage along the fault produces an earthquake. The rocks return to their original shape, but in a new location.

Where do earthquakes occur?

They occur along both new and preexisting faults in places where differential stresses cause the crust to break

Normal faults

Associated with divergent plate boundaries

• not associated with large earthquakes

Reverse and Thrust faults

Associated with convergent plate boundaries

• In a subduction zone, the boundary forms a megathrust fault

• Produces the most powerful earthquakes

Strike-slip faults

Associated with transform plate boundaries

• gradual displacement called fault creep

• Ruptures result in major earthquakes

What is a fault slip?

A term used to describe the amount of displacement on the fault surface.

What are the two types of seismic waves?

Body waves: travel through the Earth’s interior

Surface waves: travel in the rock layers just below Earth’s surface

What are the two subtypes of seismic waves for Body Waves?

• Primary (P) waves are compression waves that travel through all materials in a longitudinal direction. They are fast.

• Secondary (S) waves are shear waves that can only travel through solid material, they are slower than P-waves but faster than surface waves.

How do surface waves move?

One causes up and down motion, this is similar to ocean swells. The other causes the ground to shake from side to side and this causes the greatest destruction.

Fastest to slowest rank the waves

P-waves, S-waves, Surface waves (highest amplitude)

How do you find the epicenter of an earthquake?

You compare the arrival time of the first P-wave and the first S-wave

How is intensity used to measure an earthquake?

A measure of the amount of ground shaking at a particular location based on observed property damage

AKA modified Mercalli intensity scale

How is magnitude used to measure an earthquake?

Quantitative measurement of ground motion based on data from seismic records used to estimate of the amount of energy released at an earthquake’s source

On the Richter Scale, each step represents a 10 fold increase in _______ and a 32 fold increase in _______

A 10 fold increase in wave amplitude and a 32 fold increase in energy released

What can amplify seismic waves?

Soft sediments can amplify seismic waves more than solid bedrock

What is liquefaction?

The phenomenon where loosely packed, waterlogged sediments behave as a fluid during the intense shaking of an earthquake

Seiches

Rhythmic sloshing of water in lakes, reservoirs, and enclosed basins.

What is the zone of greatest seismic activity called?

circum-Pacific belt

Where do the largest earthquakes occur?

along megathrust faults of convergent plate boundaries

How are short and long range predictions of earthquakes done?

Short: predictions based on precursors

Long: predictions based on cyclic pattern of fault/earthquake

What are Seismic gaps?

Are tectonically quiet zones along a fault where strain is currently building up.

The stored strain will be released in a future earthquake. Strain can be estimated!