Earthquakes

Overview

Usually occurs on or near plate boundaries

Most severe occurs along constructive plate boundaries e.g. San Andreas Fault – movement of Pacific and North Atlantic plates past each other.

Although can occur along all plate boundary types

Nearly 80% of all earthquakes occur in the circum-Pacific belt, most of these resulting from convergent margin activity (plates move towards each other)

Definition

Defined as the shaking, trembling or vibration caused by a sudden release of energy along faults due to movements in the Earth’s crust

Usually associated with the faulting or breaking of rocks

Aftershocks cause continuous realignments of the Earth’s crust

Focus = point inside the crust where the pressure is released

Epicentre = point on the Earth's surface above the focus

Earthquakes can be grouped into three categories based on the depth of their focuses:

• Shallow focus - foci are less than 70 km depth - most destructive earthquakes
• Intermediate focus - foci are between 70 and 300 km depth
• Deep focus - foci are greater than 300 km

Energy is released in seismic waves which spread out from the focus.

Waves are felt most strongly at the epicentre, most destruction occurs here: reduction in intensity with increased distance

Elastic-Rebound Hypothesis

First theory to satisfactorily explain earthquakes - idea that strain builds up in rock until the elastic limit (strength) of the rock is exceeded

The rock then ruptures (fails) at a point, snapping back toward an unstrained position releasing the elastic energy as seismic waves radiating outward from the break

The greater the stored strain, the greater the release of energy

Seismic waves

The rupture of rocks produces two types of waves (occurs in every earthquake):

• Body waves – primary and secondary
  • Primary wave = pressure waves caused when rock is pushed or pulled forward or backward. They travel through solids, liquids, or gases. Are the fastest waves sent out by an earthquake
  • Secondary wave = slower than primary waves, only travel through solids. Move material perpendicular to wave movement. The second-fastest wave sent out by an earthquake

• Surface waves – Rayleigh and Love

When primary and secondary waves reach the surface, some are transformed into surface waves

Move more slowly than body waves but produce the most severe ground movements, especially to buildings

Waves travel just below or along the ground’s surface

P waves arrive first, then S waves, then L and R

Seismometers used to measure earthquake power by detecting vibrations in the Earth’s crust

• Average speeds of all four waves are recorded
• After earthquake the difference in arrival times at a seismograph station of the waves can be used to calculate the distance from the seismograph to the epicentre
• Three seismograph stations are needed to locate the epicentre of an earthquake - a circle where the radius equals the distance to the epicentre is drawn, the intersection of the circles locates the epicentre

Measuring

Magnitude –…