Earthquakes

IB SL

What Are They?

• Earthquakes are a sudden, violent shaking of the Earth’s surface.

• Earthquakes occur after a build-up of pressure causes rocks and other materials to give way.

• Most of this pressure occurs at plate boundaries when one plate is moving against another.

• Earthquakes are associated with all types of plate boundaries.

Focus And Epicentre

• Focus: Refers to the place beneath the ground where the earthquake takes place.

• Deep Focus: earthquakes are associated with subduction zones.

• Shallow Focus: earthquakes are located along conservative boundaries.

• Epicentre: is the point on the ground surface immediately above the focus.

What are earthquakes and where do they occur?

Earthquakes are vibrations caused by earth movements at plate boundaries and at major fault lines (cracks in the earth’s surface).

They can occur at all 4 major plate boundaries but the most severe earthquakes are normally found at CONSERVATIVE and DESTRUCTIVE boundaries.

CONSERVATIVE

DESTRUCTIVE

Why do earthquakes happen?Why do earthquakes happen?

Primary Hazards

• These are hazards due directly to the earthquake itself, and consist of ground movement and shaking.

• Earthquakes emit body waves (travel through the earth) and surface waves.

• It is surface waves that cause most damage as they impact on what is on the surface of the earth.

• They can cause buildings to collapse and underground pipelines to be broken.

• Damage caused by surface waves varies. For example, the Mexico City quake of 1985.

• Its impact was increased four fold as the city is built on ancient lake sediments.

Secondary Hazards

• Soil liquefaction: Solid material changed into a liquid state. Damages building foundations, resulting in them sinking.

• Landslides: Often as a result of the ground shaking, even if a slope is gentle. Cause burial of people and overrun buildings.

• Tsunami (tidal waves): If the focus of the quake is beneath the sea, tsunami can occur. Ninety percent occur in the Pacific basin. The more movement of the sea floor and the shallower the focus the larger the wave that is created.

Activity

Using Essential AS Geography P42-45, answer the following…

1.Define the words Focus, and Epicentre.2.What Short, and Long-Term problems are caused by earthquakes?3.Why are so many of the world’s largest cities located on plate boundaries?4.Why do people in LEDC’s suffer more than in MEDC’s when an earthquake occurs?

Earthquake Predictability

The following are elements of an earthquake hazard:

•Time of earthquake.•Date of earthquake.•Exact location of epicentre.•Fault upon which the quake occurs.•The magnitude of the earthquake.•The economic impact of the earthquake.•The death toll of the earthquake.

For each of the elements explain how predictable it is.