Earth Science

Earthquakes(Chapter 8)

Earthquakes

General features • Vibration of Earth produced by the rapid

release of energy.• Focus – the place within Earth where

earthquake starts.• Epicenter - Point on the surface, directly above

the focus• Caused by slippage along a break in the

lithosphere, called a fault.• Fault: Fracture in earth where movement has

occurred.

Earthquake focus and epicenter

Earthquakes Types of Faults (draw and label each

fault type on right)1. Normal Fault: Fault in which the rock

above the fault plane has moved down relative to the rock below.

2. Reverse Fault: Fault in which the material above the fault plane moves up relative to the material below.

Earthquakes

Types of Faults (draw faults on right)3. Thrust Fault: Reverse fault with a dip less

than 45 degrees.

4. Strike-slip fault: A fault along which the movement is horizontal and parallel to the trend of the fault.

The Cause of Earthquakes

Deformation of Rocks• Forces within Earth slowly deform the rock

that makes up Earth’s crust causing the rock to change shape, or bend on both sides of a fault.

• When the rocks bend they store energy, much like a rubber band when stretched.

The Cause of Earthquakes

Elastic Rebound sequence of events…1. Stored energy builds as the rock is deformed.

2. Suddenly the rocks slip, or break at their weakest point (the focus) and the stored energy is released all at once.

3. The deformed rock ‘springs back’ to its original shape. This is called elastic rebound.

Elastic Rebound

EarthquakesGeneral features

• Earthquakes are often preceded by foreshocks and followed by aftershocks.

Earthquakes

Earthquake waves • Study of earthquake waves is called seismology • Earthquake recording instrument (seismograph)

• Records movement of Earth • Record of Earth’s movement is called a

seismogram

Seismograph

Earthquakes

Earthquake waves• Types of earthquake waves

• P - waves• Push–pull (compressional) motion • Travel through solids, and liquids• Greatest velocity of all earthquake

waves

P - waves

Earthquakes

Earthquake waves•Types of earthquake waves

•S - waves•Up-down motion•Travel through solids, but NOT liquids.•S - waves travel slower than P-waves

S - waves

Earthquakes

Earthquake waves• Types of earthquake waves

• Surface waves (occurs when an S-wave reaches the surface)• Travel slower than P-waves and S-waves • Move up and down as well as side to side (similar to an

ocean wave) • Most destructive seismic wave

A seismogram records wave amplitude vs. time

Earthquakes Earthquake intensity and magnitude

• Intensity• A measure of the degree of earthquake shaking at a

given locale based on the amount of damage• Most often measured by the Modified Mercalli

Intensity Scale

• Magnitude• Concept introduced by Charles Richter in 1935

Earthquakes Earthquake intensity and magnitude

• Magnitude (M)• Often measured using the Richter Scale

• Based on the amplitude of the largest seismic wave

• A tenfold increase in wave height equals an increase of 1 on the Richter scale.

• Does not estimate adequately the size of very large earthquakes.

• Scientists no longer use it routinely.

Earthquakes Earthquake intensity and magnitude

• Magnitude• Moment magnitude scale

• Measures very large earthquakes • Derived from the amount of

displacement that occurs along a fault zone

• More precise than the Richter Scale

Magnitude 5 or greater earthquakes over 10 years

Earthquakes Locating an earthquake

• Located using the difference in the arrival times between P and S wave recordings, which are related to distance (Travel-time graph)

Travel-time graph

How long does it take a P wave and and S wave to travel 1000 km?

P-wave =

S-wave =

Which seismic wave is faster?

Earthquakes

Locating an earthquake • Epicenter

• At least three station recordings are needed to locate an epicenter

• Circle equal to the epicenter distance is drawn around each station

• Point where three circles intersect is the epicenter

The epicenter is located using three or more seismic stations

Earthquakes

Causes of Earthquake Damage • Earthquake-related hazards

• Seismic shaking• Liquefaction• Landslides• Mudflows• Tsunamis

Earthquakes

Earthquake destruction • Destruction results from …

• Seismic shaking (the ground vibrating)• May jolt and twist structures• Unreinforced brick or concrete structures may

collapse • Liquefaction of the ground

• Saturated soil material turns fluid • Underground objects may float to surface

Damage caused by the 1964 earthquake in Alaska

Damage from the 1964 Anchorage, Alaska, earthquake

Earthquakes

Earthquake destructionDestruction resulting from…

Landslides and MudflowsMovement of loose rock and soil

down slopeIf the water content of the soil is

high an earthquake can start a mudflow

Landslide

Mudflow

EarthquakesEarthquake destruction

Destruction resulting from…Tsunamis

Ocean wave formed when the ocean floor suddenly shifts.

Move fast (100+ km/hr) across the open ocean

A Tsunami slows down and increases in size as it approaches shore

Formation of a tsunami

Tsunami travel times to Honolulu

Earthquakes

Assessing Earthquake risk • Earthquakes are more frequent along the

boundaries of Earth’s Tectonic plates. • No reliable method of predicting an Earthquake

occurrence in the near future.