Earth Science 8.3 Earthquake Hazards

Earth Science 8.3 Earthquake Hazards

Earthquake Hazards

Causes of Earthquake Damage

The Prince William Sound earthquake struck Alaska in 1964 and was the most violent earthquake to hit North America in the 20th century.

It had a moment magnitude of 9.2 and lasted 3 to 4 minutes. It left 131 people dead and thousands homeless damaging the major ports and towns of Alaska.

Had this earthquake happened in more populated areas, it’s affect would have been devastating.


An earthquake as powerful as the 1964 earthquake in Alaska can cause catastrophic damage.

But even less powerful earthquakes still pose a severe danger.

Earthquake related hazards include seismic shaking, liquefaction, landslides, mudflows and tsunamis.


Seismic Shaking:

The ground vibrations caused by an earthquake, called seismic shaking, are the most obvious hazard of an earthquake.

Seismic waves interact to jolt and twist structures. Buildings made of unreinforced brick may collapse.

Wood frame buildings may

remain intact but can be knocked clear off their foundations.


Seismic Shaking: Seismic shaking is generally

strongest closest to the epicenter.

Strong seismic shaking can

occur in areas of loose soil or filled in land far from an epicenter.

The filled soil magnifies the effects of the seismic waves.

Structures in such an area can experience severe damage though far from the epicenter.


Liquefication:

Where sediment layers and rock are saturated with water, earthquakes can cause a process called liquefaction.

When liquefaction

occurs, what had been stable soil suddenly turns into liquid.


Liquefication:

The liquid cannot support buildings or other structures. Buildings and bridges may settle and collapse.

Underground storage tanks or sewer lines may float to the surface.


Landslides and Mudflows:

Earthquakes can trigger

different types of mass movements. These mass movements can often do more destruction and loss of life than the initial quake.

Earthquakes often cause loose rock and soil on slopes to move. These movements are called landslides.


Landslides and Mudflows:

In areas where the water

content of the soil is high, an earthquake can start a mudflow. During a mudflow, a mixture of soil and water slides downhill rapidly burying everything beneath.


Tsunamis: A tsunamis is a wave

formed when the ocean floor shifts suddenly during an earthquake.

In 2004, a magnitude

9.4 earthquake west of the island of Sumatra in the Indian ocean produced devastating tsunamis.


Tsunamis: A tsunamis is a wave

formed when the ocean floor shifts suddenly during an earthquake.

In 2004, a magnitude

9.4 earthquake west of the island of Sumatra in the Indian ocean produced devastating tsunamis.


Tsunami’s form: when an earthquake pushes up a slab of ocean floor along a fault.

An underwater landslide

or volcanic eruption can also trigger a tsunami.

Once formed, a tsunami

resembles the ripples created when you drop a stone in the water.


A tsunami wave in open ocean is only a meter high but moves very fast; at hundreds of miles per hour.

As the wave enters

shallower water near a shore, the water slows down and the waves begin to pile up.

When the waves do hit

shore, they are much bigger due to the compression and hit with devastating force sweeping inland for miles.

Reducing Earthquake Damage

Earthquake damage depends on several factors.

Two very important factors

are the strength and duration of the seismic shaking and the materials and design of structures.

Earthquake damage and loss of life can be reduced by determining the earthquake risk for an area, building earthquake resistant structures, and following earthquake safety procedures.


The first step to reduce earthquake risk is to assess the danger or probability of an earthquake in an area. Earthquakes are most common along the boundaries of Earth’s tectonic plates.

Scientists use several methods to determine earthquake risk.

One way is to study

historical records of earthquakes and look at where they have occurred in the past.


Scientists also use devices to measure the uplift, subsidence, and strain on rocks in the fault areas.

They also study seismic gaps. A seismic gap is an area along a fault where there has not been any earthquake activity for a long period of time.

Scientists hypothesize that the buildup of strain along a seismic gap will eventually lead to an earthquake.


Scientists also look for small warning signs that an earthquake may be about to strike. They may measure water levels and pressure in wells, radon gas emissions, small changes in the electromagnetic properties of rocks even.

Efforts at short term prediction of earthquakes are at present far from adequate to allow timely warning.


Seismic Design safety:

Many modern cities in earthquake prone areas, such as San Francisco and Los Angeles, have modern building codes that take earthquakes into account.


Seismic Design safety: several examples

Steel building frames are

often reinforced with cross-braces.

Buildings are sometimes

mounted on large rubber and steel pads called base-isolators that absorb shock.

Wooden frame houses are

often reinforced and bolted to their foundations more securely.

Flexible pipes and automatic

shutoff valves prevent rupture of gas and water lines and leakage of gas or fluids.

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Earth Science 8.3 Earthquake Hazards