G 209 Notes 02-16-05

Major Hazards of Earthquakes

ground motion – when the earth moves it shakes things apart (like buildings and houses) and moves things of off tables and shelves which can cause injuries.

liquefaction – conversion of formerly stable soil to a fluid mass, causing the structures built on it to fall over

landslides – when the earth shakes it can shake dirt down mountains

fire – indirect result of earthquakes, often triggered by broken gas lines or fallen power lines

tsunamis – when earthquakes displace seafloor you get tsunamis

The severity of damage from an earthquake depends on your position relative to it. It also depends on the depth of the focus. Foci of enough depth can have no effect to the people on the surface. The distance from focus, material properties and nature of the building all factor into how much damage is done.

Ground Shaking

Frequency differs for different seismic waves.

High frequency body waves shake low buildings more and high buildings less. Low frequency waves shake high buildings more and low buildings less.

Construction depends on damage. Wood is more flexible and holds up well provide the nails don’t wiggle out.

Unreinforced masonry is very vulnerable, the bricks fall over and are very damaging to humans.

Reinforced concrete with steel beams is very good.

Bridges often fall down during earthquakes because of ground displacement. When roads are broken apart it makes it difficult if not impossible for emergency crews to get in.

Earthquakes often disrupt utilities so there’s no gas or water (to put out fires).

The stories of buildings shake at different frequencies. Often the first story only will be effected and all the other floors come tumbling down. Often the upper stories will fall down and the first floor will be relatively intact.

Soil Liquefaction

The shaking of the molecules starts them to bounce around like a fluid and then fluid is let in.

The land sinks with liquefaction as the soil is compressed down after the water leaves.

In Alaska all soil on the bay liquefied and flowed down to the bay.

You get sand boils, subsidence (dropping of ground) disrupts the ground water and often redirects it. You get little sand volcanoes and little creeks of sand showing up in peoples yards and driveways.

In Nagasaki the buildings were built to withstand the shaking but they fell over like dominoes (in one piece).

Ground water rushes to the surface and the loss of underlying loose material moves structures.

Lateral spreading – water flows to either side and pulls the ground with it. This results in a big crack on the surface not caused by plate movement or fault breakage.

Liquefied soil puts pressure on retaining walls and can cause them to tilt. Increased water pressure causes collapse of dams.

Landslides

1970 – Peru earthquakes was relatively small and didn’t result in any damage from shaking but a big landslide ensued and wiped out 2 towns. In Alaska a little earthquake cause a landslide that went a mile across an uninhabited plain.

Fire!

Broken gas lines are the biggest problems. Lanterns and candles get knocked over, sparks from down power lines all contribute to creating the fire. Broken water mains and blocked roads make it hard to fight a fire. You need a cistern for water.

Tsunamis

When the seafloor abruptly deforms and displaces water you get a tsunami. The speed is related to water depth. The shallower the water the slower the wave goes but the closer the waves get to together.

Shikolan, Kuriti islands, a wave carried a boat 70m inland and deposited debris everywhere.

Geomorphological Changes (changing of the shape of geological features)

Earthquakes make relatively permanenent changes in the land. They can break rocks and leave a fault. The plate boundary can go away but the fault stays and you get intraplate earthquakes