Earthquake Hazards

Credit: U.S. Geological Survey Earthquake Hazards Owain Thomas

An earthquake occurs when rocks break. Plate movements put the rocks under pressure. The energy stored in the rocks is released as earthquake waves. Credit: U.S. Geological Survey The rocks break, this is called a fault. The earthquake waves lose energy as they pass through the surrounding rocks. The point where the earthquake starts is called the focus. When the waves reach the surface, they cause a great deal of damage. The area above the fault is called the epicentre. If the focus is near the surface, the damage will be greater.

Credit: U.S. Geological Survey Exercise 1 Write a short paragraph to explain what causes earthquakes. pressure rocks fault plate movement epicentre waves break stored energy focus

Credit: U.S. Geological Survey California is one of the most seismically active parts of the planet - this is the story of one of the most famous earthquakes ever:

California has many powerful earthquakes because of its location. This part of the USA is sitting on a plate boundary, Marked by a huge system of faults, including the San Andreas Fault. Credit: U.S. Geological Survey North American Plate The North American plate is moving one way and the Pacific plate is moving the opposite way. Pacific plate

The 1906 San Francisco earthquake This a picture of the City Hall in San Francisco after the massive 1906 earthquake. Credit: U.S. Geological Survey Credit: U.S. Geological Survey On the 18th of April 1906 a massive earthquake hit San Francisco. It was caused by movement along part of the San Andreas Fault. The section of the fault that moved was about 430 km (267 miles) long.

The earthquake was very powerful. Well constructed buildings built on solid rock were not badly damaged but many of the houses were built on soft, weak sand. Credit: U.S. Geological Survey During an earthquake, the shaking makes sand grains move around knocking into each other, almost like particles in a liquid. The sand changes into “quicksand” and buildings simply sink and collapse. The earthquake cracked gas pipes and very soon there were fires all over the city. Many of the buildings that did not collapse in the earthquake were destroyed by the fire afterwards. 28,000 buildings destroyed 350,000 people homeless 2500 (estimated) people dead

The power of the earthquake depends on: • How much the fault moved. • The length of the fault. • How much energy built up in the rocks before it • moved. Credit: U.S. Geological Survey Its effects also depend on: • The depth of the earthquake focus. • The types and quality of buildings on the surface. • The behaviour of people. Exercise 2 Copy this information into your book.

Predicting earthquakes is very difficult because there are so many different factors involved. One way is to monitor the behaviour of the ground. Credit: U.S. Geological Survey The scientists measure the distance between two points fixed into the ground. As the rocks are put under pressure, this distance changes. Eventually, the rocks break causing an earthquake.

Credit: U.S. Geological Survey Predicting earthquakes Radon gas given off. Water level in wells changes suddenly. Minor earthquakes - foreshocks. Electromagnetic phenomena - earthquake lights.

Sometimes, animals behave strangely before an earthquake. Credit: U.S. Geological Survey (This is a joke, cows do not say “woof” even before an earthquake!)

Credit: U.S. Geological Survey Exercise 3 Make a list of the possible ways of predicting earthquakes. ground movement foreshocks animals radon water levels earthquake lights

Credit: U.S. Geological Survey The End

Earthquake Hazards