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

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

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. The North American plate is moving one way and the Pacific plate is moving the opposite way. Pacific plate North American Plate Credit: U.S. Geological Survey

This a picture of the City Hall in San Francisco after the massive 1906 earthquake. The 1906 San Francisco earthquake 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. Credit: U.S. Geological Survey

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. 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 Credit: U.S. Geological Survey

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. 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. Credit: U.S. Geological Survey

Predicting earthquakes is very difficult because there are so many different factors involved. One way is to monitor the behaviour of the ground. 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

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

Sometimes, animals behave strangely before an earthquake. (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 radon animals water levels earthquake lights foreshocks Credit: U.S. Geological Survey

The End Credit: U.S. Geological Survey