The point where an earthquake occurs is called the focus (see FIGURE1 ). Earthquakes can occur near the Earth’s surface or up to 700 kilometres below. The shallower the focus, the more powerful the earthquake will feel. Energy travels quickly from the focus point in powerful seismic waves , radiating out like ripples in a pond. The seismic waves decrease in strength as they travel away from the epicentre . There are three main types of seismic waves. Primary waves , or P-waves, are the first waves to arrive, and are felt as a sudden jolt. Depending on the type of rock or water in which they are moving, these waves travel at speeds of up to 30 000 kilometres an hour. Secondary waves , or S-waves, arrive a few seconds after the P-waves and travel at about half the speed. These waves cause more sustained up-and-down movement. Surface waves radiate out from the epicentre and arrive after the main P-waves and S-waves. These move the ground either from side to side, like a snake moving or in a circular movement. Even very strong buildings can collapse as a result of these combined movements.
SkillBuilder discussion Geographical inquiry 1. Study FIGURE2. Discuss the interconnection between plate boundaries (faults) and earthquakes.
FIGURE2 Earthquakes in our region, 1900–2013
2. Identify which of
Australia’s neighbours experience earthquakes and have volcanoes. 3. Describe why Australia experiences few major earthquakes.
Source: USGS
Measuring earthquakes Earthquakes are measured according to their magnitude (size) and intensity. There are three ways we measure earthquakes. The Richter scale The Richter scale was first developed in 1935 and has been, until recent times, the main way we measure the magnitude of an earthquake. It operates as a logarithmic scale where each number on the scale is approximately ten orders of magnitude larger than the previous number.
TOPIC14 Geomorphological processes and hazards
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