Earthquakes
Introduction
This chapter looks at earthquakes and their connection to plate tectonics. It also covers seismic waves and devices used for measuring them.
The history of studying earthquakes
Thousands of years ago people thought that earthquakes were caused by angry gods who were 'shaking the Earth'. After carefully observing and studying earthquakes, scientists of ancient China realised that earthquakes are caused by powerful forces of nature. The Chinese developed the first device that could roughly predict earthquakes. The Ancient Greek scientist Aristotle came up with a theory that earthquakes are caused by heavy winds.
See image 1.
Robert Mallet was a nineteenth century scientist who managed to measure the speed at which earthquakes spread. Later, in Italy, Luigi Palmieri built a device that could measure the strength of the earthquake. It was installed near Mount Vesuvius with another at the University of Naples. It was only at the beginning of the twentieth century, however, when the origins of earthquakes were fully understood and scientifically explained. The Scientist Alfred Wegener was the one who came up with the theory of plate tectonics and their connection with earthquakes.
Origins of earthquakes
Most earthquakes and volcanic eruptions happen along the boundaries of tectonic plates. The most tectonically-active area on Earth is the Pacific Ring of Fire which is the place where several tectonic plates meet. Most earthquakes and volcanic eruptions registered today happen in this area. Areas like this are also called seismically and volcanically active. See image 2.
Forces that push tectonic plates can build up for decades. When that energy is released, the plates slip over each other, sending strong shocks all around them. Earthquakes occur when these energy shocks become stronger than the forces that hold the Earth's surface material together. The shaking and vibrating of the Earth's surface is called an earthquake.
See animation 1.
The underground starting point of an earthquake is called the focus. It can be as deep as 600 kilometres. The aboveground starting point of an earthquake is called the epicentre. The zone around an earthquake's epicentre is damaged the most. In this area the ground cracks and shifts in all directions.
Foreshocks are the small shocks that mark the movement of rocks beneath the Earth's surface. Strong earthquake shocks often follow. These series of shocks are called 'seismic waves.'
Seismic waves
There are two types of seismic waves:
- P wave or primary wave - is the fastest kind of seismic wave.
- S wave - is the secondary wave. That means that it is the second wave you would feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock.
Measuring earthquakes
The power of an earthquake is measured by the Richter scale for their magnitude. A seismograph is a device that registers and measures an earthquake. The goal of a seismograph is to accurately record the movements of the ground during an earthquake. New, modern seismograph technologies help scientists to better understand and predict earthquakes. The main problem that must be solved in creating a seismograph is that when the ground shakes, so does the instrument that is supposed to measure it, so seismographs must be very heavy. According to the Richter scale, anything below 2.0 is not noticeable by people and is called a microquake. Microquakes happen very often. Moderate earthquakes measure less than 6.0 on the Richter scale. Earthquakes measuring more than 6.0 can cause serious damage. The maximum quake rating ever measured is about 8.9. See image 3.
One of the worst earthquakes recorded happened on September 1, 1923 in Japan. Its power was about 8.3 on the Richter scale and it killed about 143 000 people.
Some people believe that animals may feel vibrations from an earthquake before humans do. Animals might start behaving differently just minutes before an earthquake.
