Understanding Earthquakes: Origins, Measurement, and Risk
Understanding Earthquakes
Earthquake Origins
As an earthquake originates:
- Stress builds up.
- Tension increases, and materials deform.
- Materials fracture, releasing energy that vibrates the ground.
Measuring Earthquakes
Seismometers detect even very weak earthquakes that go unnoticed by people, drawing graphic seismograms. The magnitude of an earthquake is the amount of energy released, measured on the Richter scale, which is open-ended and has no limit.
Intensity vs. Magnitude
Intensity and magnitude are not the same. Intensity measures the effects of an earthquake, which are greatest at the epicenter and diminish with distance. Each earthquake has one magnitude, but its intensity varies. Intensity is measured on the Mercalli scale.
Plate Tectonics Theory
- The lithosphere is divided into 7 major and a dozen smaller plates.
- Heat in the Earth’s interior causes material beneath the lithosphere to move.
- Lithospheric plates are dragged by this movement, causing continents to shift over time.
- Plate movements generate friction, faults, and continental collisions, causing earthquakes.
The Earth’s internal heat agitates the lithospheric plates, generating earthquakes.
Ocean Ridges
Ocean ridges are where new lithosphere is formed. Plates diverge, and hot material from the Earth’s interior fills the gaps. Many small magnitude earthquakes occur here.
Subduction Zones
Subduction zones are where lithosphere is destroyed. Converging plates are forced beneath each other. These areas have high seismicity, with frequent and high-magnitude earthquakes.
Transform Faults
Transform faults are places where lithosphere is neither created nor destroyed. Lateral displacement of plates occurs, often resulting in large earthquakes.
Understanding Seismic Risk
Risk is the probability of damage or catastrophe to a population or property in an area, caused by a natural event. The natural phenomenon itself is not a risk until it threatens human interests.
Risk = Hazard x Exposure x Vulnerability
Risk depends on three factors:
- Hazard: The magnitude and frequency of a potentially catastrophic event.
- Exposure: The number of people and goods that may be affected.
- Vulnerability: The susceptibility of a community to damage.
Risk depends not only on the phenomenon but also on human decisions. Reducing the catastrophic effects of a natural event requires understanding the process and characteristics of the area. This allows for prediction and prevention.
Prediction is based on statistical data indicating the likelihood of a phenomenon in an area and relies on precursor events.
Prevention: Adopting measures to minimize the effects on people and property if an event occurs.
What are Earthquakes?
Earthquakes are ground vibrations caused by the abrupt release of energy stored in rocks under strain. They originate when large masses of rock fracture or when there is a new displacement along existing fractures, called faults. The hypocenter is the point of origin, and vibrations are transmitted in all directions. The epicenter is the point on the Earth’s surface directly above the hypocenter.
Earthquake Origins (Repetition)
As an earthquake originates:
- Stress builds up.
- Tension increases, and materials deform.
- Materials fracture, releasing energy that vibrates the ground.
Measuring Earthquakes (Repetition)
Seismometers detect even very weak earthquakes that go unnoticed by people, drawing graphic seismograms. The magnitude of an earthquake is the amount of energy released, measured on the Richter scale, which is open-ended and has no limit.