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Unlock This CourseEarthquakes are one of nature's most powerful and destructive forces. They happen when the Earth's crust suddenly moves, releasing enormous amounts of energy that can shake the ground for hundreds of kilometres. Understanding earthquakes is crucial because they affect millions of people worldwide and can cause devastating damage to communities.
Key Definitions:
Earthquakes occur when stress builds up in rocks along fault lines. As tectonic plates move, they can get stuck against each other. Pressure builds up until the rocks suddenly break and slip, releasing energy as seismic waves. This is like snapping a stick - it bends under pressure until it suddenly breaks.
When an earthquake occurs, it produces different types of waves that travel through the Earth at different speeds. Understanding these waves helps scientists locate earthquakes and measure their strength.
The fastest seismic waves that travel through both solids and liquids. They cause a push-pull motion and arrive first at monitoring stations.
Slower waves that only travel through solids. They cause an up-and-down or side-to-side shaking motion and arrive second.
The slowest but most destructive waves that travel along the Earth's surface. They cause the rolling motion that damages buildings.
Scientists use two main scales to measure earthquakes, each telling us different information about the event's characteristics and impact.
Developed by Charles Richter in 1935, this scale measures the magnitude (strength) of an earthquake based on the amplitude of seismic waves recorded by seismographs. It's a logarithmic scale, meaning each whole number increase represents a tenfold increase in wave amplitude and about 32 times more energy release.
Unlike the Richter scale, the Mercalli scale measures the intensity of an earthquake based on observed effects and damage. It uses Roman numerals from I to XII and describes what people actually experience during the earthquake.
Not felt to felt by few people. Hanging objects may swing slightly.
Felt by most people. Dishes break, windows crack, some structural damage occurs.
Panic, serious damage to buildings, ground cracks, total destruction possible.
Earthquakes don't occur randomly around the world. They follow clear patterns that are closely linked to the boundaries between tectonic plates. About 90% of all earthquakes happen along these plate boundaries.
The Pacific Ring of Fire is a horseshoe-shaped zone around the Pacific Ocean where about 75% of the world's active volcanoes and 90% of earthquakes occur. This area includes countries like Japan, Indonesia, Chile and the western coast of North America.
Plates slide past each other horizontally. Example: San Andreas Fault in California produces frequent earthquakes as the Pacific and North American plates grind past each other.
Plates collide, often creating the most powerful earthquakes. Example: The 2011 Tōhoku earthquake in Japan (magnitude 9.1) occurred where the Pacific plate subducts under the North American plate.
Plates move apart, typically producing smaller earthquakes. Example: Mid-Atlantic Ridge where the Eurasian and North American plates separate.
Date: 12 January 2010
Magnitude: 7.0 on the Richter scale
Focus depth: 13 km (shallow)
Cause: Movement along the Enriquillo-Plantain Garden fault zone
Impact: Over 220,000 deaths, 1.5 million people displaced, capital Port-au-Prince devastated. The shallow focus and poor building standards made this earthquake particularly deadly despite its moderate magnitude.
Several factors influence how destructive an earthquake will be, including its depth, magnitude, duration and the local geology of the affected area.
The depth of an earthquake's focus significantly affects its impact on the surface. Shallow earthquakes (less than 70km deep) are generally more destructive than deep ones because the seismic waves have less distance to travel and lose less energy.
The further you are from the epicentre, the weaker the shaking becomes. However, local soil conditions can amplify or reduce the effects. Soft sediments can amplify shaking, while solid bedrock tends to reduce it.
Date: 11 March 2011
Magnitude: 9.1 on the Richter scale
Focus depth: 32 km
Cause: Thrust faulting on the boundary between the Pacific and North American plates
Impact: Nearly 16,000 deaths, massive tsunami waves up to 40m high, Fukushima nuclear disaster. This was the most powerful earthquake ever recorded in Japan and the fourth most powerful worldwide since modern record-keeping began.
While scientists cannot predict exactly when an earthquake will occur, they can identify areas at high risk and monitor seismic activity to provide early warnings.
Seismographs detect and record ground motion during earthquakes. Modern digital networks can pinpoint an earthquake's location within minutes and issue tsunami warnings if necessary. Countries in earthquake-prone areas maintain extensive monitoring networks.
Japan's early warning system can provide up to 60 seconds of warning before strong shaking arrives. This brief time allows trains to stop, elevators to halt at the nearest floor and people to take cover. While short, these warnings can save thousands of lives.
Communities in earthquake-prone areas have developed various strategies to reduce risk and prepare for future events.
Strict construction standards ensure buildings can withstand earthquake shaking. Japan and California have some of the world's most advanced earthquake-resistant building codes.
Regular earthquake drills teach people how to "Drop, Cover and Hold On" during shaking. Schools and workplaces practice evacuation procedures.
Families keep emergency kits with water, food, first aid supplies and communication devices. Communities develop evacuation plans and identify safe meeting points.
Date: 17 January 1994
Magnitude: 6.7 on the Richter scale
Focus depth: 18.4 km
Cause: Thrust faulting on a previously unknown fault
Impact: 57 deaths, over $20 billion in damage. This earthquake led to major improvements in building codes and emergency response procedures in California. It demonstrated that even moderate earthquakes can cause severe damage in urban areas.