Why are there so many earthquakes at plate boundaries?
Most earthquakes occur at or near tectonic plate boundaries. These are larger segments of the earth's crust and upper mantle that move relative to each other in a colliding, separating or side-wise motion. However, this movement is not a continuous slow motion, but occurs in small discrete, sudden steps that over time accumulate the large-scale plate motion.
The reason is because friction holds the smaller and larger segments along the boundary together for most of the time, causing tension to accumulate gradually. When the accumulated stress overcomes the strength of the rock, the two packets of rock move suddenly to a new stable position. This process is known as a tectonic earthquake - it releases energy in the form of seismic waves that lead to ground shaking in areas above the quake.
What category of earthquake is considered an emergency?
Most earthquakes are small and cause no damage, but occasionally, there are bigger ones that can cause considerable damage to people and infrastructure. How much damage a quake does depends a lot on a number of factors including:
- depth beneath the surface
- duration and motion of the shaking
- local geology
- population density
- overall building standards of the affected area
- risk awareness and preparedness
Quakes below magnitude 5 rarely cause damage, while quakes above around magnitude 7 almost always result in extensive damage, even in areas with very good building standards such as in the western US or Japan. Quakes from magnitudes 8 often result in wide-spread or even catastrophic destruction.
What kind of damage can an earthquake do?
The ground shaking of earthquakes can cause soil liquefaction, landslides and avalanches, the opening of fissures in the ground, deformation of the ground, cracks in walls and other structures leading to their collapse. As secondary and often even more severe damage, this can lead to fires, gas leaks, or even tsunamis if the sea floor is moved in a way to displace a large mass of water.
How dangerous are earthquakes?
Earthquakes are among the (if not THE) most dangerous type of natural disaster.
According to data from the World Health Organization (WHO), quakes caused nearly 750,000 deaths globally in the period 1998-2017, more than half of all deaths related to natural disasters.
How many people are affected by earthquakes every year?
Each day, many thousands of people feel an earthquake somewhere. Stronger quakes can be felt by millions of people.
According to data from the World Health Organization (WHO), nearly 750,000 people were killed by quakes and more than 125 million people injured, made homeless or displaced or otherwise seriously affected by earthquakes during the 20 years from 1998-2017.