Last days there were many news about earthquakes at different locations: China, Russia, Mexico and more. There are even articles suggesting explanation a new theories about earthquakes. This blog presents a brief introduction about earthquakes. Further blogs will provide deeper information, but basic concepts are important in order to fully understand deeper information.

What is an earthquake?
An earthquake is a sudden release of energy in the Earth's crust that creates seismic waves, which in turn shake the ground causing destruction. It happens when two blocks of the earth suddenly slip past one another. The surface where they slip is the fault plane. The location below the earth’s surface where the earthquake starts is the hypocenter, and the location directly above it on the surface of the earth is the epicenter.

There are three main types of earthquakes: Plate tectonics, Intraplate earthquakes and volcanic earthquakes.

Plate tectonics. Earth's crust is broken into tectonic plates which are about 100 Km thick and are constantly moving. An earthquake occurs when the rocks break and move as a result of stresses caused by plate movements. In areas where plates collide, earthquakes can occur down to depths up to 700km. In areas where plates slide past each other, earthquakes are shallower.

Intraplate earthquakes. Intraplate earthquakes are earthquakes that do not occur on plate margins. They are caused by thrust faulting due to the rocks being squeezed or compressed. The movement of the tectonic plates causes the rocks away from their margins to be compressed , generating intraplate earthquakes.

Volcanic earthquakes. Molten rock, i.e., magma, is stored in volcanoes. As this magma moves upwards, it can fracture the rock it squeezes through, causing earthquakes, usually with magnitudes not much greater than 5.0. Sometimes the magma collects in a high level reservoir prior to a volcanic eruption and as it moves around it causes bursts of continuous vibration, called volcanic tremor.

How to detect earthquakes
Erathquakes are detected and measured with seismometers (Image 1). A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight that are suspended from a frame that moves along with the earth's surface. As the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. If a recording system is installed, such as a rotating drum attached to the frame, and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce a history of ground motion, called a seismogram.
Image 1. Seismometer. Image source: Iris

Modern research seismometers are electronic, and instead of using a pen and drum, the relative motion between the weight and the frame generates an electrical voltage that is recorded by a computer. By modifying the arrangement of the spring, weight and frame, seismometers can record motions in all directions.

The Earth's Hazard Program from the USGS has a link reporting earthquackes from all over the world. For intance, in the last 7 days (up to April 23) has 309 reported earthquakes. Moreover, the program also has a real time earthquake map which updates its database every minute.

References and Additional reading

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