It was in China, in the year 132 AD, when the first instrument to measure seismic movements was devised, called ‘Houfeng Didong Yi’ for Earthquake Early Warning. The mechanism of the ancient seismograph was simple: eight dragons mounted on the surface of a cylindrical urn which pointed their heads in eight opposite directions and held in their mouths eight copper balls, which, with the earth movements, released said balls towards eight toads with heads raised and mouths open in the opposite direction to the mouths of dragons. During an earthquake, the tremor caused a pendulum located inside the cylindrical urn to release the copper ball that it held in its mouth, which in turn would fall into the mouth of the toad located just below the dragon, emitting a sound that would help the sages to determine in which direction the earthquake had taken place.


The first seismic instrument in the world for Earthquake Early Warning: Houfeng Didong Yi.


Hundreds of years passed before technology could be revolutionary again. In 1855, Luigi Palmieri developed a rudimentary seismograph formed by several U-shaped glass tubes filled with mercury, with the occurrence that, during an earthquake, this heavy and liquid metal would oscillate, engraving its movement on a strip of paper. But this idea did not last long and in less than 30 years it was replaced by Thomas Gray and John Milne when they built the first modern seismograph.

Horizontal pendulum seismograph created by Thomas Gray and John Milne in 1880.

Today, several devices that can help us quantify the energy released by an earthquake using modern seismographs. Instead of using a marker and drum, today’s devices have small mechanisms in which the relative movement between a weight and a base generates an electrical voltage that is recorded and digitized by a computer; when it trembles, the displacement of the mass generates electricity by moving a coil within a magnetic field of a magnet, producing a current in the coil that will be proportional to the speed of movement of the ground.

The “Broadband” seismographs are the most used nowadays and are capable of detecting earthquakes whose epicenter is located thousands of kilometers away. This system allows extending the bandwidth and linearity of the seismometers because it does not allow large movements of the mass that bend the springs or levels. The output signal of these systems has an extended dynamic range because the electromagnetic transducers have to a wide range.

Modern seismic station with seismometers, accelerometers, solar panels, GPS and transmission antennas.

Seismographs are instruments with high sensitivity which can extend the speed of the land movement thousands of times, either because of a small nearby earthquake or a large one. If an earthquake occurs very close to the instrument, the seismograph becomes saturated and uses other devices like accelerographs.

Unlike seismographs, regardless of the magnitude of the earthquake, a characteristic of accelerographs is to record the acceleration of the terrain expressed as a fraction of the earth’s gravity. It records the intensity of the movement produced by tremor at a certain site.


Seismic alerts


When an earthquake occurs seismic waves radiate from the epicenter in every direction. The technology exists to detect moderate to large earthquakes so quickly that an alert can be sent to locations outside the area where the earthquake begins before strong shaking arrives. Such seismic alerts are available for your business thanks to Epicenter, and for personal use directly in your smartphone, the SkyAlert App.

Earthquake Early Warning utilizes many seismic sensors distributed over a wide area where earthquakes are likely to occur. This network of sensors sends data to a central site where ground motion signals are analyzed, earthquakes are detected and warnings are issued. The network approach may take more time than the single station approach because it requires multiple stations to detect the earthquake before alarms are sent, but this makes it less prone to errors than the on-site approach. Using a network of seismic sensors also has the advantage that the system is constantly exercised and tested as detected, and warnings small earthquakes daily. For this reason, the system maintains a high level of readiness. Finally, only a distributed network of sensors is capable of characterizing large, complex earthquakes as they evolve. Thus, estimates of intensity and extent of shaking gain accuracy as more data are recorded and analyzed.

The West Coast States routinely experiences small and moderate earthquakes that do little or no damage. In the vast majority of cases, an EEW system will alert users the ground is about to shake.

It is important to mention that an Earthquake Early Warning is not an earthquake prediction, but an alert system and that the warning time depends on how far you are from the epicenter.

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