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What is a Seismometer?

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  • Written By: C. Martin
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  • Last Modified Date: 26 November 2016
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A seismometer is a device designed to measure movement in the Earth. Seismometers are typically used to measure seismic waves originating from an earthquake or explosion, and traveling through the ground as waves of force in the rocks and soil. The basic design of seismometers is typically based on two objects, or masses. The first object is called the frame, and it moves along with any movements of the ground. The second object is usually referred to as an inertial mass, and this mass tends not to move when the ground and the frame move.

Typically, the inertial mass is attached to the frame in such a way that it can move relative to the frame. By measuring the movement of the inertial mass relative to the frame, it is possible to calculate the magnitude of the waves in the ground that triggered the movement. Usually, the movements of the inertial mass are very small, so often a seismometer will include some mechanism to amplify the movement of the inertial mass in order to facilitate more accurate measurements.

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An early example of a seismometer was a device that was used in 1906 to analyze an earthquake that occurred in San Francisco. This seismometer had a stationery frame from which a pendulum was suspended. Attached to the pendulum was a stylus, which rested on a glass plate that had been covered in soot. As movements in the ground caused the pendulum to swing, the stylus recorded the movement on glass. Devices like this one, called an earthquake seismometer, could help scientists to map out the likely point where an earthquake might have originated.

A seismometer measure is often based on a scale of measurement called the Richter Magnitude Scale. This is a logarithmic scale where each unit of seismometer measure indicates a seismic wave ten times stronger than the previous unit. For example, an earthquake measuring a six on the Richter scale is ten times more powerful than an earthquake measuring a five on the Richter scale.

Modern seismometers are typically highly sensitive electrical devices that utilize electrical sensors, highly effective amplifiers, and electrical recording of the output measurements. These instruments are usually categorized as broadband, short period, or long period. A broadband seismometer is usually the most flexible device, as it typically has the ability to measure a very large range of different seismic waves. Short period and long period seismometers are usually restricted to measuring waves in a certain range, but can be extremely sensitive.

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lluviaporos
Post 3

I actually ran across a guide online the other day on how to build your own homemade seismometer. I'm not sure whether or not it would actually be of any use to you to have one though.

It would definitely be a neat project to do with your kids. Most areas have very accurate and up to day listings of recent earthquakes online as well. If you build your own, it would be easy enough to check it against the local website to see whether or not it was very accurate.

Of course, you would have to be in an area where there were regular earthquakes, or other earth movements in order for kids to be really interested in the outcome of the project!

umbra21
Post 2

@irontoenail - Technically, I'm not sure that was a seismometer, as it doesn't really measure the strength of the earthquake, but only the direction. I've heard it called a seismoscope though. It was invented during the Han dynasty.

On the other hand, if the balls only drop when the earthquake is a certain strength, that could be said to be measuring it. But I've heard it was so sensitive it would occasionally drop a ball even though there wasn't an earthquake happening.

Still, it was an incredible invention, considering it was made almost 2000 years ago and worked pretty well.

To be honest though, I think they thought of it as more of a curiosity than something that could be used to help people who had been affected by earthquakes in a more timely manner.

irontoenail
Post 1

There is a fantastic ancient example of a seismometer in my local museum. Actually I think it is just a replica of one, but it is still really cool.

It's made out of iron and has dragons with balls in their mouths. In theory, if there was an earthquake the way it has been designed will make the dragon which is facing the direction of the earthquake drop its ball.

It's a Chinese invention, from hundreds of years ago. Of course, it can't predict earthquakes, but still, for a leader to be able to know where the earthquake came from, and therefore, where was likely to have been strongest hit would be invaluable, particularly back when there were no telephones or ways of easily communicating across distances.

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