How does a seismograph detect seismic activity?

Ever wondered how scientists track local earthquakes without the equipment itself trembling? A seismograph is a device designed to sense and document ground movements caused by earthquakes and volcanic eruptions.

The First Earthquake Detectors

In 132 C.E., Chinese polymath Zhang Heng designed the earliest known earthquake detector. His seismoscope, resembling a large vase encircled by dragons, featured frogs at the base. When seismic activity occurred, a ball would drop from the dragon's mouth into a frog, making a sound.

In 1875, Italian physicist Filippo Cecchi created the first seismograph, which used pendulums to trigger a clock and recording device, allowing the measurement of an earthquake's duration.

What Is a Seismograph?

A seismograph is a tool used by scientists to track earthquakes. Its primary function is to accurately capture seismic waves during an earthquake.

In cities, you might feel the ground tremble when a large truck or subway train passes. To avoid interference from such movements, seismographs are installed in dedicated stations and anchored to bedrock to ensure accurate data.

The Challenges of Seismographs

One of the main challenges in building such an instrument is that both the ground and the seismograph shake during an earthquake. This is why most seismographs use a pendulum mechanism to isolate the instrument from ground motion.

You can create a simple seismograph by suspending a heavy weight from a rope above a table. By attaching a pen to the weight and securing a piece of paper on the table, the pen will record vibrations in the Earth's crust as the weight moves.

If you use a roll of paper (a rotating drum) and a motor that gradually moves the paper across the table, you will be able to track tremors over time. However, only larger tremors would be visible in this setup.

A real seismograph uses levers or electronics to amplify the signal, enabling even the smallest tremors to be detected. A large mechanical seismograph may feature a weight of over 1,000 pounds (450 kg), which drives levers to greatly enhance the pen's movement.

Short-period seismographs are designed to detect the high-frequency seismic waves of nearby earthquakes, while long-period seismographs capture the lower-frequency waves from distant quakes. In the case of very large earthquakes, strong-motion seismographs are used, but they amplify seismic activity to a lesser extent.

Types of Seismic Waves

Seismic waves are categorized into two groups: body waves and surface waves. Body waves, such as P and S waves, travel through the Earth’s interior, while surface waves move along the Earth's exterior, causing greater surface movement.

P waves, or primary waves, are longitudinal in nature, shifting back and forth much like a spring that’s been compressed and then let go. S waves, also known as secondary waves, are transverse, moving vertically like ripples on water.

Seismograph vs. Seismometer: What's the Difference?

Seismographs and seismometers are tools used to detect seismic activity, with the primary difference being that seismographs record the data. The term "graph" is derived from the Greek word "graphos," meaning 'to write.' The recording itself is referred to as a seismograph.

Initially, seismographs captured ground vibrations on analog surfaces such as photographic paper or magnetic tape. However, in modern times, many seismographs utilize digital technology.

What Is the Richter Scale?

The Richter scale is a commonly used method for assessing earthquakes. As a logarithmic scale, it represents magnitudes in powers of 10. For instance, an earthquake measuring 4.0 on the Richter scale is ten times stronger than one with a 3.0 magnitude.

On the Richter scale, any reading under 2.0 is imperceptible to most people and is classified as a microquake. These tiny quakes happen frequently. Moderate quakes usually register under 6.0. Earthquakes above 6.0 can inflict substantial damage.

The most powerful earthquake recorded since 1900 hit Chile on May 22, 1960, registering a magnitude of 9.5.