You may be surprised at the diversity of sensors that are able to detect seismic waves. Everything from the accelerometer in your cell phone to multi-million dollar satellites orbiting at altitudes of over 12,000 miles are capable of recording seismic vibrations. Surfers who visit the tsunami-prone Mentawai islands in Sumatra often invert an empty glass bottle on the floor before retiring, creating a very rudimentary ground-motion sensor in the absence of cell-phone coverage in that remote Indonesian region! This article will focus on three of the most ubiquitous seismic sensors in use today: broadband seismometers, strong-motion seismometers, and the Global Navigation Satellite System (GNSS).
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Not all seismometers are created equal because not all ground motions are the same. When a seismic observatory such as the center records a ground motion, that recording may be processed in a variety of ways to achieve different purposes, and different types of seismometers excel at different tasks. The workhorse sensor for the center is the broadband seismometer. These extremely sensitive devices record seismic energy across a large range of frequencies (thus the “broadband” term). They usually record seismic velocity on three channels: up-down, east-west, and north-south. This allows them to capture the complexity of seismic waves as they move through the three-dimensional earth. These sensors excel at “listening” to the earth for weak motion—very small ground motions that are induced by small local or regional earthquakes, or very distant earthquakes. When the center’s website reports, for example, a magnitude 1.1 earthquake in the Brooks Range, that detection was made possible by our network of broadband seismometers. Broadbands are also very astute at recording seismic noise. This may not seem very useful, but this ambient, or background, noise is special. It is the “hum of the earth,” induced primarily by wave action at the coasts, and seismologists use it to study the geological structure of the rocky crust. Broadband seismometers form the backbone of the center’s monitoring capability.
From the above description, it may seem that broadbands are the only sensors that a seismologist would need. However, they do have a crucial limitation. Because they are optimized for recording faint, weak-motion signals, they do not do well in the large ground motions induced by strong or nearby earthquakes. Strong shaking will cause broadband seismometers to “clip”, or go off scale (see Figure 1). This phenomenon is similar to what happens if a stereo sound-system is too loud for its speakers, causing them to buzz and not properly reproduce the music. This is unfortunate, because it means that broadband sensors perform poorly, at least locally, at recording big earthquakes, and big earthquakes are the most societally impactful seismic events. After all, the definition of strong motion is ground motion that impacts people or their environment. Fortunately, the center operates another class of sensor called a strong-motion seismometer. As the name implies, while these do not excel at detecting the very faint seismic signals that are captured by broadbands, they have a large dynamic range, and can record even very large ground motions. This makes them extremely well-suited to engineering applications; dense networks of these sensors will often be located in urban environments such as Anchorage (see Figure 2) or around critical infrastructure such as power plants and the Trans Alaska Pipeline. In fact, although there is overlap, you can roughly think of broadbands seismometers as used for scientific purposes, while strong-motion sensors are for engineering.
Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.
Seismograph is another Greek term from Seism - "the shakes" and Grapho - "I draw". It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.
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The PASSCAL Instrument Center supports the following types of seismometers:
- Broadband Sensors - are three-component seismometers capable of sensing ground motions over a wide frequency band. These sensors are most-often used in passive experiments.The flat-to-velocity portion of the bandwidth is generally about 0.01 Hz (100 sec) to 25Hz. With sufficient signal, however, the 120-sec velocity transducers in the PASSCAL fleet can record signals with periods many thousands of seconds long, such as earth tides.
- Intermediate Sensors - are three component seismometers with corner periods in the 30- to 40-second range (distinct from the 120- to 240-second range of PASSCAL's truly broadband fleet). These sensors, like their broadband cousins, are capable of sensing ground motions of much longer period than their corner periods, if the long-period amplitudes are sufficient.
- Short-Period Sensors - are rugged three-component seismometers that cover higher-frequency bands (usually 1 Hz to 100+ Hz). These sensors are used in both passive and active-source experiments. The sensors, themselves, can be either feedback seismometers requiring power or conventional, passive seismometers requiring no external power.
- High-Frequency Sensors - are very rugged seismometers that cover even higher frequency bands (e.g. 4.5 Hz to 100+ Hz). These sensors are most-often used in active-source experiments and are often referred to as geophones.
- Accelerometers - also known as strong-motion sensors, are designed to measure the large amplitude, high frequency seismic waves typical of large local earthquakes, and operate in the frequency band 0 Hz to 100+ Hz.
To compare details of different sensors in the PASSCAL fleet, see the Sensor Comparison Chart.
Graph of example signal amplitudes and periods along with approximate sensor response amplitude/period space
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