Question 1

1. What is a seismic sensor?

Accepted Answer

Answer: Seismic sensor is an instrument used for measuring ground motion. It measures ground motion in a dynamic manner. Seismic sensors can be of displacement type, or velocity type, or acceleration type. Before the invention of modern sensors, earthquakes used to be measured by the displacement of a stylus representing amplified ground motion. Modern seismic sensors include geophones that measure velocity or accelerometers that measure acceleration of the ground.

Question 2

2. How does a seismic sensor work?

Accepted Answer

Answer: Seismic sensors can be of inertial mass type or piezoelectric types. In inertial mass type seismic sensor, the mass is kept stationary because of its inertia or by applying external means. Because of an earthquake, the support of the sensor vibrates producing a relative motion between the mass and the frame of the sensor. This relative motion is used to measure the earthquake magnitude. In piezoelectric type seismic sensor, a piezoelectric material is placed between the inertial mass and the casing of the sensor. Due to ground motion, base of the sensor experiences an oscillation. Due to the motion of the ground, a pressure differential is created across two planes of the piezoelectric material. Due to piezoelectric property, the pressure differential generates voltage across mutually perpendicular planes. This voltage is used to quantify seismic motion.

Question 3

3. What is seismic vibration sensors?

Accepted Answer

Answer: Seismic sensors can be of displacement type, velocity type, or accelerometer type. Seismic vibration sensors are those that measure vibration of the ground. Seismic vibration sensors are mainly of two types: Broadband sensors Strong motion sensors Broadband sensors are used to detect seismic signals over a wide frequency range (thus the name broadband). Usually, the frequency range is from 0.001 Hz to 100 Hz. Broadband sensors are capable of capturing extremely low frequency seismic waves. But the amplitude range is relatively low. Higher amplitude signals are clipped in these types of sensors. On the other hand, strong motion sensors are capable of measuring large amplitude signals. These sensors, however, don’t capture very low frequency signal.

Question 4

4. What is the function of seismograph?

Accepted Answer

Answer: Seismographs provide a low cost approach for recording ground motion. Seismographs are in use to detect earthquake for a long time. Before the use of sensors to detect earthquake, seismograph provided a convenient method to record earthquakes. Seismograph in its simplest form is composed of a simple pendulum attached to a recorder by a mechanical arrangement. A large mass is suspended from a cable and the cable is attached to a casing that rests on the ground. When the ground shakes because of an earthquake, it also shakes the casing. But the mass remains stationary for some time after the excitation because of its inertia. Thus, a relative motion is observed between the mass and the casing. This relative motion is amplified and recorded on a moving drum. From the amplitude of the recorded signal, earthquake intensity is predicted. This is how seismographs detect an earthquake.

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