Application for Seismometers in the study of earthquakes

Published Jan-20-2013
Application for Seismometers in the study of earthquakes:-
The science of seismology is based on the idea of using a sensor to detect an earthquake and to measure the magnitude of an earthquake. Seismology also focuses on the effects of an earthquake, for example a tsunami. This sensor is made up of an inertial mass component that is mobile relative to the frame of the sensor held to the frame by a spring. This attachment helps to block out any movements once the motion of the sensor frame stabilizes. The aim is to record the motion of the mass relative to the frame of the sensor. The idea being that any motion from an external source will move this frame. By being able to measure the motion of the base relative to the mass, the whole transition is transformed into an electrical voltage that is then recorded on a magnetic tape. Therefore, by using this sensor, the movement of the ground can be measured.With over 90 per cent of earthquakes occurring under water, the ocean-bottom seismometer (OBS) (see Figure 3) was designed for measuring movement under water. The seismometer is only part of the instrument used to measure earthquakes; the sensor is attached to additional components to help run the sensor, such as a data logger that will require batteries, a spherical weight to sink the sensor onto the ocean floor, and a controlled acoustic release and flotation device to lift the instrument back to the surface of the ocean. Based on the knowledge that seismic waves travel through the earth’s interior, an earthquake can be detected by a seismometer from long distances.Types of Ocean-bottom Seismometers (OBS)
Short-period earthquake seismometer
Smaller motion earthquakes have a much higher frequency making it difficult to monitor these movements in one single measurement – this type of motion requires the measurement of such movements up to 100 times per second. The Woods Hole Oceanographic Institution (WHOI) D2 is a modern type of OBS, containing a six-month battery life, measure seismic waves in horizontal and vertical directions, whilst recording small and frequent earthquake activity, such as fissure vent activity.

Long-period earthquake seismometer
Larger seismic waves are rare so the Long-Period OBS is used to record these earthquakes with a time period estimated to range from approximately 40 seconds to 0.1 seconds. WHOI long-deployment OBS and Scripps long-deployment OBS are both used for longer periods of time (up to one year or longer if necessary) to monitor the earth’s motion.

Challenges in the Application of Seismometer
OBS instruments are challenging to put in place whilst maintaining a high degree of accuracy, which when descending through thousands of meters of water can alter the quality of data recorded. The aim is to set these sensors on the bedrock of the ocean rather than a cluster of sediment which can cushion the motions being measured (i.e., the cushion will provide an inaccurate reading of the seismic wave).

Another disadvantage to the short-term OBS instrument is the short life span: this type of sensor has to be reset over a 30 day cycle. However, these instruments are small in size making the recovery of this sensor from the ocean a relatively easy task compared to long-term deployment seismometers. Furthermore, recording data detected by the seismometers requires a large amount of memory of the drive holding the information; this also consumes the battery life for such short-term use sensors.

The design and method of application for seismometers is ever-evolving to try and eliminate limitations to such sensors, with a rather interesting research area opening up for NASA and a cluster of academic institutes identified collaboratively as the READI network. This group has initiated research on an earthquake-warning system that works by the use of satellite data from a global positioning system (GPS) device.

When focusing on the commonly affected areas by earthquakes, countries such as Japan, can use this system to help take more accurate measurements of an earthquake with timely warnings to prevent fatal displacement of water waves following an earthquake (i.e., a tsunami), a natural occurring disaster seen in March 2011. The idea of this GPS navigated system, will involve positioning a series of GPS receivers along the North American coastal area, stretching these out to other heavily affected areas including California, British Columbia, and Canada. These sensors will work in synchronicity to record real-time motion activity moving through the ground as a reflection of local or distant seismic wave pattern reflecting the shift between two tectonic plate boundaries. Compared to the seismometer which finds it difficult to provide an accurate reading where you can distinguish between magnitude 8 and magnitude 9, a reading from the GPS satellite system will directly measure this without ambiguity.