16-01-2013, 09:11 AM
Satellite-Based Tsunami and Earthquake Early Warning System
Abstract:-
A satellite-based communication system is introduced to give early warning of tsunamis and earthquake by direct measurement of tsunamis in the open ocean and s-waves followed by real-time reporting to warning centers. This alert system for tsunami and earthquakes transmits their magnitude values to a central place via satellite communication to deliver alert signals. The tsunami alarm system consists of bottom pressure recorder (BPR) that can record seismic waves followed by tsunami waves and transfer this data to the warning dissemination centre through satellite communication media. The earthquake alarm system consists of reporting terminal which is very similar to the tsunami reporting terminal, except that an accelerometer is used in place of the Bottom pressure recorder. After receiving warning a prerecorded or instantly-recorded, siren appended with voice/text messages is transmitted to the warning receivers, placed in different locations, using a common satellite channel.
Introduction:-
Our country recently witnessed the havoc played by tsunami. This natural phenomenon showed to the world the raw and untamed power of nature. Very few people knew about tsunami until it struck. Even those who are involved in this field never thought of tsunami striking in Indian Ocean. And that is the reason why we could not do anything even though we had plenty of time to broadcast alert warning to our coastal areas. The term ‘tsunami’ was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word made of ‘tsu’ and ‘nami’ characters where ‘tsu’ means ‘harbour’ and ‘nami’ means ‘wave’. Also in the past, the scientific community referred to tsunamis as ‘seismic sea waves’. ‘Seismic’ implies an earthquake-related mechanism of generation. Although tsunamis are usually generated by earthquakes, these are less commonly caused by land slides, infrequently by volcanic eruptions and very rarely by a large meteorite impact in the ocean.
Earth quake (seismic) waves
When an earthquake occurs, it releases energy in the form of waves that radiate from that earthquake source in all directions. Different types of energy waves shake the ground in different ways and travel through the earth at different velocities.
The fastest wave is called the primary wave or p-wave. Like sound wave, this is compression in nature. It compresses and expands material in the direction it is traveling. P-waves move at the rate of 8 km per sec. These move the earth up and down perpendicular to the direction of their motion. S-waves are slower than p-waves and move at half the speed of p-waves i.e., km ps. Vertical ground motion generated by s-waves is highly damaging to the structures.
Locating an epicenter
U needs a seismogram from 3 different seismic stations (See fig 3). Examine the seismograph and determine the elapsed time between the arrival of the first p wave and the1st S-wave. Knowing the s-p time, u can determine the distance to the epicenter from the seismic station using the time distance graph. (See fig 2) On a map, draw around the seismic station a circle with a radius equal to the distance to the epicenter. Repeat for other 2 seismic stations. The point where the circles meet is the epicenter.
Magnitude of an earthquake
The equation for Richter magnitude is:
ML=log10A (mm) + Distance correction Factor
Where ‘A’ is the amplitude in millimeters, measured directly from the photographic paper record of the Wood-Anderson seismometer –a special type of instrument. Thus after you measure the wave amplitude, you have to take its logarithm and scale it according to the distance of the seismometer from the earthquake, estimated by the s-p time difference. The s-p time, in seconds, makes delta. Measure the distance between the first p wave and the first save. In this case, the first p and s waves are 24 seconds apart. Find the point for 24 seconds on the left side of the chart shown in fig 4 and the mark that point. According to the chart, this earthquake’s epicenter was 215 km away.
Measure the amplitude of the strongest wave. The amplitude is the height of the strongest wave. On this seismogram, the amplitude is 23 mm. Find 23 mm on the right side of the chart and mark that point. Place a ruler on the chart between the points you marked for the distance to the epicenter and the amplitude. The point where your ruler crosses the middle line on the chart marks the magnitude of the earthquake. This earthquake had a magnitude of ‘5’.
Tsunami waves
A tsunami is a series of ocean waves generated by a rapid, large-scale disturbance of the sea water, associated primarily with earthquakes occurring below or near the ocean floor or much less frequently by volcanic eruptions, landslides, undersea slumps and meteor impact. Tsunamis can propagate across whole oceans; distinguishable only by their great length from crest to crest, which may be up to 100 km, with the duration between successive peaks being anything from five min to an hour. This creates a sea surface slope so gentle that the wave usually passes unnoticed in deep water. As tsunami enters the waters of coastlines, the velocity of its waves diminishes and wave height increases.
Tsunami alarm system
From the above text, it is clear that two types of waves are generated; seismic surface waves that induce vertical motion of the sea floor and tsunami waves that cause displacement of the sea surface. Both the waves can be recorded by a special type of equipment called bottom pressure recorder (BPR) deployed at the bottom of the ocean.
The waves are represented as one of the two distinct packets of energy as shown in fig 5. The first packet is composed of the seismic waves that traveled at 11,000 km/hour to arrive at the PR only minutes after the event. The second packet shows the tsunami waves that traveled at 800 km hour to arrive 70 minutes after the earthquake.
Bottom pressure recorder
The Pacific Marine Environmental Laboratory (PMEL) has developed a bottom pressure recorder (BPR) that can record seismic waves followed by tsunami waves. This BPR is being used for the deep-ocean Tsunami observation in Pacific Ocean project.
Working of tsunami warning system:
An array of reporting systems (as shown in fig 7) is to be deployed in the Indian Ocean such that it covers the entire coastal areas. A large number of reporting systems will give near real-time and accurate direction of killer waves. However, this will also increase the cost and maintenance of the total system.
Functioning of the system begins with the detection of an earthquake, which has a magnitude and location that make it potentially capable of generating a tsunami. When the earthquake occurs, it generates seismic surface waves (SSWs) that travel in all the BPR’s one by one. The BPR nearest to the epicenter will be struck first.
Once a BPR detects SSWs, it will start transmitting this data to the warning centre through the satellite communication unit and start frequently monitoring and transmitting the average sea-level data until the sea comes to its normal state because tsunami waves may be more than one Peak level is the height of tsunami wave.
The satellite communication unit adds GPS data to the BPR data as the GPS measures the height of waves, confirming the rise of the sea level, and also helps to interpret the BPR data. But when a GPS can give height of the waves, why the BPR is required? It has been found that some times the surface buoy jumps five to six meters for actual wave height of one meter.
The time between SSWs and tsunami depends on the distance between the epicenter and the reporting terminal. The Indian meteorological department (IMD) continuously monitors earthquake activities through observatory centers deployed in different part of India. On the arrival of SSWs, the warning centre will be continuously in touch with the IMD to find out the location of the epicenter and its magnitude, which takes 15 to 20 minutes.
Time to alert for tsunami:
The first example is that of a tsunami occurring in Sumatra. Within three minutes of the start of the earthquake at Sumatra, all the reporting systems will be struck by surface waves one by one. The surface waves will first strike the reporting terminal no. 1, then 4 followed by 1. After that, it will strike at the same time terminal no’s 2 and3, followed by 5 and 6, then 8.
The warning centre will receive all the data one by one and thus know locations of all the reporting terminals. The computer can simulate all the data and find out a rough direction and distance of the epicenter. Here the epicenter may be at point no 1 or away from it. Finding out the exact distance is not possible because of the BPR’s limitation of recording p waves. As reporting terminal no 1 is far away from the Indian coastal areas, the warning centre can wait for 15 to 20 min for confirmation of the location of the epicenter and the magnitude of the earthquake.
Earthquake alarm system:
Whenever an earth quake occurs, most of the casualties are due to structural damage caused by s-waves followed by l and r waves. These waves travel at a speed of 3 of 4 km per second in all directions. A house located 40 to 50 km away from the epicenter of a high magnitude earthquake would therefore be hit by s-wave after 10 to 12 seconds. It will take 10 to 15 seconds to damage the house if it is poorly structured. Therefore, a person gets a total tie of 20 to 25 seconds to go to a sager place if an alarm is raised just 10 seconds before the killer waves hit that house.
Working of the earthquake alert system:
In the event of an earthquake of r-scale magnitude greater than 6, s waves will hit the reporting terminal or terminals nearest to the epicenter. As the wave moves away from the epicenter, it will hit other terminals. The terminals, in turn, will transmit this data to the network manager through the satellite channel.
The network manager automatically selects from the computer the places where warning is to be disseminated, and those would be 40 to 400 km away from the reporting terminal depending on the magnitude of the earthquake. It transmits the prerecorded siren or audio to selected warning receivers stationed 40 to 400 km away from the reporting terminal. The network manager also receives earthquake data from different reporting terminals one by one and that confirms the event of earthquake.
Conclusion:-
The Pacific Marine Environmental Laboratory (PMEL) has developed this technology and it is being used for the project deep-ocean tsunami observation in Pacific Ocean. This single alert system for both tsunami and earthquake senses changes of the water-level pressure on the sea floor caused by tsunami in sea and seismic wave magnitude due to an earthquake on the earth surface. It transmits these discrete magnitude values to a central place via a satellite communication network, and uses computer-based decision making to deliver alert signals to the identified receivers placed at different towns, cities and coastal areas. The system can be configured with the available resources in India. India can also go for the development of such a system with the help of renowned R&D institutions of India.