Thu, 22 Feb 2018 10:02:32 +0000
By Ronald Lwamba
There are hundreds of seismic data recording stations throughout the world. In order to locate the epicentre of an earthquake, you need to estimate the time interval between the arrival of the P and S waves (the S-P interval) on the seismograms from at least three different stations.
You have to measure the interval to the closest second and then use a graph to convert the S-P interval to the epicentre distance. On the sample seismogram, the vertical lines are spaced at 2 second intervals. The S-P time interval is about 36 seconds. You can now determine the distance from each seismic recording station to the earthquake’s epicentre using the known times of travel of the S and P waves.
Examine the graph of seismic wave travel times. There are three curves on the graph: The upper curve shows S wave travel-time plotted against the distance, the centre one shows P wave travel time versus distance, and the lower one shows the variation in distance with the difference of the S and P travel times. If the S-P interval is 36 seconds then using the available S-P curve, this corresponds to a distance of about 355km.
Once you have the epicentre distances, you can draw circles to represent each distance on a map. The radius of each circle corresponds to the epicentre distance for each seismic recording station. Once you have drawn all three circles and located the point where all three intersect, you will have successfully located (triangulated) the epicentre of the earthquake.
The table below briefly describes earthquake effects corresponding to various magnitude levels and also gives an estimated number of earthquakes of different magnitudes that happen in the world each year. It can be observed from this table that a large majority of earthquakes (900,000) are of magnitude 2.5 or less (very minor earthquakes, usually not felt). Great, catastrophic earthquakes (magnitude 8 or greater) happen once in 5 to 10 years.
Depending on their magnitude, earthquakes are classified into categories ranging from minor to great.
Therefore, the earthquake that occurred with its epicentre in Ngwerere/Chibombo and felt by Lusaka residents on Thursday morning, 2nd November, 2017, with a magnitude measuring 4.7 on the Richter scale can be classified as a light earthquake.
The assessment of earthquake intensity on a descriptive scale depends on actual observations of earthquake effects. Observations on the performance of building structures, natural phenomena, and human perceptions are essential for evaluating the earthquake intensity. Intensity of an earthquake depends on the distance from the epicentre, and also on the local soil conditions, geology and topography. In a typical case, however, the largest intensity is observed in the vicinity of the epicentre and it diminishes with the distance.
The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to buildings, and finally—total destruction. Numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes. The one currently used is the Modified Mercalli Intensity (MMI) Scale. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.
Detailed specifications of various intensity levels related to the MMI intensity scale are presented in the table below.
A major difference between the earthquake intensity and magnitude lies in the fact that the magnitude of an earthquake is determined based on measuring the ground motion with instruments (seismographs), whereas the intensity of an earthquake is determined based on observations of earthquake effects on building structures and human perceptions.
Another essential difference between a magnitude and intensity of an earthquake lies in the fact that magnitude is a unique indicator of a size of an earthquake — each earthquake is characterized with a single value which indicates its magnitude. But when it comes to earthquake intensity, each earthquake is characterized with various intensities, depending on the location of a particular site with respect to the epicentre.
For example, Canada’s largest historic earthquake, the Queen Charlotte Island earthquake of August 22, 1949 was characterized by a magnitude of 8.1 on the Richter scale. The same earthquake was characterized with MMI intensities ranging from III to over VII, as illustrated in the figure below, depending on the location of a particular site and it is zero in locations where it is not felt.
The MMI (Modified Mercalli Intensity) intensity in areas close to the epicentre of this earthquake is VII or higher “cows were knocked off their feet, and a geologist with the Geological Survey of Canada working on the north end of Graham Island could not stand up”, whereas in Prince Rupert the MMI intensity is VI, “windows were shattered and buildings swayed.” Thus, coming to our Chibombo earthquake here in Zambia the MM Intensity was higher in Chibombo than in Lusaka and probably zero in Kafue but the magnitude still remains 4.7 on the Richter scale.
This description of the Modified Mercalli Intensity scale is from the U.S. Geological Survey pamphlet The Severity of an Earthquake (1986).
XII Damage total. Lines of sight and level distorted. Objects thrown into the air.
According to the social media, this is how some Lusaka residents felt the earthquake that had its epicentre in Chibombo:
.I felt it and it shook my house. It was from west to east with a sound of a big aeroplane taking off. It was quite massive. It is luck that it wasn’t an earthquake.
.I felt it here in Chilenje South, sounded like a huge plane at first. Doors and windows started
.I felt it here in Chawama too. I ran out of the house in panic. Kikiki
.I felt like it is a cargo plane passing and the vibration was so strong things started moving on the roof
.I equally felt it here in Lusaka Northmead area. We all came out of the offices just to check on what had happened.
.I was in the bathroom time when it happened, water had some movement felt the tub shaking; I too was scared for a moment.
From these observations from Lusaka residents, I would give the Chibombo earthquake an MMI of IV (please note the use of Roman numerals) in Lusaka. The MMI (Modified Mercalli Intensity) intensity in Chibombo area, which was close to the epicentre of this earthquake, must have been higher.
Thankfully, Zambia has yet to experience a major or great earthquake but perhaps it is just a matter of time, maybe not in our life time, considering that it is in close proximity to the Great Rift Valley and Lake Kariba and Lake Itezhitezhi, which are capable of causing earthquakes, which is usually referred to under the acronym, RIS, i.e., Reservoir Induced Seismicity (or earthquakes).
However, earthquakes are common in Japan, but the 1995 Kobe earthquake stands out. It spelled the end of the road for a section of an asphalt highway, a fleet of trucks, and even a huge crane which collapsed under the force of the earthquake.
The magnitude 7.2 earthquake was one of the worst in Japan’s long history; 6,433 people were killed and damages topped $100 billion. But on 11th March, 2011, Japan experienced an even bigger earthquake, later estimated by the Japanese Meteorological Agency to be 9.0 in magnitude, which struck 130 kilometres east of the city of Sendai on the nation’s north eastern coast. The forces of this earthquake, the fifth most powerful in the past century, set off a giant wave, called a tsunami, that engulfed villages, destroyed buildings and drowned and crushed people who lived there [source: Green].
The tsunami resulted from a reverse fault type quake, which in this case occurred when the North American tectonic plate slid over the Pacific Plate, releasing a huge amount of built-up pressure. Seawater was then lifted up from the seabed, pushing the sea increasingly higher as it moved closer to land. [Source: Yomiuri Shimbun, March 13, 2001] The earthquake and tsunami also badly damaged a six-reactor nuclear power plant in Fukushima, 241 kilometres north of Tokyo, destroying the backup generators that powered its cooling systems and causing a dangerous release of radiation that forced people in the region to flee. In all, the quake claimed the lives of 20,896 people, according to the U.S. Geological Survey.
Though earthquakes have terrorized people since ancient times, it’s only been in the past 100 years that scientists have come to understand what causes them, and to develop technology to detect their origin and measure their magnitude. In addition, engineers and architects have worked to make buildings more resistant to earthquake shocks.
