Earthquake or tremor: What's the difference?

Last week tremors of the earth on which we lay all the fruits of our labour sparked hundreds of conversations on what exactly is taking place in the country. Some thought they are signs of the end of time. Traditionalists believe their god “Walumbe” was passing by while the scientists think otherwise. More interesting were the tense arguments in taxis and around boda boda stages on whether what occurred was an earthquake or a tremor.
According to geological reports from Entebbe Metrology Centre, we experienced two, three to five seconds tremors of magnitude 5.7 and 5.4 on the Richter scale on July 3, at 10.21pm and 1.22am the next morning. These were after an earlier one on July 2 that shook parts of Kampala and areas along the Albertine Grabben. Most affected were areas of the Lake Albert region, 71km from Hoima in Uganda and 50km from Bunia town in the Congo. Fortunately, no deaths were registered.
For the elderly people in Bunyoro region that Wednesday’s events were a chilling memory of the 1966 earth quake in Toro which is recorded as the most deadly, claiming lives of 154 people, shaking the earth for three to five minutes with a maximum of 6.8 on the Richter scale, associated with a 20km surface faulting about five feet deep, according to Mr Isaiah Tumwikirize, a senior officer at the Department of Geological Survey and Mines in Entebbe.

Earthquake vs tremors
Earthquakes are the vibrations caused by rocks breaking under stress against an underground surface called a fault plane while a tremor is an involuntary movement of earth surface caused by stress in the underground rocks.
They are both signs of seismic movement within the earth. Tumwikirize, says, “The three tremors are a result of the activeness of the Eastern and Western Rift valleys.”
He adds that since Uganda lies between the two arms of the rift valley, she is prone to such movements. The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake. These are put into a formula which converts them to a magnitude, which is a measure of the energy released by the earthquake. Alex Binego a senior engineering geologist at Fichtner Consulting Engineers in Ggaba, explains how the quakes are measured. For every unit increase in magnitude, there is roughly a 30 fold increase in the energy released. For instance, a magnitude 6.0 earthquake releases approximately 30 times more energy than a magnitude 5.0 earthquake, while a magnitude 7.0 earthquake releases approximately 900 times (30x30) more energy than a magnitude 5.0.
A magnitude 8.6 earthquake releases energy equivalent to about 10,000 atomic bombs of the type developed in World War II. Fortunately, in Uganda out of the 30 or more movements we have experienced in the western region as lies in the rift valley, less than 10 are earthquakes.
Binego says the earthquake magnitude was traditionally measured on the Richter scale. It is now often calculated from seismic moment, which is proportional to the fault area multiplied by the average displacement on the fault.
“The focus of an earthquake is the point where it originated within the Earth,” he says. For the case of last week’s earth tremors, measurements were taken from an epicentre which was 57 kilometres west of Kigorombya in Hoima District.
The amplitude of the shaking caused by an earthquake depends on many factors, such as the magnitude, distance from the epicentre, depth of focus, topography, and the local ground conditions.
“We advise the public to follow the building codes so that they design structures that can withstand earthquakes,” Mr Tumwikirize said. Mr Binego also cautioned that buildings in wetlands and hilltops should follow the right criteria. “If the people or buildings are on soft ground such as old river sediments, the MM intensity experienced may be one to two units higher, if on solid rock, it may be one unit lower. The intensity with which the earthquake is felt may also be higher on hilltops,” he explained.
Apart from causing shaking, earthquakes of magnitude 4.0 or greater may also trigger landslides which can cause casualties. Binego adds that in areas which have water-saturated sediments beneath them, large earthquakes, usually with a magnitude 6.0 or greater may cause liquefaction (the process by which something becomes liquid). The shaking causes the wet sediment to become quicksand and flow. When the earth therefore begins to sink, this may cause buildings to topple, and the sediment may erupt at the surface from craters and fountains.