Earthquakes - Scientific Background and Realities

BY DR. RANJITH Premalal De Silva, President/ Geo-Informatics Society of Sri Lanka

THE first anniversary of the tsunami falls on December 26. It was caused by a massive seaquake off the coast of Indonesia. Here we take a look at earthquakes.

The public is in fear of possible reoccurrence of the tragic events in the new future. Predictions based on personal judgments without proper factual scientific information released to the public by certain media could worsen the situation.

The slight tremor experienced in the central highlands was attributed to an early warning of a forthcoming disaster by some geologists.

In this background, an attempt is made here to explain the scientific background and realities behind the occurrence of earthquakes so that readers would get a basic understanding of this devastating phenomenon.

Earthquakes occur when energy stored in the rocks are released suddenly. This release of energy causes intense ground shaking in the area near the source of the earthquake. When this occurs in the sea, waves known as seismic waves created by the released energy could make sea water waves travelling thousands of kilometres.

Since there is less resistance on water for this energy transmission, the energy could loose its power when it hits large barriers of land mass. The movement of large volume of sea waves is called as tsunami.

The source of an earthquake is called as its focus and the epicentre is the point directly above focus on the land surface. The epicentres of the recent episodes of earthquakes are located in a well known earthquake zone in the world earthquake distribution map known as seismicity.

The depth to the foci of the recent earthquakes are reported to be less than 20 km from the surface suggesting the cause for earthquakes as diverging earth plate boundaries at oceanic ridges.

As the boundary is along the north south direction, it is natural to expect a shock wave propagation along east west direction.

This is the reason for the devastating impact of this tidal waves or tsunamis on our coast. Magnitude of earthquakes is measured by a scale known as Richter introduced by seismologist Charles F. Richter.

The Richter magnitude involves measuring the amplitude or the height of the largest recorded wave at a specific distance from the earthquake. Each unit increase or increase of 1 in the scale shows tenfold increase of the height of the wave.

However, it is not correct to say that each increase of 1 in Richter magnitude represents a tenfold increase of the size of the earthquake. However, the better measure of the impact of an earthquake is the measure of its energy which causes the impact. Each unit increase in the Richter's scale, the energy released increases by 31 times.

The largest recorded earthquake prior to the December 26 disaster was recorded at 8.6 on the Richter scale in Alaska. More catastrophic earthquakes would have happened in the past but has not been recorded scientifically.

Accordingly, the earthquake recorded at 9 on the Richter's scale is the highest recorded earthquake in the world.

In another comparison, the estimated energy release of Hiroshima atomic bomb was equivalent to an earthquake of magnitude 5.5 on Richter's scale. The energy release of the December 26 earthquake is 30000 times that of the Hiroshima bomb.

Although the measurement of earthquakes looks like a complicated process, it is measured by a simple instrument called seismometer.

The instrument records the vibrations of the earth on a paper using a simple principle of relativity or relative movement. Yet there is no mechanism to predict the earthquake in advance. However, within minutes of its occurrence, it can be reported and warnings can be issued for the safety of public.

In case of Tsunamis, depending on the distance from the epicentre, a time lag exists between the record of the earthquake and the devastating impacts. Therefore, Tsunamis can be predicted and the impacts can be minimized with proper planning.

Unfortunately, in our case, the impact of the earthquake was not seriously considered when it had been reported and moreover, even if we issue warnings to the public, there was no mechanism in place to disseminate the information and evacuate people from areas of high vulnerability.

The degree of impact by Tsunamis depends on the distance to the origin of the earthquake. The distance to the epicentre of the earthquake is determined based on the relative velocities of different waves generated due to the earthquake.

Primary waves travel faster than the secondary waves from the epicentre. The delay of the secondary waves with respect to the primary waves in reaching a certain monitoring station helps us to determine the distance.

When there are three monitoring stations, the exact location of the epicentre can be estimated using simple geometric relationships.

In general, areas with loose unconsolidated sediments are subject to more intense shaking than solid bedrocks. This again depends on the direction of wave propagation. If intense shaking takes place, damages can be caused by secondary impacts depending on the stability of the area and types of construction.

However, it is noted that Sri Lanka is located in a very safe zone and earthquakes which can occur in the country are not a serious threat for us. However, Tsunamis can obviously cause much havoc in the country as we experienced.

It is the responsibility of the scientific community specialized in geosciences to get together and acquire the global knowledge available in these disciplines. This would assist us in developing an early warning system within the country.

This system can then be directly linked with the regional network of Asia and Pacific. In addition to an early warning system, there should be systems in place for disaster preparedness, defense and mitigation.

Advanced technologies like Remote Sensing and Global Positioning Systems (GPS) could provide us dynamic information required for these activities.

Large amount of foreign aids have been spent on procuring equipment for Remote Sensing and GPS and it is doubtful whether we have got the intended benefit returns from the investments on these equipment.

Further, we have been signatories to global forums for climate change and associated impacts. Global warming and climate change occur over thousands of years and people can adapt to this gradual change over time.

Large number of seminars and workshops were organized in the country to discuss the impacts of global warming and sea level rise while neglecting much more catastrophic episodes of earthquakes and related impacts.

It is time for us to realize that there is no real benefit for us in taking the global themes forward without realizing the priorities and the needs of the country.

Geo-Informatics Society of Sri Lanka (GIS-SL), the largest professional body of geospatial professional in the country is ready to support the government in the efforts to establish disaster management framework which should include Advanced Early Warning System, proper disaster preparedness plan, continuous monitoring and updating the data and metadata bases, strengthening the disaster defense mechanisms and disaster impact mitigation action plans.