Geology of Earthquakes L. R. K. Perera, Head/Department of Geology, University of Peradeniya, Peradeniya.

1. Earth and Its Active Interior

The earth’s interior has several compositional zones, namely, the core (metallic solid), the outer core (liquid), the mantle (solid), and the crust (solid) (Fig. 1). The continents and the ocean-floor rocks represent the outer zone known as the crust. The continents are made up of granitic (less dense) rocks and the ocean bottoms are underlain by basaltic (dense) rocks. The crust and the upper part of the mantle make rigid ‘plates’ of rock ‘floating’ on the lower mantle (Fig. 2). The plates or parts of plates are identified as ‘continental’ or ‘oceanic’ based on the differences in their make-up mentioned above. Although solid, the lower mantle rocks have the ability to ‘flow’ under the very hot conditions prevailing in the earth’s interior. This hotter material in the lower mantle ‘flows’ up, and down as it cools in a cycle (Fig. 3), like water in a boiling kettle, but in a solid state. As the mantle flows, it drags, pushes, and pulls apart the ‘plates’ of rock ‘floating’ on it. The moving ‘plates’ collide, slide pass one another, or are pulled-apart to make new plates due to flow in the mantle. The plate movements are very slow (5-10 cms/year) but are effective for hundreds of million years. The theory of Plate Tectonics deals with the plates and their movements.

2. Plate Tectonics, Earthquakes, Volcanoes and Tsunamis The theory of Plate Tectonics best explains earth’s natural phenomena of very short duration, like earthquakes and volcanism, and much longer duration processes like ocean opening and closing, continental drift, and mountain building. In fact, the theory reveals that the short duration phenomena are offshoots of long duration processes that continue for hundreds of million years due to plate movements. The crust is made up of about a dozen Plates (Fig. 4). On the basis of interaction between those during movement, three kinds of plate boundaries have been identified. (a) Convergent plate boundaries, (b) Divergent plate boundaries, (c) transform fault boundaries (Fig. 5). At convergent boundaries two continental plates, or two oceanic plates, or an oceanic and a continental plate collide. During the collision oceans gradually close, and heavier oceanic plate sink into the mantle under the continental/oceanic plate. Mountain ranges develop along the collision zone. Under the severe pressure mounting on rocks during collision and sinking, the blocks of rocks fracture and move relative one another at depth generating earthquakes (Fig. 6). The most destructive earthquakes occur at convergent plate boundaries. At much great depths the sinking rocks melt and come out to the earth surface as lava flowing out from volcanoes. At divergent boundaries a continental plate splits into two and move away from one another opening an ocean. Lava coming out from the mantle solidifies to generate new oceanic crusts, and at the same time pushing the continental fragments further apart. Relative movement of blocks of rocks during widening of the ocean leads to earthquakes but as not destructive as at convergent plate boundaries. Along transform fault boundaries continental or oceanic plates of rocks slide pass one another (Fig. 7) and as a result of friction earthquakes are generated but there is no volcanism. One comparing the present-day plate boundaries with a world seismicity map (Fig. 8) understands the relationship between plate boundaries and earthquakes mentioned above. Notably, there is a very high intensity of occurrence of earthquakes around the margin of the Pacific Ocean. This is a convergent plate boundary. In the middle of the Atlantic Ocean lies a divergent plate boundary where there is lesser incidence of earthquakes compared to that in the Pacific. Sri Lanka lies in the middle of the Indo-Australian plate (Fig. 4) away from the very high seismic zones, thus, is free from disastrous earthquakes. But geo-scientists believe that a new plate boundary has been developing in the Indian Ocean south of Sri Lanka during the last eight (8) million years. Some less intense earthquakes and minor tremors felt in Sri Lanka may be originating in this newly developing seismic zone. A tsunami is mainly generated by an under-sea earthquake whenever there is a vertical, relative displacement between blocks of rocks on the sea-floor. If the displacement is lateral a tsunami will not be generated. Thus, tsunamis are not associated with all under-sea earthquakes.

3. Earthquakes and Seismic measurements

An earthquake releases a huge amount of energy in the form of waves known as seismic waves. Three kinds of seismic waves originate from the focus of an earthquake. (a) P-waves, (b) S-waves, (c) surface waves. P and S waves are also known as body waves as they travel through the body of the earth, whereas the surface waves travel along the earth surface. It is the latter waves that cause huge destruction to property during an earthquake. Geo-scientists record the earthquake waves on seismometers. The three kinds of waves travel at different speeds and reach a seismometer one after the other. By measuring the travel time difference between P and S waves received by a given seismometer and by measuring the amplitude of the wave, one calculate the distance between the earthquake and the seismometer and the magnitude of the earthquake in Ritcher scale. Seismic wave data received from at least three such seismometers, which are several hundred kilometers apart, is necessary to locate the position of the earthquake.

4. Seismic Monitoring Equipment (SME) in Peradeniya

The Government of Japan through JICA donated Seismic Monitoring Equipment to the Department of Geology, University of Peradeniya for teaching and research purposes. The system of equipment was not intended to be serving as a Regional Seismic Monitoring Center providing information on earthquakes to the general public in Sri Lanka (in the manner Meteorology department of Sri Lanka operates) or to the international community. It can be developed to that capacity if and only if 24-hour, 365-day monitoring of seismicity in the region can be financially supported to provide, (a) the man-power to run the system, (b) maintenance cost of equipment, (c) licensing and updating relevant computer software, (d) an uninterrupted power supply, (e) maintenance of communication links to be used in emergencies etc. Above all providing sufficient office space and security for the system is a matter for concern. As at present Department of Geology is overstretched with respect to its building requirements and the seismic equipment is occupying a laboratory space used by other scientific instruments accessed by both researchers and students. This is not a good practice given the scientific value of this equipment, and particularly if the authorities expect future functioning of this equipment to serve the national needs. However, this arrangement cannot be avoided as at present due to problems of space. Similarly, components attached to this system installed in other universities should to be protected and given adequate security and the responsibility has to be handed over to identified officials in those institutions. The seismometers attached to this system are capable of receiving seismic waves of regional and local earthquakes, after the earthquakes have occurred. Four seismometers are stationed at, Matara, Mihintale, Olluwil, and Peradeniya, within university premises, to receive seismic signals. Any signals received are channeled through telephone lines to the main station at the Department of Geology, University of Peradeniya, where the seismic data can be analysed. Analysis of the data will reveal the location of the epicentre of an earthquake and its magnitude. It should be also mentioned that the seismic equipment donated by the Government of Japan to the Department of Geology cannot be used to predict earthquakes or tsunamis. Prediction of an earthquake means revealing the location of the epicenter of an earthquake, its intensity (magnitude in Richter Scale), and time of occurrence in advance (see below). Even countries like the United States of America possessing very high technological capabilities, have not successfully predicted an earthquake, even though zones of high seismicity (earthquake activity) around the globe are very well known to the geo-scientists. As mentioned above, tsunamis are generated mainly by earthquakes beneath sea-floor and would be predictable if an earthquake causing a vertical displacement of the sea-floor could be predicted. Because earthquakes cannot be predicted tsunamis are even more unpredictable. But centers like the Pacific Ocean Tsunami Warning Centre can identify a tsunami in the middle of the ocean, after it has been generated by an earthquake, and warn the coastal communities before it reaches the shore. If Sri Lanka is to install a tsunami warning system in any part of the Indian Ocean in the near future, the Seismic Monitoring Equipment (SME) at Peradeniya is going to be of crucial significance in assisting any Tsunami Monitoring and Warning exercise. After an earthquake, Peradeniya SME will receive seismic signals ahead of any indications of a tsunami on the Tsunami Monitoring and Warning System (TMWS). Thus, TMWS can be on the look-out for a tsunami more closely on their system of instruments. Thus, Peradeniya SME should be made capable of raising an alarm of a possible tsunami both manually and automatically to the officials at the TMWS. Thus, any future TMWS in the Indian Ocean will largely benefit from closely cooperating with SME at Peradeniya. That will not only ensure efficient functioning of the TMWS but that will enable them to make accurate predictions and warnings on time.

5. Earthquake prediction Earthquake prediction research was initiated nearly 30 years ago in a number of countries. First presumed precursors to subsequent large and small earthquakes were studied with the hope that these precursory phenomena would appear before many, if not all, subsequent events. Among the phenomena considered were things such as anomalies in the ratio of P-to S-wave velocities, magnetic fields, resistivity, tilt, emission of noble gases. Some of these hopes have either evaporated or have proved extremely difficult to document. But even today new precursors, such as, increase of the concentration of chlorine and other ions in well waters before the Kobe earthquake, are added to the list and some avenues of phenomenology have continued to be pursued: clustering and anticlustering of earthquakes, creep measurements, changes in the attenuation factor, paleoseismicity methods, etc. A second line pursued in prediction research was the presumption that small earthquakes were scaled-down versions of large ones, and hence the supposition was made that the study of small earthquakes would reveal important truths about large ones. This direction of research has also undergone modification over the years. It was found that earthquake fractures take place in a pre-stressed solid in which fluctuations are significant perturbations of the uniformity of the stress field. But small fractures take place in the shadows cast by the stress field of the larger and the largest fractures. Also, the details of the fracture and of the properties of nearby rocks are not resolved observationally in smaller earthquakes, but is clear in the case of large earthquakes. Thus, today the paradigms have shifted to the study of strong earthquakes and away from more numerous small earthquakes. Earthquake prediction research has been divided into three time intervals: (i) short-term predictions - a day to a few hundred days before a strong earthquake, (ii) intermediate-term predictions - one year to one decade, (iii) long-term predictions - longer than a decade before great earthquakes. Prospects for short-term prediction is rather low because of its local nature: one would have to be fortunate to have instruments within short range of the future focus of a strong earthquake. Intermediate-term prediction research has centered on identification of patterns of earthquake occurrence by magnitude, time, and/or location prior to strong earthquakes. Here too one relies on well-instrumented case histories of precursory clustering and anti-clustering presumed to be associated with future large earthquakes. On the long time scale, the question that is asked is whether given faults, and especially those that support the largest earthquakes, rupture periodically or not. Here the evidence is derived from analysis of ancient faulting episodes and the interaction between the geometry of faults and the seismicity of the largest events.

6. Educating the Public on Natural and Man-made Hazards

The national tragedy on 26th December 2004 has highlighted the need to acquire public warning systems to face any such future calamities. But if the public is not educated, prepared and trained to respond to a warning, even the most sophisticated systems may not deliver the expectations. Almost all natural, accidental and man-made hazards may be identified if the first signals can be sensed and given serious thought. That may give sufficient time to alert everybody and take necessary precautions to avoid casualties. Sometimes, vigilant and knowledgeable public may raise the alarm of a danger before the warning systems sound. Thus, it is vital to educate the public on the causes, early signals, and dangers all kinds of natural, accidental, and man-made hazards. That will give the leading cutting edge to protect the public from any future danger and build confidence in them to face any situation. The kind and the scale of a hazard and those that would be affected may depend on the circumstances. It could be an earthquake, a tsunami, a landslide, a flood, a natural or man-made fire, a poisonous gas leak/attack, thunder and lightning, micro-organism related health hazard etc. Educating the public on these is the responsibility of the university academics, scientists of national research institutes, organisations and agencies. The scientists of all these organisations should identify the natural and man-made hazards coming under the purview of their institutions. A collective effort must be made by all the institutions to educate the general public with the cooperation of all available expertise. A National Hazard Monitoring and Mitigating Centre could co-ordinate between different universities and national institutions cooperating in such endeavour. The idea of having such centre is to facilitate the flow of knowledge directly from expertise to the general public. Provincial or divisional Hazard Educating Centres could be the vehicle taking the knowledge to people at village level. Personnel trained at postgraduate level and research minded has to man Hazard Educating Centres around the country to cater the public. They should be completely aware about possible hazards a given area is likely to face and should be able to delineate most hazard prone GS divisions that will be first affected. Thus, the government must be encouraged to initiate Provincial or divisional Hazard Educating Centres closely co-operating with administrative set-up (Provincial/ Divisional secretaries, Pradeshiya Sabhas) to educate the public as well as to coordinate action during any hazardous event. The mechanisms to be adopted to educate the communities may range from hazard awareness educational TV programmes, public seminars to programmes designed for school children, preparing hand-outs and booklets to educate the public, conducting frequent hazard responding drills, and train personnel at postgraduate level to man Hazard Educating Centres around the country.

7. Evacuation and related problems

The coastal areas of Sri Lanka are densely populated and an evacuation exercise during a tsunami for example requires movement of large population to high ground during a short period of time. It is reported that nearly 5 million people are living in coastal areas affected by the recent tsunami. If the general public is allowed to occupy the first 100 meters from the shore, firm plans for evacuation should be put in place and tested regularly, assuming that a tsunami will strike in the middle of the night. Such plans can be facilitated and bring the expected out come if, (i) an effective siren warning system is erected and tested frequently in preparatory drills, (ii) people in each and every household is taught which route to take for safety during an evacuation. (iii) exit routes are free from obstacles (eg. parked vehicles) and well lit at the time of an evacuation. One of the commonest problems arising during an evacuation is the reluctant attitude of the general public to leave behind their valuable house hold items.