Disaster Management in SL - need for dissemination of information

N.U.Yapa Memorial Lecture, November 23, 2011 at OPA Auditorium, Colombo 7

Dr. Kapila Dahanayake- Professor Emeritus (Geology), University of Peradeniya

Being an earth scientist, I would like to concentrate on geology-related disasters such as earthquakes, tsunamis, volcanic eruptions and landslides. My discourse will be mainly on dissemination of information vital for understanding these geological phenomena in disaster management programmes.

The earthquakes refer to vibrations that occur suddenly on earth. This happens when seismic waves travel inside and outside the earth at speeds of 15,000 to 20,000km per hour causing the ground to shake. These waves originate due to a fault or fracturing of rocks that occurs within the earth. The location of fracturing is identified as the focus of the earthquake. During a fault, two blocks of rock move relative to each other. This movement can be vertical or horizontal or at an angle. The focus can be located at depths of several hundreds of kilometers from the surface. The epicenter of an earthquake is identified at a point on the earth’s surface or ocean bed directly above the focus. An earthquake may generate a tsunami if (a) its focus is located at a point less than 10km below the ocean bed (b) the fault movement is vertical and (c) its magnitude-M is generally greater than 6 or 6.5. A layman on the sea beach feeling a strong earthquake should immediately observe the sea level and if it gradually starts to increase, then he should go to high ground for safety.

Figure 1 - Major tectonic plates and their boundaries (From US Geological Survey Professional Paper 1240-B)

The earth trembles during an earthquake with differing severities depending on the magnitude of the earthquake and the underground geological conditions.

The magnitude of the earthquake is calculated using data from seismometer networks located in different parts of the world. Magnitude as measured on the Richter Scale developed in 1935 can have negative and positive values. Earthquakes (tremours) of magnitudes 1 to 4 may not be felt by humans. Those above 6 can be categorized as dangerous.

They can cause strong ground motions and buildings will collapse causing death to inmates. At the slightest trembling of earth those living in single floor buildings should run to open areas such as playgrounds for safety. The people living in multi storey buildings should not panic but stay calm and should find shelter under preferably a hard table top or near the corners of buildings as death is caused by falling debris from ceiling. They should not crowd and jam stairways and should avoid using lifts. Earthquakes of magnitude above 6.5 may develop tsunamis that can kill vast populations living in coastal areas sometimes thousands of kilometers from the epicenter like the December 26, 2004 Boxing Day undersea earthquake near Sumatra in Indonesia(which registered a magnitude of 9.3 on the Richter scale) that killed more than 31,000 in Sri Lanka alone. In all, more than 200,000 people perished in 11 countries including Indonesia, Thailand, India etc.,

Tsunamis

Tsunami is a Japanese word meaning harbor waves-A tsunami travels outwards from the epicenter of a tsunamigenic earthquake (with magnitudes of generally over 6.5 on the Richter Scale as a series of waves. Its height in the deep ocean is typically about 30 cm but the distance between wave crests can be very long –more than 75km. Its speed depends on the depth of water and increasing/decreasing ocean depth. In areas of oceans where the water depths reach more than 3000m, the tsunami travels at speeds of 500 to 1000km per hour. The speed of a tsunami decreases as water depth decreases when it reaches the coast. As they reach shallow water the tsunami speed can be just a few tens of kilometers per hour. At the shore, tsunami will behave differently depending on the shape of the coast line and the state of the tide. In some instances, the tsunami may induce flooding in low lying coastal areas. In other instances, tsunami floods come as a vertical wall or long train carrying several thousands of metric tonnes of turbulent water causing destruction to buildings, ships, vehicles. The high momentum (mass x velocity) generated by the massive body of water can destroy foundations of buildings and cause collapse of sea walls and twisting of rail tracks. These destructive floatation and drag forces can move, maim, destroy or move buildings, vehicles, ships or even moving trains that come their way.

Surviving tsunamis

In coastal areas, the largest earthquake may kill fewer people than the tsunami that may follow. Some hints for survival are discussed.

Heed natural warnings- An earthquake may serve as a warning that a tsunami is coming by rapid fall or rise of waters. Observe the sea and check for sea level changes if you live near sea.

Head to high ground and listen to radio/television for official tsunami warnings. Remember a still higher wave may come followed by a first wave. In case of a tsunami wave, abandon belongings. Save your life, not your possessions. If a tsunami warning is given leave your cars behind and go to high ground. Do not stay inside vehicles.

Do not count on the roads. When fleeing a tsunami caused by a nearby earthquake, you may find roads broken or blocked.

Go to an upper floor or roof of a sturdy building. As a last resort, climb up a strong tree if trapped on low ground building and unable to reach high ground.

If swept by a tsunami, look for something that floats and climb on to it. Use it as a raft.

Volcanic eruptions and their relation/s to earthquakes

Volcanic eruptions and earthquakes display the powerful forces of nature and they can be extraordinarily destructive. On average about 55 of earth’s 550 or so historically active volcanoes are in eruption each year. In 1988, there were 99 strong earthquakes worldwide above magnitude 6.0 many of them causing extensive damage. The world’s earthquakes and volcanoes are not randomly scattered over the earth’s surface. Most are concentrated along edges of certain continents(e.g western margin s of the Americas) . According to the theory of plate tectonics, we know that the Earth is a very dynamic planet. Its outermost shell (or lithosphere) is made up of a patchwork of a dozen or so (12 or 13 ) large, cool, rigid slabs (called tectonic or lithospheric plates. They are identified as oceanic and continental plates (Fig.1). They are moving horizontally relative to one another at speed varying from less than 1 to 10cm per year. The plates average about 80 kilometers thick and are composed primarily of the Earth’s thin surface layer (crust)and the topmost layer of the Earth’s interior (part of the mantle).

The plates are thicker under the than under the oceans. The plates are sliding and colliding with each other in very slow motion on top of the asthenosphere- a 200km thick layer of the mantle that is hot, weak and capable of viscous flow characterized by convection currents thereby providing a mechanism for the plates to glide on top where the plate grind against each other, stress builds up and is relieved periodically through earthquakes that occur when rocks break along faults. Near a plate boundary, molten rock material or magma rises to the surface and is erupted along fissures or vents to form volcanoes (Fig. 1).

Distribution of earthquakes and volcanoes

Waves of the tsunami hit homes after the largest earthquake in Japan's recorded history slammed the eastern coast Picture: AP

The theory of plate tectonics provides a tool for explaining the distribution of earthquakes and volcanoes. They are related to boundaries between tectonic plates and geologists recognize three principal types:

(i) Divergent boundaries
(ii) Convergent boundaries and
(iii) Transform boundaries (Fig. 1)

Divergent plate boundaries

They occur where the oceanic plates are moving away from each other e.g mid oceanic ridges are impressive sub marine mountain ranges identified in all ocean floors. The ocean floor is spreading apart at these ridges and hot magma from Earth’s mantle is injected to form new oceanic crust. Almost all of world’s divergent plate boundaries and associated abundant volcanoes are hidden by the oceans. The exception is Iceland where mid-ocean ridge is exposed above sea level and active volcanoes are found on land. Earthquake activity along mid-ocean ridges is low and earthquakes occur at shallow depths.

Convergent Plate boundaries :

These occur when two similar or different plates collide. Most boundaries formed due to two different plates with different densities -oceanic and continental generate subduction zones because one of the plates sinks or subducted beneath the other forming a deep trench in the ocean floor. Collision of continental plate with a continental one forms the Himalayas and such convergence produces shallow and intermediate depth earthquakes but little volcanic activity. Earthquakes occur at the converging junction of two plates producing over 75 percent of world’s earthquakes at these boundaries. To be continued