Tsunamis and the Early-Warning System Kapila Dahanayake, Senior Professor of Geology, Department of Geology, University of Peradeniya, Peradeniya.

Tsunamis- An Introduction

Tsunamis (a Japanese word meaning harbour waves) are systems of ocean gravity waves that are caused by sudden vertical movement of a large area of sea floor during an undersea earthquake. A vertical disruption of the water column can result from such a vertical tectonic displacement of the sea bottom along a zone of fracture in the earth’s crust. In a very large tsunamigenic earthquake like the 26/12 event, 100,000 sq km or more of the sea floor may have got displaced up to several meters.

Tsunamis and associated earthquake ground shaking differ in their destructive characteristics. Ground shaking causes destruction in the vicinity of the fault (e.g near Sumatra in the 26/12 event) whereas the tsunamis can cause destruction locally and at very distant locations (e.g Somalia). Other triggering mechanisms of tsunamis include volcanic eruptions in the ocean, displacement of submarine sediments, coastal landslides or even large scale man-made detonations or meteor impacts. Tsunamis are often called Tidal Waves but this term is a misnomer. Unlike regular ocean tides, tsunamis are not caused by the tidal action of the Moon and Sun.

A tsunami travels outwards from the epicenter of a tsunamigenic earthquake (with magnitudes generally over 6.5 on the Richter Scale) as a series of waves. Its height in the deep ocean is typically about 30cm but the distance between wave crests can be very long-more than 100km Its speed depends on the depth of water and increasing/decreasing ocean depth. In areas of oceans where the water depths reach more than 3km the tsunami speeds travels at speeds of 500 to 1000km per hour. The speed of a tsunami decreases as water depth decreases As they reach shallow water around islands or on a continental shelf, the height of the wave can increase many times, sometimes reaching levels of more than 25m.Near shore, a tsunami will slow down its speed to just a few tens of kilometers per hour. At the shore, the tsunami will behave differently depending on the near shore bathymetry, shape of the coast line and the state of the tide. In some instances, a tsunami may induce relatively benign flooding particularly in low lying coastal areas. In other instances, it can come onshore as a vertical wall of turbulent water that can be very destructive. In most cases there can be a draw down of sea level either preceding or in between crests of tsunami waves that results in a receding of the shoreline, sometimes by a kilometer or more. Destruction from tsunamis is the direct result of three factors: inundation, wave impact on structures and erosion. Strong tsunami-induced currents have led to the erosion of foundations, collapse of bridges and sea walls. Floatation and drag forces can move houses and overturn vehicles including moving trains. The floating debris including boats and cars can become dangerous projectiles and cause considerable damage

The disastrous 26/12 event was a shallow earthquake with its focus at a depth of about 10km below the earth’s surface and characterized by vertical motion causing the slip of about 1000km of the plate boundary .It took place on 26th December 2004 at about 7.00 a.m (Sri Lanka time) in a seismically active region at the plate boundary separating Indian-Australian and E Asia plates. The epicenter was located on the sea bed off the west coast of northern Sumatra island. It is the fifth most powerful in the world in the last 100 years registering a magnitude of 9.0 on the Richter Scale and the worst in 40 years causing more than 300,000 (?) human casualties in 12 countries of the region. A 9.2 strong earthquake hit Alaska in1964. The damages were caused by earth shaking that happens in any earthquake (e.g in regions close to the epicenter of the earthquake like Sumatra ) and seismic sea waves (tsunamis- or tidal waves) that originated with this tsunamigenic earthquake and struck coastal regions causing unprecedented destruction to life and property in the Indian ocean region where such events were rare. Only five tsunamis had been reported in the last 500 years. The 26/12/2004 tsunamigenic earthquake with 300,000 human casualties has taken the biggest human toll recorded so far in the world by any tsunami. The earlier high casualty tsunami from the Pacific was reported from Chile in 1868 with 25000 deaths. However a tsunami that killed about 70,000 people was reported from Portugal in 1775.In Sri Lanka, according to ancient chronicles a tsunami had killed several thousands of people during the reign of King Kawantissa after an earthquake that had caused large scale subsidence of land in the western part of the country during 2n to 3rd centuries B.C (?)

Need of a Tsunami Early Warning System TTsunamis or earthquakes can be neither prevented nor predicted. An early warning system that can give sufficient hours of advance notice to coastal communities is the primary way for effective mitigation of this disaster.. Water level gauges are an essential element of a tsunami warning system. They can be used to confirm the existence or non existence of tsunami waves following an earthquake. To be effective for warning purposes, water level gauges should be located near the tsunami source region to get the most rapid confirmation of the event. Forecasting tsunamis requires adequate understanding of the phenomenon, good and quick collection of earthquake and sea level data and accurate and fast assessment and interpretation of data. A tsunami warning center such as those in operation around the Pacific ocean could have saved thousands of people who died in the Indian Ocean earthquake. Indian ocean countries like Sri Lanka and India did not have such a facility mainly because large tsunamis were in the recent past extremely rare in the Indian Ocean. Not only Indian ocean bordering countries but also countries around Atlantic ocean are currently not protected against possible future tsunami threats. Only countries around the Pacific Ocean where more than 90% of all tsunamis recorded have occurred in the past presently have access to an elaborate state –of- the- art tsunami monitoring/warning system. In the Pacific ocean, a few tsunamis are reported in any given decade . Between 1975 and 1998 there have been at least eighteen (18) in the Pacific and the adjoining seas resulting in significant human casualties and damage to property. In countries like Japan or USA, people living in Pacific coastal areas are warned and taught to move away from coastal regions after a tsunamigenic earthquake. The Pacific Tsunami Warning Center (PTWC) operated by the US in Hawai is supported by more than 2 dozens of member countries including Australia, Canada, Chile, China, Japan, USA etc., and these countries have many seismic stations, water level reporting stations and dissemination points (the 3 principal components of a tsunami warning system) scattered in the Pacific Basin. Functioning of the system begins with the detection by any participating seismic laboratory of an earthquake of sufficient size to trigger alarms, set at the threshold value of 6.5 on the Richter Scale. PTWC collects seismic data, locates the earthquake and computes its magnitude. When reports from water level stations show that a tsunami has been generated posing a threat, a warning is transmitted to the relevant dissemination centers within shortest possible time. In the aftermath of the massive and disastrous tsunamigenic earthquake of 26/12 we have to learn from the long and successful post tsunami experiences of the Pacific Rim countries and take appropriate steps to establish a tsunami early warning facility in Sri Lanka / SAARC region and be prepared for any possible such future threat from the ocean. Such a proposal should indeed be given high priority. Until the installation of such facilities, educating the public about what to do if such a tsunami disaster recurs is of cardinal importance.

Landslides of Sri Lanka KKapila Dahanayake, Senior Professor of Geology, Department of Geology, University of Peradeniya, Peradeniya.

Introduction

Landslides refer to a significant geological hazard involving a wide variety of mass movement landforms and processes. The term ‘landslide’ denotes downward and outward movement of slope-forming materials composed of natural rock, soils, artificial fills , or combinations of these materials. The moving mass may be produced by any one of the three principal types of movement: falling, sliding, or flowing, or by their combinations under gravitational influence. Landslides therefore refer to a geological phenomenon which involves downward movement of rock boulders, rock debris, sands, clays and finer soils. Landslides can take place on unstable slopes in conjunction with other geological hazards like earthquakes or volcanic eruptions or hydrologic hazards like floods. Some landslide movements can be slow, subtle and almost undetectable on a short time basis. However, some movements like earthflows can be swift and devastating.

Recent Landslides

Landslides cause major economic loss and human casualties in many parts of the world. During the last three decades extensive losses to life and property have been reported in the highlands of Sri Lanka during periods of continuous rains. More than 100 people lost their lives during the recent (May 2003) floods in the Ratnapura district when intense precipitation of more than 300mm fell within a period of few hours on unstable hill slopes. Landslides have now become the principal natural hazard in Sri Lanka replacing floods. It has been noted that highland terrains characterized by intense precipitation and underlain by weak weathered feldspar-rich gneisses with closely spaced joint systems are susceptible to landsliding. Such hillslopes invariably show thick soil profiles (5m to 75m) In Sri Lanka, landslides are common in the highlands where appropriate rock types, relief and rainfall are available. However, landslides causing damage to life and property have been reported with increasing frequency in the last three to four decades in populated hilly areas with elevations exceeding about 100m above MSL in the districts of Badulla, Galle, Hambantota, Kandy, Kegalle, Matale, Matara, Moneragala, Nuwara Eliya and Kalutara. Most of Sri Lankan landslides are caused by the percolation of rain water into the fractures of weathered rocks (e.g gneisses). The landslides are thus associated with accumulating and/or flowing water on or below the surface in mountainous terrains. Landslides are thus associated with surface or subsurface streams or water flows originating from the mountains. Newly formed landslides and reactivated ancient landslides have been observed in the Sri Lankan highlands. Remnants of ancient landslides can be identified in their hundreds, if not, thousands along the hill slopes of many a location in the highlands. These old landslides are in many instances stable under present geomorphic and climatologic conditions.

Identification of Old Landslides

They can be identified due to following characteristics: (i) presence of erratic boulders-displaced fragments of rocks-sometimes rounded but identified easily by different orientations of their bedding/foliation (ii) uneven hummocky topography (iii) disrupted drainage patterns and frequent accumulations of water (iv) Since time immemorial such landslides have been stabilized by ancient Sri Lankan villagers who have converted them into paddy fields where there is a system of drains to control rain water so as to arrest future reactivations (Fig. 6) (v) Tea plantations in the highlands are in most instances old landslides stabilized by networks of stepped drains and stone that facilitate the flow of rain water to lower reaches. Ancient landslides (old earth/debris flows) at locations now stabilized and overgrown with vegetation can get disturbed by forest denudation and construction of whole villages/road networks. These old landslides can get reactivated and move due to internal accumulations and subsurface flow of water. At times of continuous rains characterized by intense precipitation, new landslides will develop on locations where weak rocks (e.g weathered gneisses characterized by dense fractures and high feldspar contents) are underlain by thick weathering profiles (Fig.8).

Signals of Future Landslides

Landslides unlike earthquakes can be predicted if signals are observed early. People living on hill slopes with a history of landsliding could save their lives if the following signals of impending landslides are taken note of during periods of rain and take suitable action to evacuate to safer locations - (a) widening of cracks in the surface soils or floor/walls of the dwelling and additional development of cracks (b) slanting of trees, lamp posts and electric posts towards the downward side of the hill slope (c) sudden withering of coconut, arecanut and other similar trees due to downward displacement of their root systems (d) disappearance of water from the upper reaches of the slope and reappearance at locations downhill spouting sometimes warmer water as a spring (e) sounds of water movement below the floors of dwellings- due to internal flow of water (f) slow downward movement of boulders that exist in the vicinity of the dwelling.

Control of Landslides

In terrains like those in Sri Lanka where landslides are triggered by rainfall, it is good to remember the dictum- Control of landslides is control of rainwater. In landslide-prone hill slopes, the rain water should not be allowed to percolate into the body of the landslide. This could be achieved by (i) sealing the fractures of associated rocks and construction of suitable retaining structures, if necessary, to arrest the percolating waters (ii)construction of drains parallel and perpendicular to the landslide axis to drain out the rain water as early as possible to the nearby stream or drainage system (iii) growing suitable grass cover on the surface of the landslide or potential landslide to prevent percolation of rainwater by deflecting the same to a nearby drain. (iv) quick removal of accumulated water in a landslide body by using horizontal piping or electric pumps and (v) growing trees that will absorb water. The best defense against any natural hazard is education. The author in association with the Disaster Management Centre in Colombo has been conducting awareness campaigns during the last few years to educate the relevant officials and people living in the landslide-prone areas about the threat of landslides and the potential signals that appear before an imminent landslide