standard Earthquake preparedness and geohazard vulnerability in Chile

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Ground crack induced by fault reactivation during the 2010 earthquake near Santiago. © Sergio Sepulveda

Dr Sergio Sepulveda from the University of Chile is visiting Durham University as a Cofund Senior Research Fellow through the Institute of Advanced Study, to work with the International Landslide Centre at IHRR that is led by Professor Dave Petley and Dr Nick Rosser from the Department of Geography. Sepulveda is from Chile, one of the most seismically active parts of the world that regularly experiences earthquakes, landslides, debris flows, volcanic eruptions and other geohazards.

Sepulveda is a leading researcher on landslides in South America and is well-known in the field. Most of the fatalities caused by natural hazards in South America are from earthquakes, floods and landslides that affect both urban and rural communities. Sepulveda is working closely with colleagues in IHRR to identify the vulnerability of populations in Latin America and the Caribbean to landslides, in order to acquire a better understanding that would lead to developing measures to help reduce fatalities, and is testing a number of volcanic soils from Chile to understand the role they play in landslides and other hazards.

Research at Durham

Working with Prof Dave Petley, Sepulveda is studying records of fatal landslides that have occurred in Latin America and the Caribbean using the Durham Fatal Landslide Database, a global record of landslide- induced fatalities from the past 10 years. The database is a useful tool for identifying vulnerability to landslide hazards, ‘…there is a very strong correlation between population density and fatal landslides, and most of them are induced by heavy rainfall’, said Sepulveda. Along with this research Sepulveda is also studying the geomechanics of landslides themselves with Prof Dave Petley and Dr Matthew Brain.

The researchers are investigating the properties of volcanic soils from central Chile that have demonstrated some special behaviours during recent earthquakes, such as amplifying the seismic signals from the earthquake that can cause structural damage, landslides and soil liquefaction.

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Liquefaction-induced ground cracks from 2010 earthquake in southern Chile. © Sergio Sepulveda

‘The aim is to have a closer look at how these soils behave during an earthquake. What are the changes that the seismic waves of the earthquake make on the strength of the soils? Prior to an earthquake they tend to be robust, but apparently during or after an earthquake their behaviour changes’, said Sepulveda.

This is the first time volcanic soils from Chile will be tested using the International Landslide Centre’s dynamic back pressured shear box, a unique, geotechnical lab facility based in Geography that runs a series of dynamic tests that simulate saturated conditions and earthquake shaking, testing the strength of the soils. Properties of the soils may amplify the signals that cause higher shaking around slopes that could collapse creating landslides, due to the higher accelerations through the slope.

Sepulveda thinks more attention should be paid to the physics of the landslide itself, particularly the different kinds of seismic waves that move through the soil that cause slopes to collapse.

‘Depending on the geometry of the slope and the properties of the soil you can get higher shaking and accelerations in some parts, which are more likely to trigger a landslide’, he said.

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Dynamic back pressured shear box for testing strength of soils involved in landslides.

Identifying hazard and vulnerability in Chile

While cities in Chile are resilient to the direct shaking on the ground from earthquakes, their secondary hazards pose a difficult challenge. ‘In Chile you don’t have many casualties relative to the size of the earthquake, but the country is less prepared in terms of landslides, soil liquefaction and tsunamis’, said Sepulveda.

On 27 February 2010 an 8.8 magnitude earthquake and subsequent tsunami led to severe damage and loss of life along the coast of south central Chile. According to Sepulveda, damage to buildings in many areas was mainly caused by soil liquefaction rather than the direct shaking from the earthquake, a severe secondary hazard that was ‘re-discovered’ during that time. Strict building codes in Chile account for the shaking from earthquakes, but not liquefaction. Also, in the case of the tsunami, ‘people were not well-prepared’.

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Flood and wind damage from 2010 tsunami in Talcahuano, Chile.

The 2010 tsunami caught communities living along the coast by surprise. While a tsunami warning system was in place at the time, authorities hesitated to sound the alarm, but since then Sepulveda says that there has been much more effort dedicated to tsunami preparation and evacuation. This has proven to be the case after the recent 8.2 magnitude earthquake that also caused a tsunami.

Chile’s capital city, Santiago, lies in a large valley at the foot of the Andes Mountains. The landslides and debris flows that are most likely to affect the city first take place high up in the mountains. During heavy rain fall, the soil, rock and other material is transported through gullies and ends up at the edge of the city where people live.

Sepulveda has done work for the government of Chile to map landslide and debris flow hazards that are caused by rainfall and earthquakes, but says there is still much work to be done because ‘there is not much data to work with’ and ‘no historical records’. Setting up rainfall gauge stations in the mountains is one way researchers could create historic records and find a threshold for rainfall that causes landslide hazards.

Building codes and preparedness save lives

Geoscientists and others who work to prevent fatalities caused by earthquakes usually always stress that earthquakes don’t kill people, but poorly constructed buildings do. Chile is a prime example of how better building practices can save lives. It has building codes in place that are comparable to many developed countries that experience seismic hazards.

Chile has been seen as an exemplar for earthquake safe building that has been highlighted by the United Nations Office for Disaster Risk Reduction (UNISDR). The World Risk Index issued by the UN in 2011 reported that Chile had ‘continuously established and enforced better building regulations’.

This is likely why the 8.2 magnitude earthquake that struck northern Chile earlier this month has only led to six recorded deaths, three of which were from heart attacks and one by an accident during the tsunami evacuation. Combine this with well-planned warning and evacuation plans for the subsequent tsunami, and the risk to the populations vulnerable to the hazards are significantly reduced.

‘The recent earthquake in northern Chile confirmed the strength of our building code, with limited damage mainly in old adobe houses and very few casualties. Although the tsunami was small, people evacuated quite swiftly even though it was during the night, showing that preparation from several drills in recent years worked well. A large earthquake, actually bigger than this, has been expected in this region for several years, and the lessons from the failed evacuation in 2010 seemed to be learned. Let’s hope that they are not forgotten with time, as usually happens with natural disasters’.

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