The 29 September earthquake near Samoa, which caused a fatal tsunami would have been a notable seismic event even if it had not claimed so many lives (at least 149 according to recent figures). The quake measured 8.0 on the Richter scale, and on average the world experiences only one quake of magnitude 8.0 or above per year. Less than 17 hours later there was a magnitude 7.6 quake just offshore of Sumatra, which devastated the city of Padang. This did not cause a tsunami, probably because it was too deep, but many more lives have been lost (at least one thousand and rising, mainly in collapsed buildings).

In an average year, we would expect about 20 quakes exceeding magntiude 7.0, so naturally the news media got rather excited as to whether the two quakes were linked. Had the Samoan quake caused the Sumatra quake? I spent quite a while on Wednesday evening talking to various print and broadcast journalists by telephone and on Skype, and was chauffeured down to London at 4am on Thursday to do a live interview on GMTV just after 6am. Some journalists also cottoned on to a magnitude 5.8 quake in Peru nine hours after the Sumatra quake, and looked for a link between all three...

The answer is that the events are most unlikely to be linked. Look at the map below: the epicentres of the Samoan and Sumatra quakes are more than 7500 km apart, and they are not even on the same plate boundary. The Samoan quake occured in what is popularly called the 'Ring of Fire' This (almost) circles the Pacific ocean, and is a system of trenches where the Pacific ocean floor is being subducted (pushed down below) continents or island arcs, running from New Zealand, northwards past Tonga (where the ocean floor is being subducted westwards, and hence the Samoan quake), thence onwards to the Philippines, Japan, Kamchatka, Alaska and down the west coast of the Americas to Chile. The magnitude 5.8 Peru quake was on the 'Ring of Fire' too, where the ocean floor is being subducted east below South America) and was entirely unremarkable because we expect three or four quakes exceeding magitude 5.0 somewhere in the world every day. Melting processes in subductioin zones, not directly related to earthquakes, feed the volcanoes that give the 'Ring of Fire' its name. An earthquake happens when strain that has built up over decades or centuries is released by the sudden, violent, slippage of a fault. The most that can be said is that a distant quake might be capable of precipitating a quake that was going to happen soon anyway. It certainly cannot cause one that was not fairly imminent.

Strictly speaking, the Sumatran quake was not on the 'Ring of Fire', despite what you may read or hear in many new reports, but was caused by the same sort of process. Here, the Indian ocean floor is being subducted northeastwards below Sumatra and Java. It was a large quake further along this plate boundary that caused the 26 December 2004 Indian ocean tsunami that claimed nearly 300,000 lives, and a smaller (magnitude 7.7) quake just offshore of Java that caused a smaller tsunami in July 2006.  There was a very intelligent report about many of these issues on the BBC's Newsnight on Thursday.

The epicentres of the Sumatran (on the left) and Samoan (on the right) quakes.  They are more than 7500 km apart, and a very complex system of tectonic plate boundaries lies between. [Map made in Google Earth]

Attention now must focus on rescue efforts, but I hope it will subsequently switch to enquiries into why buildings in Padang were not better able to withstand the shaking caused by the earthquakes. It seems that hospitals and schools collapsed. These are buildings with large rooms, and hence large extents of unsupported roofs and ceilings, but it is well known how to make such structures earthquake resistant. All too often, it turns out that seismic building codes have been flouted. I have blogged previously about shoddy school buildings in regions at risk from earthquakes, for example in the magnitude 7.9 quake in Sichuan, China on 12 May 2008 . Fortunately the local time when the quake struck Padang was after 5 pm, so presumably the schools were fairly empty - but not so the hospitals.

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I left a cold and snowy UK earlier this month to go back to Central America to continue the geophysical measurements in Costa Rica and Nicaragua. The journey over was long but uneventful and this time US customs decided not to open the cases (breaking the locks) to check that the instruments were all in order with the paperwork. Why, oh why can’t they have a transit lounge at Miami so that you don’t have to immigrate for just a few hours on the way to Central America?

On arrival in Costa Rica, the weather was warm, but windy and the volcanoes, which should dominate the skyline, were obscured in cloud. The next couple of days were spent preparing equipment, talking with colleagues at the local observatory, OVSICORI (Observatorio Vulcanológico y Sismológico de Costa Rica) part of the National University, and giving a lecture on our results so far.

We spent a day trying to get up Turrialba volcano, but the weather was so bad that a landslide caused by the rain blocked the road and after an hour or so of queuing and waiting, although we got through, it was clear that it was too dangerous to proceed. It is so frustrating to get so close and then not to get to the summit to make our measurements. Turrialba has increased its gas output over the last few years, and we have been monitoring the gravity field at the summit region to see whether a new batch of magma is rising or whether this is just an escape of gas from the magma body, which has been cooling at the summit since the last eruption in 1866.

Masaya volcano from the airport.
[Image by Hazel Rymer © copyright Hazel Rymer]

On Sunday, it was time to leave the relatively cool breeze in Costa Rica and go up to Masaya, Nicaragua, where the elevation is lower and the climate at this time of year is much drier and hotter. The volcano is persistently degassing and we are working with a groups of Earthwatch volunteers to collect geophysical and ecological biodiversity data on the environmental effects and changing activity at Masaya Volcano.

Earthwatch volunteers taking gravity, GPS and magnetic measurements.[Image by Hazel Rymer © copyright Hazel Rymer]

We have a network of gravity stations, which we measure every year and we have shown a correlation between a reduction in gravity at the summit crater area and an increase in gas flux. Our earlier work has been published already and is freely available – "Gravity changes and passive SO2 degassing at the Masaya caldera complex, Nicaragua".

Volunteers installing a tiltmeter at the bunker near the crater.[Image by Hazel Rymer © copyright Hazel Rymer]

This year, we are setting up continuously recording gravity meters, making dynamic gravity measurements, magnetic, differential GPS and SP measurements to investigate the level of the sub-surface magma and the amount of gas within it. We are also conducting biodiversity studies to investigate the effects of the persistent degassing on the flora and fauna.

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We have a long term research project at active volcanoes in Central America. Our primary goal is a better understanding of the environmental and ecological hazards posed by gas emissions at persistently active volcanoes. Armed with this understanding, our second goal is to develop strategies to mitigate the environmental and ecological risk at these sites. We have chosen to conduct this interdisciplinary study at Masaya (Nicaragua) and Poás (Costa Rica) volcanoes because of the contrasting environmental conditions at each and the persistent, low level of eruptive activity. The aim is to track and quantify the volatile flux at each volcano from the source magma, through the volcanic plume, to the local environmental sinks in the soil and water, and the flora and fauna.

The local environmental effects of pyroclastic flows and lavas are obvious in their coverage and destruction of the land surface. Persistently active volcanoes by their very nature erupt in a regular manner and effects over time are not so obvious. These volcanoes may erupt magma - for example Stomboli (Italy) and Arenal (Costa Rica) typically erupt explosively every 20-30 minutes throwing magma tens to hundreds of metres into the air. For the most part however, persistently active volcanoes emit gases rather than rock.

We are investigating the processes that control volatile flux from magma and quantifying the long-term environmental and ecological effects of background degassing at these two persistently active volcanoes. The aim is to identify the relationship between acid rain and dry deposition of sulphur and to find out how this varies with local climate, soil type and volcanic activity. The idea is to uncover the path and ultimate fate of volatiles erupted at Masaya and Poás volcanoes from their magmatic source, through the gas plume and into the ecosystem. This will lead to a better understanding of the hazards posed by gas emissions at persistently and intermittently active volcanoes. Information on the transport mechanisms of pollutants will allow for more effective mitigation procedures to be adopted including (i) cultivation of acid tolerant crops to neutralise soil, (ii) evacuation of livestock and (iii) advice on the full evacuation or time-limited exposure for the human population as necessary.

As part of the monitoring programme, we are currently in Costa Rica and on 8^th January 2009 there was a 6.2 magnitude earthquake at Poás volcano while we were working on the crater rim. We were making gravity and biological diversity measurements at the time.

There was intense shaking of the ground for several seconds and it was very hard to remain standing. New fractures opened up around the crater rim and there were several rock slides as parts of the crater wall crashed down to the crater floor. We moved back away from the rim and sheltered behind boulders to wait for aftershocks or in case there was an eruption. The degassing from the crater increased in intensity and the landslides and shaking caused sulphur pools in the crater bottom to be disturbed so that the lake changed colour on the surface as yellow sulphur streaks appeared on it.

We climbed out of the crater area and felt a few more aftershocks. Colleagues in the crater bottom also emerged safely. The visitor centre at Poás suffered some damage with broken windows. Further down from the summit, local villages were severely affected. Houses were destroyed, some completely disappeared in landslides and floods. More than 40 people lost their lives in these events and emergency services were hampered by blocked roads due to fallen trees, landslides and collapsed bridges. After shocks are still occurring more than 24 hours after the main event.

We will be going back to the volcano over the next few days to resume our work. The gravity measurements demonstrate that there has been an increase in sub-surface mass, which we interpret to be shallow intrusions of magma beneath the active crater. We are also measuring the rate of deposition of sulphur around the crater area and also the effect on biodiversity.

Hazel Rymer talking to a group of students on the crater rim of Poás volcano.
[image by Michelle Spinosa © copyright Michelle Spinosa]

About the author

Dr Hazel Rymer is Senior Lecturer In Environmental Geophysics at the Open University. A founder member of the OU’s Volcano Dynamics Group, her research is focussed on identifying the processes that trigger eruptions.

Subscribe to Hazel Rymer's posts