Tag Archives: Tsunami

Seismic menace of the Sumatra plate boundary

More than a decade after the 26 December 2004 Great Aceh Earthquake and the Indian Ocean tsunamis that devastating experience and four more lesser seismic events (> 7.8 Magnitude) have show a stepwise shift in activity to the SE along the Sumatran plate boundary. It seems that stresses along the huge thrust system associated with subduction of the Indo-Australian Plate that had built up over 200 years of little seismicity are becoming unlocked from sector to sector along the Sumatran coast. Areas further to the SE are therefore at risk from both major earthquakes and tsunamis. A seismic warning system now operates in the Indian Ocean, but the effectiveness of communications to potential victims has been questioned since its installation. However, increasing sophistication of geophysical data and modelling allows likely zones at high risk to be assessed.

Recent Great Earthquakes in different segments of the Sumatra plate margin (credit: Tectonics Observatory, California Institute of Technology http://www.tectonics.caltech.edu/outreach/highlights/sumatra/why.html

Recent Great Earthquakes in different segments of the Sumatra plate margin (credit: Tectonics Observatory, California Institute of Technology http://www.tectonics.caltech.edu/outreach/highlights/sumatra/why.html

One segment is known to have experienced giant earthquakes in 1797 and 1833 but none since then. What is known as the Mentawai seismic gap lies between two other segments in which large earthquakes have occurred in the 21st century: it is feared that gap will eventually be filled by another devastating event. Geophysicists from the Institut de Physique du Globe de Paris and Nanyang Technological University in Singapore have published a high-resolution seismic reflection survey showing the subduction zone beneath the Mentawai seismic gap (Kuncoro, A.K. et al. 2015. Tsunamigenic potential due to frontal rupturing in the Sumatra locked zone. Earth and Planetary Science Letters, v. 432, p. 311-322). It shows that that the upper part of the zone, the accretionary wedge, is laced with small thrust-bounded ‘pop-ups’. The base of the accretionary wedge shows a series of small seaward thrusts above the subduction surface itself forming ‘piggyback’ or duplex structures.

Seismic reflection profile across part of the Sumatra plate boundary, showing structures produced by past seismicity. (credit: Kuncoro et al. 2015, Figure 3b)

Seismic reflection profile across part of the Sumatra plate boundary, showing structures produced by past seismicity. (credit: Kuncoro et al. 2015, Figure 3b)

The authors model the mechanisms that probably produced these intricate structures. This shows that the inactive parts of the plate margin have probably locked in stresses equivalent to of the order of 10 m of horizontal displacement formed by the average 5 to 6 cm of annual subduction of the Indo-Australian Plate over the two centuries since the last major earthquakes. Reactivation of the local structures by release of this strain would distribute it by horizontal movements of between 5.5 to 9.2 m and related 2 to 6.6 m vertical displacement in the pop-ups. That may suddenly push up the seafloor substantially during a major earthquake, thereby producing tsunamis. Whether or not this is a special feature of the Sumatra plate boundary that makes it unusually prone to tsunami production is not certain: such highly resolving seismic profiles need to be conducted over all major subduction zones to resolve that issue. What does emerge from the study is that a repeat of the 2004 Indian Ocean tsunamis is a distinct possibility, sooner rather than later.

A tsunami and NW European Mesolithic settlements

About 8.2 ka ago sediments on the steep continental edge of the North and Norwegian Seas slid onto the abyssal plain of the North Atlantic. This huge mass displacement triggered a tsunami whose effects manifest themselves in sand inundations at the heads of inlets and fjords along the Norwegian and eastern Scottish coasts that reach up to 10 m above current sea level. At that time actual sea level was probably 10 m lower than at present as active melting of the last glacial ice sheets was still underway: the waves may have reached 20-30 m above the 8.2 ka sea level. So powerful were the tsunami waves in the constricted North Sea that they may have separated the British Isles from the European mainland by inundating Doggerland, the low-lying riverine plain that joined them before global sea level rose above their elevation at around the same time. Fishing vessels plying the sandbanks of the southern North Sea often trawl-up well preserved remains of land mammals and even human tools: almost certainly Doggerland was prime hunting territory during the Mesolithic, as well as an easily traversed link to the then British Peninsula. Mesolithic settlements close by tsunami deposits are known from Inverness in Scotland and Dysvikja north of Bergen in Norway and individual Mesolithic dwellings occur on the Northumberland coast. The tsunami must have had some effect on Mesolithic hunter gatherers who had migrated into a game-rich habitat. The question is: How devastating was it.

English: Maelmin - reconstruction of Mesolithi...

Reconstruction of Mesolithic hut based on evidence from two archaeological sites in Northumberland, UK. (credit: Lisa Jarvis; see http://www.maelmin.org.uk/index.html )

Hunter gatherers move seasonally with favoured game species, often returning to semi-permanent settlements for the least fruitful late-autumn to early spring season. The dominant prey animals, red deer and reindeer also tend to migrate to the hills in summer, partly to escape blood-feeding insects, returning to warmer, lower elevations for the winter. If that movement pattern dominated Mesolithic populations then the effects of the tsunami would have been most destructive in late-autumn to early spring. During warmer seasons, people may not even have noticed its effects although coastal habitations and boats may have been destroyed.

Splendid Feather Moss, Step Moss, Stair Step Moss

Stair-step moss (credit: Wikipedia)

Norwegian scientists Knut Rydgren and Stein Bondevik from Sogn og Fjordane University College, Sognda devised a clever means of working out the tsunami’s timing from mosses preserved in the sand inundations that added to near-shore marine sediments. (Rydgren, K. & Bondevik, S. 2015. Most growth patterns and timing of human exposure to a Mesolithic tsunami in the North Atlantic. Geology, v. 43, p. 111-114). Well-preserved stems of stair-step moss Hylocomium splendens still containing green chlorophyll occur, along with ripped up fragments of peat and soil, near the top of the tsunami deposit which has been uplifted by post-glacial isostatic uplift to form a bog. This moss grows shoots annually, the main growth spurt being at the end of the summer-early autumn growing season. Nineteen preserved samples preserved such new shoots that were as long as or longer than the preceding year’s shoots. This suggests that they were torn up by the tsunami while still alive towards the end of the growing season, around late-October. All around the North Sea Mesolithic people could have been returning from warm season hunting trips to sea-shore winter camps, only to have their dwellings, boats and food stores devastated, if indeed they survived such a terrifying event.

Judging earthquake risk

The early 21st century seems to have been plagued by very powerful earthquakes: 217 greater than Magnitude 7.0; 19 > Magnitude 8.0 and 2 >Magnitude 9.0. Although some lesser seismic events kill, those above M 7.0 have a far greater potential for fatal consequences. Over 700 thousand people have died from their effects: ~20 000 in the 2001 Gujarat earthquake (M 7.7); ~29 000 in 2003 Bam earthquake (M 6.6); ~250 000 in the 2004 Indian Ocean tsunami that stemmed from a M 9.1 earthquake off western Sumatra; ~95 000 in the 2005 Kashmir earthquake (M7.6); ~87 000 in the 2008 Sichuan earthquake (M 7.9); up to 316 000 in the 2010 Haiti earthquake (M 7.0); ~20 000 in the 2011 tsunami that hit NE Japan from the M 9.0 Tohoku earthquake. The 26 December 2004 Indian Ocean tsunamis spelled out the far-reaching risk to populated coastal areas that face oceans prone to seismicity or large coastal landslips, but also the need for warning systems: tsunamis travel far more slowly than seismic waves and , except for directly adjacent areas, there is good chance of escape given a timely alert. Yet, historically http://earthquake.usgs.gov/earthquakes/world/most_destructive.php, deadly risk is most often posed by earthquakes that occur beneath densely populated continental crust. Note that the most publicised earthquake that hit San Francisco in 1906 (at M 7.8) that lies on the world’s best-known fault, the San Andreas, caused between 700 and 3000 fatalities, a sizable proportion of which resulted from the subsequent fire. For continental earthquakes the biggest factor in deadly risk, outside of population density, is that of building standards.

English: A poor neighbourhood shows the damage...

A poor neighbourhood in Port au Prince, Haiti following the 2010 earthquake measuring >7 on the Richter scale. (credit: Wikipedia)

It barely needs stating that earthquakes are due to movement on faults, and these can leave distinct signs at or near to the surface, such as scarps, offsets of linear features such as roads, and broad rises or falls in the land surface. However, if they are due to faulting that does not break the surface – so-called ‘blind’ faults – very little record is left for geologists to analyse. But if it is possible to see actual breaks and shifts exposed by shallow excavations through geologically young materials, as in road cuts or trenches, then it is possible to work out an actual history of movements and their dimensions. It has also become increasingly possible to date the movements precisely using radiometric or luminescence means: a key element in establishing seismic risk is the historic frequency of events on active faults. Some of the most dangerous active faults are those at mountain fronts, such as the Himalaya and the American cordilleras, which often take the form of surface-breaking thrusts that are relative easy to analyse, although little work has been done to date. A notable study is on the West Andean Thrust that breaks cover east of Chile’s capital Santiago with a population of around 6 million (Vargas, G. Et al. 2014. Probing large intraplate earthquakes at the west flank of the Andes. Geology, v. 42, p. 1083-1086). This fault forms a prominent series of scarps in Santiago’s eastern suburbs, but for most of its length along the Andean Front it is ‘blind’. The last highly destructive on-shore earthquake in western South America was due to thrust movement that devastated the western Argentinean city of Mendoza in 1861. But the potential for large intraplate earthquakes is high along the entire west flank of the Andes.

Vargas and colleagues from France and the US excavated a 5 m deep trench through alluvium and colluvium over a distance of 25 m across one of the scarps associated with the San Ramon Thrust. They found excellent evidence of metre-sized displacement of some prominent units within the young sediments, sufficient to detect the effects of two distinct, major earthquakes, each producing horizontal shifts of up to 5 m. Individual sediment strata were dateable using radiocarbon and optically stimulated luminescence techniques. The earlier displacement occurred at around 17-19 ka and the second at about 8 ka. Various methods of estimation of the likely earthquake magnitudes of the displacements yielded values of about M 7.2 to 7.5 for both. That is quite sufficient for devastation of now nearby Santiago and, worryingly, another movement may be likely in the foreseeable future.

Landslides and multiple dangers

English: A rock landslide in Guerrero, Mexico....

A landslide in Guerrero, Mexico in August, 1989. (credit: Wikipedia)

Just as modern humans were establishing a permanent foothold in Britain and engaging in the transition to settled farming and livestock husbandry disaster struck some of the most attractive Mesolithic real estate. Around 8 000 years ago the east coast of Scotland, from the Shetland Isles to the Firth of Forth, was struck by a tsunami as big as that affecting the north eastern island of Honshu in the Japan archipelago in 2011. It washed over low lying islands of Shetland and Orkney and roiled up the great inlets or firths of eastern mainland Scotland to leave thick sand deposits containing carcases of whales and other large sea mammals. At that time, Britain was joined to the rest of Europe by marshy lowlands linking East Anglia and the Netherlands dubbed ‘Doggerland’ at the southern end of a huge gulf that became the North Sea. Final sea level rise removed that initial gateway to Britain, so we cannot judge what damage the tsunami wrought, but tools and animal bones dredged from the area show that it was full of game and people. A disaster, but not one linked to seismicity. The driving force has been recognised in a series of submarine scars off the west coast of Norway that witness massive slides of sediment on the sea bed area known as Storegga. Similar scars around the Hawaiian Islands and those making up the Azores and Canaries in the mid Atlantic bear witness to many large slippage events, on the sea bed and from the islands themselves. Recognising signs of past tsunami damage in coastal areas worldwide reveals plenty of cases triggered by landslides rather than earthquakes.

The March 2011 Sendai tsunami and those which ravaged lands around the Indian Ocean in late 2004 formed because of vertical movements on major faults that dropped or shoved up the oceanic crust itself. Yet any sudden change in the shape of the sea floor will displace all the ocean water above, the difference from seismic tsunamis lies in the energy source: instead of tectonic plate forces, gravitational potential energy is released by slumps and slides. That may happen because of erosion producing unstable steep slopes, build up of sedimentary piles, large outpourings of lavas or slopes being destabilised by minor earthquakes or release of gases from the sediments themselves. The Mesolithic submarine slide at Storegga may have been set in motion by massive release of methane from gas-hydrate deposits, and such is the extent of scarring of the sea floor there that it must have happened before and may do so again.

1755 copper engraving showing Lisbon in flames...

Copper engraving showing the 1755 Lisbon tsunami overwhelming ships in the harbor. (credit: Wikipedia)

Realisation of the potential for tsunamis to be triggered by submarine and coastal and slides has spurred bathymetric studies in a number of likely areas, including the Gorringe Bank that lies on the Atlantic floor just west of the Iberian Peninsula. It is tectonic in origin but has a thick veneer of sediment brought by Iberian river systems. On its northern flank is a 35 km long scar of a slip that moved 80 km3 of sediment (Lo Iacono, C. And 11 others 2012. Large, deepwater slope failures: implications for landslide generated tsunamis.  Geology, v. 40, p. 931-934). The Spanish-British-Italian group estimate that the slip would have generated a 15 m tsunami most likely to have affected the Iberian coast south of Lisbon. Conditions for slides of si,ilar magnitude still exist on the Gorringe Bank. One unstable system ripe for collapse is present far out in the Atlantic on the south-east coast of the island of Picos in the Azores (Hildenbrand, A. et al. 2012. Large-sale active slump on the southeast flan of Picos Island, Azores. Geology, v. 40, p. 939-942). This is in a coastal area where repeated volcanism has piled up lavas on the flanks of the island’s main volcanic edifice. Failure has already started, with a number of prominent arcuate scars having developed. The Picos slide moves very slowly sideways but vertical displacements ar estimated at up to a centimetre a year. The volume of the slowly moving mass is an order of magnitude less that the fossil slide on the Gorringe Bank. Yet should it fail entirely, the slopes involved, the absence of water’s slowing effect and the height of the mass might ensure comparable energy is delivered to the Atlantic Ocean, though the likely trajectory of tsunamis would be parallel to the coast of Africa rather than directly towards it.

Landslides of all kinds, though hazardous, have long been thought to be less of a risk to life globally than the more spectacular seismic and volcanic hazards, but there are few data to support that view. In an attempt to assess the annual risk properly, David Petley of Durham University, UK ‘mined’ world-wide landslide records for the seven years since 2004 (Petley, D. 2012. Global patterns of loss of life from landslides. Geology, v. 40, p. 927-930). There were more than 2600 recorded slope-failures that killed people and caused a total of more than 32 thousand fatalities: ten time more than previous vague estimates. This is a minimum because many landslides occur in very remote areas, especially in the mountainous regions of China and the Himalaya. The number of fatalities accompanying each event shows distinct signs, on a country-by-country basis, of a relationship with population density. Several international agencies are emerging that aim at means of measuring disaster risk, one being the Integrated Global Observing Strategy for Geohazards (IGOS).

Search on for past tsunamis

Wandoor is a small village and beach near the ...

Relics of the 2004 tsunami on the coast of South Andaman Island. Image via Wikipedia

Spurred by the horrific scenes and death toll wrought by tsunamis following  the 26 December 2004 Sumatran and 11 March 2011 Sendai giant earthquakes, environmental geologists are beginning to look for signs that can reveal past tsunamis in order to evaluate risk from region to region. Before the 11 March disaster Japanese scientists had in fact traced signs of a tsunami in 869 CE and showed that it had reached almost as far inland as that following the Sendai earthquake. There are a number of geological features that mark the wake of a tsunami: dislodgement of huge boulders on rocky shores; signs of powerful scouring of sallow marine sediments as water recedes from the land; chaotic sediments made up of a jumble of clasts; sediments associated with high-energy flow interleaved with those that mark long periods of low energy deposition; marine faunas unexpectedly found in otherwise terrestrial sediments.

Shortly after the 2004 Indian Ocean tsunamis Indian and Japanese scientists visited the Andaman Islands, which were at the northern end of the megathrust deformation, to seek onshore signs of previous catastrophes (Malik, J.N. et al. 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India. Geology, v. 39, p. 559-562). They discovered a layer of ripped-up lumps of mud set in a sandy matrix dumped on a low-energy black mud, the sandy unit showing inclined stratification that dips inland. All the evidence pointed to deposition by a tsunami. An earlier event reveals swamping of older non-marine sediments by the black mud unit that contains brackish-marine diatoms; a probable result of sudden subsidence linked to an earthquake affecting the Andamans in much the same was as did that of December 2004. The mud had also been intruded by a body of structureless sand , probably resulting from liquefaction as a result of the seismicity. Dating the events using radiocarbon methods proved difficult. Although dating of the earlier event suggested an event around 1670 CE, carbon from the later one gave much older ages, suggesting that the tsunami had ripped up older sediments and redeposited them. However it may be correlated with the major Arakan earthquake of 2 April 1762 close to the coast of Myanmar.

Evidence of this kind can easily be overlooked, and rather less research centres on recent coastal-zone sediments than on sedimentary rocks of the distant past. Areas where such signs of neotectonics have been sought assiduously are those surrounding coastal nuclear installations, but largely to check for evidence of recent faulting that may indicate potential seismic threat but not tsunamis. Clearly it was that kind of threat that decisively put the Japanese Fukushima Daiichi nuclear power station out of action and almost resulted in complete melt-down in March 2011, and severely set back construction of an advanced fast-breeder reactor on the eastern coat of India at Kalpakkam, near Chennai in 2004.