Anne Hinton

The Tectonics of the Sumatran earthquakes

The world has heard much about the Boxing Day tsunami of 2004 and the vast loss of life associated with this, including more than 200,000 deaths in Sumatra, Indonesia, alone.

However, the cause of the tsunami

- the great earthquake, magnitude 9.2 on the logarithmic

Richter Scale - has not been widely discussed. That

a second great earthquake, of magnitude 8.7, occurred

shortly afterwards, on 28th March 2005, which resulted

only in a local tsunami, suggests that there is

something 'big' ongoing in the tectonic world in

this part of the globe.

What is behind the earthquakes? Will there be more

in this area? Will there be any more devastating

tsunami? Read on before you plan a holiday to this

surfers' paradise…

Figure 2: Uplifted coral on Pulau Bugi (Bugi island, also sometimes spelled Bogi) in the Hinako islands. Uplift here was ~1.75 m.

The earth has been recognised as having an outer

'shell' composed of 'plates' that move around on

the molten surface beneath. Off western Sumatra

the plate tectonic situation is that the Indo-Australian

plate is moving against the Eurasian plate, which

is being dragged beneath the Eurasian plate, in

what is termed a 'subduction zone', down into the

heat of the earth's molten mantle.

Scientists at Caltech (the California Institute

of Technology) are monitoring land movements in

Indonesia through a network of Global Positioning

System (GPS) points, which assess changes to the

earth's elevation against readings from satellite

positions. Land movements do not just occur 'coseismically'

(with earthquakes); there may also be deformation

('bending') of the earth's surface 'interseismically'

(between earthquakes), at a rate that is so slow

as to be imperceptible to the human eye. GPS data

provide the information on these otherwise indistinguishable

movements.

Figure 3: The photo shows ~60 cm of subsidence at Haloban village in the Banyak islands.

What has happened in this area is that the movement

between the two plates has become partially 'locked'.

Strain builds up over centuries, deforming the earth's

surface, and is released in the large earthquakes,

which also cause surface deformation, but at an

instantaneous timescale.

The Caltech scientists, lead by Professor Kerry

Sieh, published their latest discoveries in the

journal Science on 31st March this year. The island

of Simeulue, off the Sumatran coast, marked both

the 'end' of the earth movements from 26th December

and the 'beginning' of those of 28th March 2005.

There is not normally the chance for scientists

to study such areas in detail as they are usually

on the ocean floor.

Figure 4: The top panel shows slow interseismic subsidence along the southwestern coast of Nias (notice submerged palm plantation). The bottom panel shows coseismic uplift in March 2005 of about 2.5 m.

Incidentally, it was the long length of fault rupture

- movement occurred over a total distance of 1,600

km - beneath the deep ocean waters that created

the world-travelling tsunami wave of 26th December.

Uplift of the sea floor in December 2004 was 6m,

nearly twice that of the amount in March 2005. (The

wave reached Britain on 27th December, with unusual

water level changes of + or - 10 cm being recorded

at the Newlyn tide gauge in Cornwall, according

to data reported by Phil Woodworth and colleagues

of the Proudman Oceanographic Laboratory at Liverpool

University.) Much of the 400km of length of the

fault rupture on 28th March, however, occurred in

the vicinity of land, so only a local tsunami was

generated on this occasion, as there was no large

volume of water displacement involved.

Kerry Sieh, Rich Briggs and their co-workers examined

both the GPS land elevation change data recorded

on Simeulue Island, and the island of Nias to the

south, and the coral reefs in those locations. Uplift

of the land meant that many corals were raised out

of the water, so they were unable to survive. The

scientists found that the northern part of Simeulue

was raised by more than a metre on 26th December

2004. On 28th March 2005 it was the turn of southern

Simeulue, and areas to the south of it, to undergo

a similar amount of uplift.

Why should the movement of the earth's crust have

stopped with northern Simeulue on 26th December

2004 and have started immediately to the south on

28th March 2005? The Caltech scientists believe

that the dislocation zone may mark what is either

effectively a 'tear' in the continental plate at

this point or differences in properties of the thrusting

behaviour. Movement differs to the north and south

of this location as a result.

What of the area to the south of that which moved

on 28th March? Signs of the interseismic deformation

of the earth's crust are clearly present. The strain

will be released at some stage - and, if the amount

of vertical movement and lateral tear in the earth's

crust is sufficiently long, it is possible that

an ocean-travelling tsunami may be generated from

this movement. However, even a local tsunami could

result in large numbers of casualties here, as the

Sumatran 'mainland' is both low-lying and heavily

populated in this area; a point highlighted by Professor

James Jackson of Cambridge University in his lecture

'Surviving Natural Disasters' at Darwin College,

Cambridge.

Will the warning systems, currently being implemented,

help? Well, probably not for Sumatra, due to the

proximity of the land to where the tsunami is generated,

as well as the lack of high ground to escape to

in some areas. Dr. Rich Briggs of Caltech commented

that the best policy in these areas is for people

to recognise the danger sign - long (30+) seconds

of ground shaking - and then head for higher ground.

Professor Sieh and his colleagues distributed information

leaflets to the Sumatran population on their visits

to the area, to educate the local population as

to the dangers.

Whilst most of these were in the local languages,

some leaflets were also in English, as this is a

popular destination for surfers from across the

world, who might, otherwise unwittingly, find themselves

surfing a rather different kind of wave from that

which they were expecting on a visit to Sumatra.

Bear this information in mind if you are considering

a holiday to that part of Indonesia!

 

- May 2006

About the Author

Anne Hinton is an independent research scientist.