Chapter 6 of "Earthquake and Tsunami":
What next?
|
Table of Contents
6.1. Earthquakes and tsunamis 6.2. Volcanoes |
6.1. Earthquakes and tsunamis
The 26 Dec 2004 earthquake released a huge amount of pent-up energy on the fault line lying to the north of its epicenter. Soon after the quake, worries were voiced by experts that the release of so much energy in the north might put extra stress on the rest of the fault line and perhaps cause a new earthquake further south. Not all agreed - "the possibility of another big tremor is very remote," said the Director General of the Geological Survey of India in February 2005. On 28th March 2005 the remote possibility became fact, only 200 km south of where the 26 December quake had been centred.
Earthquakes release energies that have accumulated on fault lines over many decades, centuries or millennia. If only part of a fault line ruptures, the unruptured sections then carry increased loads and are consequently more likely to rupture. This is why quakes often occur in clusters. Of the 10 giant earthquakes of the 20th century, seven occurred in the 15 years between 1950 and 1965, five of them in the northern Pacific margin (ref. Kanamori H., 1978, Nature 271:411, quoted in Sieh K., 2005, Nature, 434:573-574).
After some initial uncertainty, it has now been established that the entire tectonic fault line from the epicenter of 26 Dec 2004 to the coast of Burma at a length of 1400 km had ruptured. This is good news for the battered people of northern Sumatra, the Nicobars and the Andaman islands. It means that much of the pent-up tectonic energy in their area has been released and the risk of further major earthquakes with tsunamis has been reduced.
Good news for some is very bad news for others, in this case for Indonesians living on islands from southern Sumatra to Timor. The part of the fault line south of the epicentre of 26 Dec 2004 lies close to the coast of many islands, from southern Sumatra to densely-populated Java (home of around 100 million people) right along the island chain of the lesser Sunda islands, from Bali to Timor north of Australia. On Sumatra, in addition, a separate Sumatra fault line runs on land along the entire length of the island. All these islands lie close to the fault line that is now under increased pressure. Moreover, these islands have the world's most impressive line-up of volcanoes, active and dormant . Most of them lie parallel to the fault line and are connected to it (see also the article Toba volcano on this web-site). Major activitity on the faultline could start a period of increased volcanic activity.
How immediate is the danger?
For the area south of the two epicentres, palaeoseismic evidence from the fault off the western coast of southern Sumatra south of the equator indicates that giant earthquakes and tsunamis occur there every 200 to 230 years - sometimes as single huge events, sometimes as two events in (relatively) rapid succession as in 1797 and 1833. There are also records of two large earthquakes in the area in 1861 and 1907 (ref. Newcomb K.R. and McCann W.R., 1987, Journal of Geophysical Research 92:421-439, quoted in Sieh K., 2005, Nature, 434:573-57 ). The danger here clearly is considerable both long- and short-term.
For the area north of the Andamans, from Burma and beyond into the Himalaya regions, there is also increased danger. However, according to Sieh, too little is known of the long term seismic history of this area for a meaningful assessment of the risk.
|
|
Relative decrease/increase of danger from EARTHQUAKES and TSUNAMIS It should be noted that both earthquakes and tsunami are unpredictable and that "less danger" does not mean "no danger". It just means that the risk has come down a little compared to what it was before the recent earthquakes. blue
oval: reddish ovals: |
As Sieh notes:
"Because many of the giant faults in the Aceh-Andaman neighbourhood have been dormant for a very long time, it is quite plausible that the recent giant earthquake and tsunami may not be the only disastrous 21st century manifestation of the Indian plate's unsteady tectonic journey northwards."
|
|
produced by the United States Geological Survey. In the map to the left, produced by the United States Geological Survey, the seismic (not tsunami!) risk is expressed as "peak ground acceleration (PGA) on firm rock, in percent g, expected to be exceeded with a probability of 2% in a 50-year period." Note that the seismic danger is considerably higher in the Himalayas than around Sumatra.
|
Moving plates not only cause earthquakes and tsunamis, they also create volcanoes and keep them going. The fears that the long dormant and now wide awake faultline of the Indonesian coast could rekindle volcanic activity are not groundless. After all, the most destructive volcanic eruption during the entire existence of Homo sapiens has taken place on Sumatra 73,000 years ago (see chapter Toba volcano on this web-site).
However, these fears can be overdone. Many of Indonesia's volcanoes have been active and dangerous even when the plates were quiet. Among them, to name just the two most widely known, are the killer volcanoes Krakatau and Tambora. That a number of hitherto only modestly active volcanoes have started to rumble and spew more smoke, ash and lava is not surprising considering how much the responsible plates have moved during the last 3 months - but it need not be a direct consequence. There is definitively an increased danger from volcanoes but it is not a foregone conclusion that the area will be hit by a major episode of volcanic activity. Such danger has been threatening the area since prehistoric times and it is still looming. The recent plate movements have merely increased the risk by a notch. What the argument and uncertainty is about now concerns the degree by which the risk has increased.
Just how many active volcanoes there are in Indonesia, most lined up along the coast, on the tectonic fault line that has given the people of the area so much trouble lately, is shown on this map (curtesy United States Geological Survey). Note that there are also a large number of sleeping volcanoes (including Toba) not shown on the map.
Those wishing to keep an eye on volcanic activity in the area should consult http://www.volcanolive.com/volcanolive.html
The following volcanoes have become more active since 26th December 2004. Note, however, that these volcanoes have been active for many years (if perhaps not quite so strongly), so that their latest increased acitvity does not necessarily signal a major calamity to come. The World Disaster Alliance (see bulletin below) is not quite so optimistic. There is a narrow line between scare-mongering and complacency - nowhere more so than with volcanoes.
Talang Volcano
0.97 S, 100.67 E, summit elevation 2896 m
Observations of Talang volcano on Wednesday 13 April 2005:
White-gray ash plume rising 250 m above solfatara inside Kawah Mati.
Gabuo Atas crater emitted 10 m high white gas plume. Seismicity
decreased compared to the previous day. On 12 April, there were 11
deep volcanic earthquakes, 48 shallow volcanic, and 38 explosion
earthquakes. On 13 April recorded earthquakes were: 2 deep volcanic,
4 shallow volcanic, and 3 explosion earthquakes. More than 25,000
people have been evacuated from the surrounding area. Alert level 4
remains in place (out of 4). The eruption of Talang volcano was
probably precipitated by the April 10 Mentawai earthquake.
Krakatau Volcano
6.10 S, 105.42 E, summit elevation 813 m
Observations of Krakatau volcano on Wednesday 13 April 2005:
Krakatau volcano has been raised to alert level 2. Krakatau is one of
the world's most famous volcanoes. The paroxysmal eruption in 1883
created 30-40 m high tsunamis which affected the coast of Sumatra and
Java and killing an estimated 36,000 people.
Tangkuban Parahu Volcano
6.77 S, 107.60 E , summit elevation 2084 m
Observations of Tangkuban Parahu volcano on Wednesday 13 April
2005:
The alert level has been raised to 3 (out of 4) at Tangkuban Parahu
volcano in west Java, Indonesia. The Volcanological Survey of
Indonesia reports seismicity has increased at the volcano, and
sulfuric gas was strongly smelt near Ratu Crater. The summit area has
been closed to visitors. Tangkuban Parahu volcano overlooks the city
of Bandung, home to 2 million people.
While the natural forces behind earthquakes and volcanoes - moving geological plates - are relatively simple in principle, the the details, causes and results of these forces are incredibly complex and still largely unpredictable in the short run (on a human as well as a geological time scale).
On a geological time scale, "short run" could mean a hundreds of thousands of years. Toba volcano on Sumatra last exploded 73,000 years ago and may have decisively influenced development of modern humans. Geologically speaking, the Toba eruption was "very recent" while in human terms it was a very long time ago. Warnings of another disaster following a recent one, therefore, may be strictly speaking "correct" - but the next disaster need not come true for a very long time on a human time scale.
No need for panic, therefore, but an unceasing need for caution and watchfulness - and not just near fault lines or volcanoes.
[ Go to HOME
] [ Go to HEAD
OF THIS CHAPTER ]
Last changed 27 August 2006
