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EARTHQUAKES AND TSUNAMI OF 12 SEPTEMBER 2007 IN INDONESIA

EARTHQUAKES AND TSUNAMI OF 12 SEPTEMBER 2007 IN INDONESIA

George Pararas-Carayannis
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© 2007 George Pararas-Carayannis – All Rights Reserved


INTRODUCTION

On Wednesday, September 12, 2007, a great earthquake struck off the southern coast of the island of Sumatra, killing at least 9 people and generating a relatively small tsunami. Two other major earthquakes struck later. A tsunami warning was issued for the immediate area and watches for adjacent regions.

Origin Time The great earthquake struck on Wednesday, September 12, 2007 at 1110 Z (UTC), about 21:10 (AEST), (1810 local time in the Bengkulu province of Indonesia). Another major earthquake (7.1 magnitude) struck four hours later. A third earthquake with a 7.8 magnitude struck at 1145 GMT 6:45 a.m. (7:45 p.m. Wednesday ET).

Epicenter The epicenter of the great earthquake was off the coast of Southern Sumatra at 4.517 S. 101.382 E., about 185 km south-southeast of Padang, about 130 km (80 miles) SW of Bengkulu, 410 km (255 miles) SW of Jambi, 620 km (385 miles WNW of Jakarta and 695 km (435 miles) SSW of Singapore (USGS).


Magnitude
The great earthquake’s estimated Moment Magnitude was initially given as Mw= 8.2. Later, it was raised to Mw=8.4


Focal Depth
The focal depth was shallow at 30 Km (about 18.6 miles)

Aftershocks At least 10 aftershocks of magnitude 5.1 to 6.0 were felt in the region after the larger quake. At least 60 major aftershocks measuring 5.0 and occurred within the 24-hour period following the main earthquake.


Felt Motions
The earthquakes shook buildings in the Indonesian capital, Jakarta, about 605 km (375 miles) southeast of the epicenter, forcing evacuation of several tall buildings. Shaking at some locations in Singapore (about 435 miles from the epicenter) were so strong that several high-rise buildings were also evacuated. According to eyewitnesses in Singapore, the shaking lasted almost a minute.


Deaths and Damages
Several buildings in Padang, the capital of West Sumatra, collapsed and some caught fire. At least nine people were killed in the Bengkulu province and in Padang. An unknown number of people were injured or were missing. The death toll is expected to rise. Thousands of homes were damaged in Sumatra.




Tectonic Setting

For millions of years the India tectonic plate has drifted and moved in a north/northeast direction, colliding with the Eurasian tectonic plate and forming the Himalayan mountains. As a result of such migration and collision with both the Eurasian and the Australian tectonic plates, the Indian plate’s eastern boundary is a diffuse zone of seismicity and deformation, characterized by extensive faulting and numerous large earthquakes. The region off the west coast of northern Sumatra, the India plate is moving in a northeastward direction at about 5 to 5.5 cm per year relative to the Burma plate.

The Indo-Australian tectonic plate may not be as coherent as previously believed. According to recent studies reported in the Earth and Planetary Science Letters (vol 133), it apears that the two plates have separated many million years ago and that the Australian plate is rotating in a counterclockwise direction, putting stress in the southern segment of the India plate.

USGS graphic showing the migration of the Indian tectonic plate

The earthquake of September 12, 2007 occurred along the great Sunda Trench off Central Sumatra and south of the triple point junction of three tectonic plates where major earthquakes and tsunamis have occurred in the past. This is a region of active subduction where the Indian oceanic plate sinks below the Burma subplate, which is part of the Eurasian plate The relatively young (40 million years) Sunda Arc has been formed by this active subduction process. This is a region of extremely high seismicity where great and major earthquakes occur with frequency. Tectonic movements along this particular segment of the subduction boundary zone produced great earthquakes in 1797 and 1833. These had moment magnitudes estimated at 8.4 and 8.7, respectively. Of the two, there is geologic evidence that the earthquake of 1833 resulted in a much greater rupture and uplift of the outer arc islands (Sipora, and North and South Pagai Islands) and generated a destructive tsunami.

Recent Earthquakes in the Region

Since the devastating tsunami of December 2004, Indonesia has had 15 earthquakes with magnitudes of 6.3 or greater. These recent quakes killed almost 8,000 people in Indonesia. On May 26, 2006, a moderate, magnitude6.3 earthquake, 16 km south-southeast of Yogyakarta, was responsible for 5,749 deaths. On July 17, 2006, a magnitude 7.7 earthquake, 145 miles south-southwest of Tasikmalaya in Indonesia’s Java region, killed 730 people. On March 28, 2005, a great earthquake (Moment magnitude 8.7) centered about 201 km west-northwest of Sibolga, killed 1,313 people.

THE TSUNAMI



A small tsunami was generated. The maximum wave registered by the Padang, tide gauge, north of the quake’s epicenter, was about 60 cm high. The wave amplitude is measured relative to normal sea level – not crest-to trough wave height. The Padang tide gauge is located 0.9S 100.4E. The tsunami height at the tide gauge does not represent tsunami runup. It is only an instrumental recording by a gauge. Maximum tsunami runup on the open coast may have been much higher. A maximum tsunami runup height of 90 cm has been reported. The travel time of the first tsunami wave is estimated at about 20 minutes. The period of subsequent waves was about 28 minutes.


Tsunami Warning and Watch

The Pacific Tsunami Warning Center in Honolulu issued a tsunami warning to the authorities in the immediate region and a watch for all Indian Ocean areas including, north-west Australia, India, Sri Lanka, Myanmar, Bangladesh, Thailand, Malaysia, Sinagapore, Pakistan, Iran, Yemen, Oman, Mauritius, Seychelles, Madagascar, Reunion I., Comores, Crozet Islands, Somalia, Mozambique, Kenya, Tanzania, Kerguelen Islands, and South Africa. No tsunami watch was issued for the Pacific Region. Indian government authorities issued a separate tsunami alert for the coastal states of Kerela, Tamil Nadu, Andhra Pradesh, and the island territories of Andaman and Nicobar.

Evaluation of the tsunami

Folowing the great earthquakes of 2004 and 2005, it appears that there was significant transferrance of tectonic stress further south/southeast to the central region of Western Sumatra. In fact the 2005 earthquake occurred in the same region as the 1861 earthquake. The present earthquake (magnitude 8.2) of September 12, 2007 and the other two events and aftershocks (and later a fourth event) occurred even further south/southeast and within the segment that ruptured when a great (estimated magnitude Mw=8.7) earthquake occurred in 1833.

Apparently, the September 12, 2007 earthquake had a smaller magnitude and length of rupture than the 1833 event which generated a much greater tsunami. The shorter rupture (estimated roughly at about 200 km) and the smaller magnitude were the probable reasons for the smaller 2007 tsunami. Fortunately, the energy release by the other two earthquakes which occurred in sequence, helped release gradually the tectonic stress in this segment. This may have contributed to the relatively smaller tsunami that was observed Padang and elsewhere. This did not occur when the 1833 earthquake struck the same region. All the energy of the 8.7 earthquake in 1833 was released at once and the rupture zone may have been as much as 300 km or even more.

Earthquakes and tsunamis similar to the 2007 and 1833 events can be expected every hundred years or so in this segment of the great Sunnda subduction zone. This particular section of the Sunda megathrust along the western coast of the northern, central and southern Sumatra, is one of the more likely sources of destructive earthquakes and tsunamis in the region in the future.

Epicenters of Historical and Recent Earthquakes and Tsunami Generation Areas.

A repeat of single large earthquake with the same rupture and source dimensions as the 1833 event could again generate a devastating tsunami that could affect Sumatra and other distant regions of the Indian Ocean.

It is still unknown if all the stress that had built in this segment was completely eleased. It may not have been released and additional earthquakes could follow in the next months and years. Also, other seismic regions further south/southeast were probably energized by stress transferrance. Major or great earthquakes could occur further south/southeast in the near future – even sooner than expected – and have the potential of generating destructive tsunamis.

It remains to be seen if the earthquake of September 12, 2007 resulted not only from partial subduction but also from counterclockwise rotation of the Australian plate. Such rotation, with diminished vertical uplift, could account for the smaller 2007 tsunami. Field studies of uplift and lateral motions on the offshore islands would confirm if the mechanism of this 2007 event was different than that of the 1833 tsunami generation mechanism. Field studies on Sipora, North Pagai and South Pagai Islands of the outer-arc ridge of the great Sunda Arc, indicate that the great 1933 earthquake resulted in vertical uplift of up to 2.3 meters. Such extensive vertical uplift generated the greater tsunami. The uplift caused by the September 12, 2007 earthquake may have been much lessthan that of 1833.

Future Earthquakes and Tsunamis in the Region

The earthquakes of 12 September 2007, should not have been a surprise. They should have been expected. In previous evaluation in this website it was pointed out that when the 26 December 2004 earthquake occurred, the Indian plate subducted the Burma plate and moved in a northeast direction. This movement caused dynamic transfer and loading of stress to both the Australian and Burma plates, immediately to the south/southeast, on the other side of the triple junction point. As a result of this load transfer, the Australian plate moved in relation to the Burma plate and probably rotated somewhat in a counterclockwise direction, causing subsequently the great earthquake of 28 March 2005. The block that moved was relatively small in comparison. However, this movement stressloaded the segment further South/Southeast thus causing the September 12, 2007 earthquake in the same region as that which produced the great earthquake of 1833 (magnitude 8.7). That particular earthquake generated a great tsunami. The waves may have been as much as 10 t0 15 meters on the western coast of Sumatra. Luckily, most of the energy from that tsunami was directed towards the unpopulated regions of the Southwest Indian Ocean.

The earthquake of September 12, 2007 occurred sooner that was expected. Another large earthquake could occur again in the same segment in the near future (since the stress was not totally released – which is what this author believes), or in the segment further south/southeast along the southern coast Sumatra. When such an event will occur again, is not known with any degree of certainty. The only thing known with certainty is that a large destructive earthquake will occur again in this region near Padang and Sumatra’s offshore islands. Thus, a Coulomb stress transfer analysis, based on rupture parameters and the geometric distribution of aftershocks for the 26 December 2004, the 28 March 2005 and the 12 September 2007 events, could help establish the space-time evolution of stresses and could help determine both static and dynamic modifications that could possibly trigger again future large events along known faults in this region, as well as destructive tsunamis in the near future. Also, these same seismic events and deeper focus earthquakes under sumatra could activate to a greater extent Mt. Talang volcano near Padang and a large catastrophic eruption is indeed possible.

LINKS TO PAST AND RECENT EARTHQUAKE AND TSUNAMI DISASTERS IN SOUTHEAST ASIA AND THE SOUTHWEST PACIFIC OCEAN

The Great Earthquake and Tsunami of 28 March 2005 in Sumatra, Indonesia

The Great Earthquake and Tsunami of 26 December 2004 in Southeast Asia and the Indian Ocean

The Great Earthquake and Tsunami of 1833 off the Coast of Central Sumatra in Indonesia

Tsunamis of the Indian Ocean

INDONESIA 1883 : Near and Far-Field Effects of Tsunamis Generated by the Paroxysmal Eruptions, Explosions, Caldera Collapses and Slope Failures of the Krakatau Volcano in Indonesia, on August 26-27, 1883
INDIA 2002 – The Earthquake of January 25, 2001 in India

PAPUA NEW GUINEA 1998 – he Tsunami of 17 July 1998 in Papua -New Guinea

INDONESIA 1977 : The Earthquake and Tsunami of August 19, 1977

INDONESIA 1883 : The Great Tsunami of August 26, 1883 from the Explosion of the Krakatau Volcano (“Krakatoa”)

PILIPPINES 1976 – The Earthquake and Tsunami of August 16, 1976 , in the Philippine Islands

VANUATU 1999 – The Earthquake and Tsunami of November 26, 1999 in Vanuatu

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