The earthquake which devastated the city of Padang in Sumatra, Indonesia, this week, killing more than 1100 people, may have been only a hint of worse to come. Since 2004, geologists have been predicting a far nastier earthquake in the region – a shallow tremor that will rip the sea floor apart, trigger a devastating tsunami and kill far more people.(emphasis mine)
"Another earthquake is on its way, and all it will take to trigger it is the pressure of a handshake," says John McCloskey, a seismologist at the Environmental Sciences Research Institute at the University of Ulster in Coleraine, Northern Ireland.
Padang experienced a magnitude-7.6 earthquake on 30 September, just after 5 pm local time. Images of terrified relatives waiting to identify dead bodies, their T-shirts clutched over their noses to mask the stench, military officials stalking between bright yellow, zipped-up body bags and centuries-old Dutch colonial mansions obliterated in an instant have flooded around the world.
At first, geologists assumed this was the earthquake they had predicting for many years. "Padang has bad geology," explains McCloskey. "It sits 40 kilometres above the most earthquake-prone stretch of the interface between the Indo-Australian and Eurasian plates."
This interface has not experienced the stress relief of an earthquake for over 200 years, according to McCloskey's analysis of historical coral growth rings, which show no sign of seafloor uplift. GPS measurements of the rate of plate motion suggest that there has been around 13 metres of movement in this area over the same period. "A shallow earthquake at the plate interface off Padang is long, long overdue," says McCloskey.www.newscientist.com...
Several independent indicators imply a high probability of a great (M > 8) earthquake rupture of the subduction megathrust under the Mentawai Islands of West Sumatra. The human consequences of such an event depend crucially on its tsunamigenic potential, which in turn depends on unpredictable details of slip distribution on the megathrust and how resulting seafloor movements and the propagating tsunami waves interact with bathymetry. Here we address the forward problem by modelling about 1000 possible complex earthquake ruptures and calculating the seafloor displacements and tsunami wave height distributions that would result from the most likely 100 or so, as judged by reference to paleogeodetic data. Additionally we carry out a systematic study of the importance of the location of maximum slip with respect to the morphology of the fore-arc complex. Our results indicate a generally smaller regional tsunami hazard than was realised in Aceh during the December 2004 event, though more than 20% of simulations result in tsunami wave heights of more than 5 m for the southern Sumatran cities of Padang and Bengkulu. The extreme events in these simulations produce results which are consistent with recent deterministic studies. The study confirms the sensitivity of predicted wave heights to the distribution of slip even for events with similar moment and reproduces Plafker's rule of thumb. Additionally we show that the maximum wave height observed at a single location scales with the magnitude though data for all magnitudes exhibit extreme variability. Finally, we show that for any coastal location in the near field of the earthquake, despite the complexity of the earthquake rupture simulations and the large range of magnitudes modelled, the timing of inundation is constant to first order and the maximum height of the modelled waves is directly proportional to the vertical coseismic displacement experienced at that point. These results may assist in developing tsunami preparedness strategies around the Indian Ocean and in particular along the coasts of western Sumatra. sciencedirect
The great Sumatra–Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones1, 2, 3, 4. In March 2005, the Sunda megathrust ruptured again, producing an event5 of moment magnitude (Mw) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 (Mw = 8.8) and 1833 (Mw = 9.0)6, 7. Two earthquakes, one of Mw = 8.4 and, twelve hours later, one of Mw = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period.(emphasis mine)
This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non-permanent barriers, zones with locally lower pre-stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large. www.nature.com...
The best-known eruption of Krakatoa culminated in a series of massive explosions on August 26–27, 1883, which was among the most violent volcanic events in modern and recorded history.
With a Volcanic Explosivity Index (VEI) of 6,[3] the eruption was equivalent to 200 megatons (MT) of TNT—about 13,000 times the nuclear yield of the Little Boy bomb (13 to 16 kT) that devastated Hiroshima, Japan during World War II and four times the yield of the Tsar Bomba (50 MT), the largest nuclear device ever detonated.
The 1883 eruption ejected approximately 21 cubic kilometres (5.0 cu mi) of rock, ash, and pumice.[4]
The cataclysmic explosion was distinctly heard as far away as Perth in Western Australia, about 1,930 miles (3,110 km) away, and the island of Rodrigues near Mauritius, about 3,000 miles (5,000 km) away.[5]
Near Krakatoa, according to official records, 165 villages and towns were destroyed and 132 seriously damaged, at least 36,417 (official toll) people died, and many thousands were injured by the eruption, mostly from the tsunamis that followed the explosion. The eruption destroyed two-thirds of the island of Krakatoa.
Eruptions at the volcano since 1927 have built a new island in the same location, named Anak Krakatau (Indonesian: "Child of Krakatoa"). This island currently has a radius of roughly 2 kilometres (1.2 mi) and a high point around 300 metres (980 ft) above sea level, growing 5 metres (16 ft) each year.[6]
