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Tag: Science Blogs

  • Lusi sinking into its own caldera

    Two years after it first erupted, Lusi is back in the news again, this time because the area around the vent is starting to show signs of subsidence. The world’s fastest-growing mud volcano is collapsing by up to three metres overnight, suggests new research.

    As the second anniversary (May 29) of the eruption on the Indonesian island of Java approaches, scientists have found that the volcano – named Lusi – could subside to depths of more than 140 metres with consequences for the surrounding environment.

    The sudden overnight three metre collapses could be the beginning of a caldera – a large basin-shaped volcanic depression – according to the research team, from Durham University UK, and the Institute of Technology Bandung, in Indonesia.

    Their findings, based on Global Positioning System (GPS) and satellite measurements, are due to be published in the journal Environmental Geology.

    The fact that subsidence is occuring is actually no surprise; it’s a natural consequence of the eruption itself. Lusi was the result of the breaching of a subsurface aquifer – a porous, water-bearing rock horizon, which was being squeezed by the weight of all the overlying rocks. Because the cap rocks were impermeable mudstones, the water couldn’t move anywhere in response to this squeezing, meaning that the water within the aquifer was being held under extremely high pressure.

    As soon as the breach occurred, all that pressure had an outlet, causing high pressure water to burst upwards, mixing with the overlying mudstones on the way to create the lovely ooze that is currently engulfing Sidoarjo province. For those who remember Super Soakers, they work along a similar principle – you increase the water pressure by pumping air into a sealed water reservoir, resulting in a much stronger water jet when you finally pull the trigger and create a ‘breach’.

    Obviously this is going to lead to a decrease in water pressure within the aquifer; since this pressure was resisting the weight of the overlying rock – now augmented by the weight of the mud that has erupted onto the surface – a decrease will lead to compression of the aquifer, and subsidence, in the area where the eruption is occuring.

    The Environmental Geology paper by Abidin et al. observes exactly this. The GPS data presented was collected between June 2006 and September 2007, and indicates fast subsidence of 2-4 centimetres a day in the first four or five months after the eruption, and slower (and apparently slowing) subsidence of 0.1-0.3 cm a day thereafter. Even after it slowed down, that’s getting on for a metre of displacement every year, which is very fast by geological standards.

    The paper also refers to more recent measurements where large vertical displacements – 10s of cm or even metres – appear to have occurred literally overnight, which might indicate that some of the subsidence is now being taken up by extensional faulting around the vent.

    The formation of such a caldera may restrict the lateral extent of the mud flow, by creating a natural hole for it to fill (although human intervention though the building of dams is also a control here).

    Radar interferometry, which compares altimetry data collected after the eruption to altimetry collected before to highlight changes in topography, has also been used to examine elevation changes over a wider area, and confirms that subsidence has occurred up to 10 km from the central vent. Interestingly, it also shows that a sizeable area to the northeast has been uplifted since the eruption:

    The boundary between the uplifted region and the subsiding vent follows the trend of a fault which runs through the area, indicating that it has been active since Lusi first erupted. This is very interesting, because it is this fault – the Watukosek Fault – that has been cited in the argument that Lusi was a natural response to an earthquake, rather than shoddy drilling practices.

    Does this new data support that interpretation, and let PT Lapindo Brantas (or whatever they’re called now) off the hook? No, actually – the deformation associated with motion on this fault only begins after July 2006, two or three months after the eruption first started, so it in fact fairly conclusively proves that it was not involved in the initial breakthrough. Somehow, though, I doubt this will be the final word on this particular subject.

    Chris Rowan

    Sumber: Science Blogs

  • Lusi: not man-made after all?

    Regular readers of this blog should be aware of the mud volcano currently erupting in the Sidoarjo district in East Java, Indonesia, and the unsuccessful attempts to stem the flow by dropping concrete balls into the vent. Meanwhile, more and more villages, railways and factories are being engulfed, and tens of thousands of people displaced, by the encroaching mud.

    Obviously someone has to pay for the attempts at mitigation, no matter how hare-brained, and compensate those who have found their homes and livelihoods suddenly below ground level. Precisely who that someone should be has been the subject of a fair amount of legal wrangling between the Indonesian government and PT Lapindo Brantas, an exploration company who were drilling a gas well in the area at the time of the eruption.

    At first glance, it doesn’t look good for Lapindo Brantas: not only is the main vent of Lusi is a mere 200 m from their well, but it also turns out that the lower section of the borehole had not been ‘cased’, or sealed off from the surrounding rock with steel jacketing. In the paper which provided the background to my original post on Lusi, Richard Davies and his co-authors suggested that this lack of protection was directly responsible for the eruption: when the borehole penetrated a sealed limestone aquifer, it released a surge of high pressure water back up the drill hole, fracturing and mixing with the overlying mudstones and sending the a torrent of hot mud racing up to surface.

    But there is a complication. Two days before Lusi erupted, a magnitude 6.3 earthquake hit central Java. Could this have had something to do with the eruption of Lusi? Davies et al. say no, but a paper in press in Earth and Planetary Science Letters argues otherwise – and thanks to the obvious legal ramifications, this little spat is getting a little bit of media attention. In this second paper, Adriano Mazzini, of the University of Oslo, and his colleagues* propose that shaking due to the earthquake weakened a fault which runs close to the Lusi vent, and it was this structure, not the borehole, that initially provided an escape route for the overpressured water. Their Figure 4 shows the location of the fault (dashed line) in relation to the borehole (BJP1) and all the mud eruptions in the area in the first week of activity.

    This is interesting, and I can certainly see how the presence of this fault might have exacerbated matters. But it’s difficult to ignore the fact that even if teleseismic stresses acting on a buried fault did have a hand in triggering the eruption, of all the places Lusi could have erupted along that fault, it erupted at the closest possible point to a borehole which was not properly sealed, and therefore at high risk of blow-out. Could this be a coincidence? Mazzini et al. would have us believe so: they claim that the borehole had not penetrated down to the limestone aquifer at the time of the eruption (directly contradicting Davies et al.), and that, almost miraculously, the eruption has not registered in the borehole in any way at all:

    no kicks [pressure fluctuations]. were recorded at the bottomhole of BJP1 [at the time of the eruption], and no fluids erupted through the well… Borehole tests showed that there was no connection between the fluids circulating in the well and mud erupted on surface.

    Neither of these claims is supported by hard data in the paper, so where the two papers disagree on vital and important facts (particularly the depth attained and the timing and nature of pressure changes in the borehole), it is rather difficult to assess who is right. I have to say that regardless of the root cause of the eruption, the subsurface around the vent is probably now so pervasively fractured, and the borehole is such an obvious weak point, that it seems rather unlikely that you see no sign of the eruption in the borehole fluids.

    You’d also think that the eight experts consulted by the Indonesian police, who must surely have had access to the borehole records, would have been a bit less confident in implicating the drilling if this was the case. But then, when there are bills and blame to be assigned, the science is always going to be stretched to breaking point – especially since a law-suit demands a black-and-white answer to what may turn out to be a complicated question: ‘would Lusi have erupted, and erupted so spectacularly, if the Lapindo Brantas borehole been properly sealed?’.

    The answer to that may well be yes, even if the May 27th earthquake does prove to be a factor.

    Chris Rowan 

    Sumber: http://scienceblogs.com/highlyallochthonous/2007/08/lusi_not_manmade_after_all.php