by Mary Caperton Morton Friday, January 12, 2018
On May 29, 2006, a massive mud eruption in East Java, Indonesia, began spewing as much as 180,000 cubic meters — the volume of 72 Olympic-sized swimming pools — of hot muddy debris each day from several vents, quickly burying nearby villages and forcing the relocation of more than 60,000 people. Almost 12 years later, the eruption, nicknamed Lusi, continues to produce more than 80,000 cubic meters of mud a day, and nobody knows how long the oozing will continue. In a new study, however, scientists have gotten the clearest look yet of the roots of the mud volcano and its possible connection to a nearby volcanic complex that may be driving the eruption.
Mud eruptions are found in many places around the world, some occurring, as Lusi does, where a volcanic hydrothermal system meets underground reservoirs of organic-rich sedimentary rocks, bakes these rocks and then carries debris to the surface as hot, viscous mud. Lusi is unique for its sheer scale: It is by far the largest active mud eruption on Earth, with debris constantly spurting up to 10 meters into the air from a 100-meter-wide crater surrounded by a 600-meter-wide zone of hot mud. Approaching the volcano is impossible, so scientists must rely on drones and remote sensing methods to study the ongoing eruption.
Previous geochemical studies conducted on the mud, water and gas emitted from Lusi have indicated a possible link to the nearby Arjuno-Welirang volcanic system, whose youngest cone lies 10 kilometers to the southwest. “We have been studying Lusi intensively since it began erupting in 2006, and all the geochemical signs point to a close connection with the neighboring volcanic complex, but what was missing were images of the subsurface” to help resolve the connection, says Adriano Mazzini, a geoscientist at the University of Oslo in Norway and a co-author of the new study, published in the Journal of Geophysical Research: Solid Earth. In 2015, Mazzini and his colleagues installed a network of 31 seismometers around Lusi and the Arjuno-Welirang volcanic complex, letting the network collect data for two years.
“This study focuses on seismic tomography, which allows us to reconstruct the subsurface structures in three dimensions,” Mazzini says. The seismic data confirmed the volcanic connection suggested by the geochemical work, he says. Below Lusi, the team found a vertical hydrothermal conduit that penetrates at least 6 kilometers deep and intersects the Watukosek Fault System, which runs northeast to southwest between the volcano’s northernmost magma chamber and Lusi. “The fault is acting as a pathway for the hydrothermal fluids and magma to migrate inside the sedimentary basin and finally reach the surface at the Lusi site,” Mazzini says.
“I am not yet convinced this is clear evidence of a connection to the volcanic system,” says Richard Davies, a petroleum geologist at Newcastle University in England who was not involved with the new study. Imaging the substructure of mud volcanoes is notoriously difficult, he says, and the data from the new study show “a high-velocity region under the mud volcano that can be interpreted a number of ways.”
One thing the data don’t help determine is how the eruption began, one of Lusi’s biggest mysteries: Was the 2006 eruption triggered by an earthquake 250 kilometers away, two days prior, or by natural gas drilling operations that were going on in the area? Davies cites daily drilling reports from the Indonesia-based Lapindo Brantas oil and gas exploration company that “provide conclusive evidence that they had an influx of fluid into the well on May 28, 2006. They shut the blowout preventers, but about 24 hours after doing so, the mud started erupting at the surface and hasn’t stopped,” Davies says. In 2009, the company was acquitted by Indonesia’s Supreme Court of causing the eruption, due in large part to scientific uncertainty about how it was triggered. (The company has taken financial responsibility for the disaster and, to date, has paid more than $550 million dollars to the communities displaced by the eruption and toward emergency response services.)
The new study does not offer conclusive evidence for how the eruption was triggered, but it does shed light on its scope, Mazzini says. “This imaging shows that this system is much more complex than a single drilling site. To build up this system of a magma chamber and fluids inside a sedimentary basin took a [long] span of geologic time. It was primed and ready to erupt, whatever the trigger might have been.”
The new study may also help improve the next round of efforts to estimate how long the mud volcano will continue to erupt, Mazzini says. “Previous studies that have tried to predict Lusi’s longevity have assumed the system was only about 2 kilometers deep. Our imaging shows it is at least 6 kilometers deep and possibly deeper,” he says. “The whole system is much more complex than we realized.”
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