Ever since the Lusi mud volcano began erupting on May 29, 2006, in the Porong subdistrict of Sidoarjo in East Java, Indonesia, scientists have been unable to agree on its cause. Two days before to the eruption, two events occurred—a magnitude 6.3 earthquake and a subsurface blowout at a gas exploration well—either of which could plausibly, or not, have triggered the eruption. Eleven years later, scientists reported in the Journal of Geophysical Research: Solid Earth (October 2017) the best proof to date through subsurface imaging studies that Lusi's eruption was initiated by the earthquake. Using 10 months of seismic data, they reconstructed three-dimensional images of the Earth's interior below Lusi and the surrounding region, showing that Lusi is connected to the neighboring Arjuno-Welirang volcanic complex. See also: Ambient noise seismic imaging; Computerized tomography; Earth's interior; Earthquake; Lusi mud volcano; Oil and gas well drilling; Seismographic instrumentation; Seismology

Landsat 7 images showing the region and extent of the mudflow around the Lusi mud volcano one month before (left) and nine years after (right; round feature in the center) the eruption. The Lusi mud volcano is the largest mud volcano on Earth. Mud volcanoes erupt hot mud, which typically consists of a mixture of steam and gas, groundwater, and a mineral slurry. (Credit: U.S. Geological Survey)

The images show Lusi, which sits above a sedimentary basin, to be above a roughly six-kilometer-deep geyser-like hydrothermal conduit (plume) connected to the volcano complex through a three-kilometer-wide passageway of faults. The researchers suspect that when the earthquake struck, it activated the fault system, allowing magma and hydrothermal fluids to flow into Lusi’s sedimentary basin. There, the hot fluids began cooking the organic-rich sediments, generating enough steam and pressure to lift a column of water, gases, and entrained sediments to the surface. Based on this mechanism, the researchers expect the eruption to continue for years. See also: Fault and fault structures; Geyser; Magma; Sedimentary rocks; Sedimentology; Volcano

Prior to this study, a common point of agreement among scientists was that never before had such a large and continuous discharge of mud, water, gases, and rocks been observed. With no end in sight, the ongoing eruption has covered an area of seven square kilometers with mud 40 meters deep in places, burying entire villages and displacing 60,000 people from their homes. At present, Lusi is discharging about 80,000 cubic meters of mud per day. Early on in 2006, its discharge (flow) rate was more than twice that volume.