The Plumbing System Feeding the Lusi Eruption Revealed by Ambient Noise Tomography

Lusi is a sediment-hosted hydrothermal system featuring clastic-dominated geyser-like eruption behavior in East Java, Indonesia. We use 10months of ambient seismic noise cross correlations from 30 temporary seismic stations to obtain a 3-D model of shear wave velocity anomalies beneath Lusi, the neighboring Arjuno-Welirang volcanic complex, and the Watukosek fault system connecting the two. Our work reveals a hydrothermal plume, rooted at a minimum 6km depth that reaches the surface at the Lusi site. Furthermore, the inversion shows that this vertical anomaly is connected to the adjacent volcanic complex through a narrow (3km wide) low velocity corridor slicing the survey area at a depth of 4-6km. The NE-SW direction of this elongated zone matches the strike of the Watukosek fault system. Distinct magmatic chambers are also inferred below the active volcanoes. The large-scale tomography features an exceptional example of a subsurface connection between a volcanic complex and a solitary erupting hydrothermal system hosted in a hydrocarbon-rich back-arc sedimentary basin. These results are consistent with a scenario where deep-seated fluids (e.g., magmas and released hydrothermal fluids) flow along a region of enhanced transmissivity (i.e., the Watukosek fault system damage zone) from the volcanic arc toward the back arc basin where Lusi resides. The triggered metamorphic reactions occurring at depth in the organic-rich sediments generated significant overpressure and fluid upwelling that is today released at the spectacular Lusi eruption site. Plain Language Summary The Lusi mud eruption started the 26 May 2006 in the northeast of Java, Indonesia. More than 11years later Lusi is still active and continuously erupting boiling mud, rock fragments, gas, and water with stunning flow rates that reached up to 180.000m(3)/d. Today Lusi occupies a 7km(2) area framed by tall embankment walls that prevent a broader expansion of the mudflows. This spectacular eruption site has been investigated by numerous studies; however, images of the deep plumbing system are still missing. Here we present the result of a broad ambient noise tomography survey aiming to image the subsurface of the Lusi eruption site, the neighboring volcanic complex, and the faulted region that connects these two structures. Results show the presence of a >6km deep hydrothermal plume below Lusi. The magma chamber imaged below the volcanic complex connects with the Lusi conduit through an elongated corridor that is oriented following the fault direction. Our results support a scenario where the Lusi eruption is fed at depth by the migration of fluids originating from the volcanic complex interacting with the organic-rich sediments present at 4.5km depth.