Lusi hydrothermal structure inferred through ambient vibration measurements

The spectacular Lusi mud eruption started in northeast Java, Indonesia, the 29th of May 2006 following a M6.3 strike slip seismic event. After the earthquake several mud pools aligned along a NE-SW direction appeared in the Sidoarjo district. The most prominent eruption site was named Lusi. Lusi is located ∼10 km to the NE of the northernmost cone of the Arjuno-Welirang volcanic complex with which it is connected by the Watukosek Fault System. In this study, we applied the HVSR method, which is a common tool used for site effect investigations as well as to infer buried structures and reconstruct sub-surface geology. The method is based on the ratio of the horizontal to vertical components of ground motion and it generally exhibits a peak corresponding to the fundamental frequency of the site. Spectral ratio results highlight a fundamental frequency band between 0.4 and 1.0 Hz in the Lusi neighborhood. We interpret these peaks as related to the velocity lithological contrast at depth between alluvial deposits and bluish grey clay. Our analysis also highlights the presence of a “depocenter” characterized by fundamental frequency up to 0.3 Hz, which is interpreted as the subsidence caused by withdrawal of mud and fluids from depth (as also shown by the comparison of the HVSR results with gravimetry data). Moreover, in the area of the Lusi vent a broad-band frequency range is related to the Lusi conduit. In this paper, we show that detailed microtremor surveys could be used as a preliminary and fast approach to locate mud conduits with sufficient precision.