ALBERT

Description: 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.

Description: The Maltese islands, approximately 314 km2 in area with a high population density, face high levels of water stress due to low amounts of rainfall and a dependence on groundwater abstraction. Until now, in-situ borehole readings have been the only method utilised to monitor the quantitative status of groundwater in the Maltese islands. This study investigates an innovative, cost-effective approach to groundwater monitoring in a small island environment by computing cross-correlations and autocorrelations of ambient seismic noise recorded on seismic networks of broadband and short-period stations in the Maltese islands. We compare the borehole readings of groundwater levels ranging from 0.28-3.39 m above mean sea level with the variations in seismic velocity (δv/v). We apply appropriate filters for the broadband and short-period stations of 0.1-1 Hz and 0.3-3 Hz, respectively. The results demonstrate clear seasonal changes in the seismic velocities, which can be correlated with changes in groundwater levels in nearby boreholes. We find that variations of the δv/v from autocorrelations are more pronounced than the cross-correlation, with maximum seismic velocity changes of ~2% and ~0.3% respectively. The quality of the δv/v deteriorates at longer interstation distances where seasonal variations are less noticeable. Presumably, this is because longer interstation paths tend to traverse more complex geology, different types of aquifers, or even the sea.

Description: ORFEUS (Observatories and Research Facilities for European Seismology, orfeus-eu.org) coordinates and promotes seismology in the Euro-Mediterranean region through harmonized collection, archival and distribution of seismic waveform data & metadata, based on services and products managed at national level by more than 60 participating seismological Institutions. ORFEUS is one of the founding members of EPOS Seismology (www.epos-eu.org/tcs/seismology) and its services are largely integrated in the EPOS Data Access Portal (www.ics-c.epos-eu.org). ORFEUS comprises: (i) the European Integrated waveform Data Archive (EIDA; orfeus-eu.org/data/eida); (ii) the European Strong-Motion databases (orfeus-eu.org/data/strong); and iii) the recently established group representing the community of European mobile pools, including amphibian instrumentation (orfeus-eu.org/data/mobile). Currently, ORFEUS facilitates access to the waveforms acquired by more than 18,000 stations including dense temporary experiments (e.g., AlpArray, AdriaArray), with strong emphasis on open, high-quality data. Access to data and products is ensured through state-of-the-art technologies - with strong emphasis on web services - clear policies and licenses, and acknowledging the crucial role played by data providers. Significant efforts are underway to enhance the existing services to tackle the challenges posed by the Big Data Era and the needs of computational seismology, and to actively encourage interoperability and integration of multidisciplinary datasets in geoscience workflows. ORFEUS also implements community services that include software and travel grants, webinars, workshops, editorial initiatives and discussion forums. ORFEUS activities are assessed and improved through the interaction with a User Advisory Group, which comprises European Earth scientists with expertise on a broad range of disciplines.