Horizontal rotation signals detected by “G-Pisa” ring laser for the M w = 9.0, March 2011, Japan earthquake

We report the observation of the ground rotation induced by the M _w = 9.0, 11th of March 2011, Japan earthquake. The rotation measurements have been conducted with a ring laser gyroscope operating in a vertical plane, thus detecting rotations around the horizontal axis. Comparison of ground rotations with vertical accelerations from a co-located force balance accelerometer shows excellent ring laser coupling at periods longer than 100?s. Under the plane wave assumption, we derive a theoretical relationship between horizontal rotation and vertical acceleration for Rayleigh waves. Due to the oblique mounting of the gyroscope with respect to the wave direction of arrival, apparent velocities derived from the acceleration/rotation rate ratio are expected to be always larger than or equal to the true wave propagation velocity. This hypothesis is confirmed through comparison with fundamental mode, Rayleigh-wave phase velocities predicted for a standard Earth model.     

Wavefield tomography based on local image correlations

The estimation of a velocity model from seismic data is a crucial step for obtaining a high‐quality image of the subsurface. Velocity estimation is usually formulated as an optimization problem where an objective function measures the mismatch between synthetic and recorded wavefields and its gradient is used to update the model. The objective function can be defined in the data‐space (as in full‐waveform inversion) or in the image space (as in migration velocity analysis). In general, the latter leads to smooth objective functions, which are monomodal in a wider basin about the global minimum compared to the objective functions defined in the data‐space. Nonetheless, migration velocity analysis requires construction of common‐image gathers at fixed spatial locations and subsampling of the image in order to assess the consistency between the trial velocity model and the observed data. We present an objective function that extracts the velocity error information directly in the image domain without analysing the information in common‐image gathers. In order to include the full complexity of the wavefield in the velocity estimation algorithm, we consider a two‐way (as opposed to one‐way) wave operator, we do not linearize the imaging operator with respect to the model parameters (as in linearized wave‐equation migration velocity analysis) and compute the gradient of the objective function using the adjoint‐state method. We illustrate our methodology with a few synthetic examples and test it on a real 2D marine streamer data set.     

HVSR technique as tool for thermal-basin characterization: a field example in N-E Italy

The Euganean thermal basin, located in the Eastern Po plain (Italy), is one of the most important thermal districts of Italy (Euganean–Berician Thermal District). The presence of hot water in the basin has brought about its exploitation over time. The thermal area has been known since the Roman Empire and is still of crucial interest for the profitable use of its hydro-geological resources. For these reasons the water cycle of the thermal area has been thoroughly studied, in particular with regard to its chemical and hydrogeological properties. The effects of well pumping, such as subsidence and water temperature steadiness, have been monitored over time. In the present work, the Horizontal to Vertical Spectral Ratio (HVSR) single station microtremor technique was applied as a non-invasive geophysical tool for the characterization of a part of the Euganean–Berician thermal district involving the municipalities of Abano and Montegrotto towns. This study represents the first correlation between the results from microtremor geophysical surveys and the thermal basin geological structure, including remote sensing data and thermal well information. More than 50 HVSR measurements from single stations were collected and analysed, in close correlation with the vast amount of available borehole logs, well exploitation data and SAR interferometry measurements collected for the monitoring of land subsidence induced by thermal water pumping. The aim of the work was to evaluate the suitability of the microtremor technique for the preliminary characterization of a thermal basin, in terms of resonance properties of the alluvial deposits covering the rocky bedrock, evaluated by means of HVSR methods. HVSR results were interpreted in terms of regional geological setting and hydrogeological properties of the subsoil. Our field results confirmed the existence of significant correlations between the resonance behaviour of the basin determined by the HVSR technique and the geological structures, suggesting that single station measurements are a promising tool for creating a rapid and preliminary non-invasive contribution to the characterization of the geothermal basin.     

Investigating the impact of surface wave breaking on modeling the trajectories of drifters in the northern Adriatic Sea during a wind-storm event

An accurate numerical prediction of the oceanic upper layer velocity is a demanding requirement for many applications at sea and is a function of several near-surface processes that need to be incorporated in a numerical model. Among them, we assess the effects of vertical resolution, different vertical mixing parameterization (the so-called Generic Length Scale –GLS– set of k–ε, k–ω, gen, and the Mellor–Yamada), and surface roughness values on turbulent kinetic energy (k) injection from breaking waves.First, we modified the GLS turbulence closure formulation in the Regional Ocean Modeling System (ROMS) to incorporate the surface flux of turbulent kinetic energy due to wave breaking. Then, we applied the model to idealized test cases, exploring the sensitivity to the above mentioned factors. Last, the model was applied to a realistic situation in the Adriatic Sea driven by numerical meteorological forcings and river discharges. In this case, numerical drifters were released during an intense episode of Bora winds that occurred in mid-February 2003, and their trajectories compared to the displacement of satellite-tracked drifters deployed during the ADRIA02-03 sea-truth campaign.Results indicted that the inclusion of the wave breaking process helps improve the accuracy of the numerical simulations, subject to an increase in the typical value of the surface roughness z₀. Specifically, the best performance was obtained using α_CH = 56,000 in the Charnok formula, the wave breaking parameterization activated, k–ε as the turbulence closure model. With these options, the relative error with respect to the average distance of the drifter was about 25% (5.5 km/day). The most sensitive factors in the model were found to be the value of α_CH enhanced with respect to a standard value, followed by the adoption of wave breaking parameterization and the particular turbulence closure model selected.     

Probability hazard map for future vent opening at the Campi Flegrei caldera,Italy

The Campi Flegrei caldera is a restless structure affected by general subsidence and ongoing resurgence of its central part. The persistent activity of the system and the explosive character of the volcanism lead to a very high volcanic hazard that, combined with intense urbanization, corresponds to a very high volcanic risk. One of the largest sources of uncertainty in volcanic hazard/risk assessment for Campi Flegrei is the spatial location of the future volcanic activity. This paper presents and discusses a long-term probability hazard map for vent opening in case of renewal of volcanism at the Campi Flegrei caldera, which shows the spatial conditional probability for the next vent opening, given that an eruption occurs. The map has been constructed by building a Bayesian inference scheme merging prior information and past data. The method allows both aleatory and epistemic uncertainties to be evaluated. The probability map of vent opening shows that two areas of relatively high probability are present within the active portion of the caldera, with a probability approximately doubled with respect to the rest of the caldera. The map has an immediate use in evaluating the areas of the caldera prone to the highest volcanic hazard. Furthermore, it represents an important ingredient in addressing the more general problem of quantitative volcanic hazards assessment at the Campi Flegrei caldera.     

Active hydrothermal fluids circulation triggering small-scale collapse events: the case of the 2001–2002 fissure in the Lakki Plain (Nisyros Island,Aegean Sea,Greece)

Rip currents are fast moving, offshore flows that have the ability to move even the strongest swimmers into deeper waters. Miami Beach, Florida is one of the most visited beaches in the USA and a sought after destination for citizens and international tourists alike. It is also known to be a rip current “hot spot.” These factors greatly increase the risk of drowning; however, no previous research has focused on beachgoer perception of rip-related risks in South Florida. Over a 12-month period, 203 public surveys were collected to determine the rip current knowledge of beachgoers at Miami Beach based on factors such as swimming ability and frequency of beach visits. The responses were analyzed by constructing a normalized component factor to determine the respondent’s comprehensive knowledge of rips, and multiple regression models were used to assess the net influences of sociodemographic and behavioral characteristics on the responses. A significant proportion of the survey respondents showed insufficient knowledge, indicating they are at risk of drowning in a rip current. Frequent beachgoer’s exposure to the beach environment, maturation, and nativity is identified as the main contributors to knowledge net of other sociodemographic compositions. The most at-risk groups were determined to be young adults, foreign tourists, poor swimmers, and those who infrequently visited the beach. Miami Beach needs to initiate a rip current safety campaign to target these at-risk beachgoers, where interventions beyond familial and educational institutions should be introduced.