In this work, global navigation satellite system (GNSS) observations from the northern tip of the Adria microplate are analysed to differentiate non-periodic (transient) tectonic signals from other ...deviations from the linear trends primarily due to hydrological loading effects. We tested a recently proposed hypothesis that a porosity wave generated by fault-valve mechanisms in a seismogenic fault in the Bovec basin (western Slovenia) propagated throughout the surrounding region. After excluding potential spatially correlated common-mode errors in the considered time series, we investigated the relationship between the GNSS observations and periodic hydrological loading variations. The tests demonstrated that subtracting the hydrological term was effective at the global scale and that the frequency band of the transient signal (1.5 <
T
< 3.5 years) was not correlated with hydrological effects at the local scale (within a few kilometres of the station). Next, the results of previous works are used to calculate the permeability values and pore-pressure state at the source of the transient signal. The permeability values for the four main rock formations in the region are consistent with independent observations for similar lithotypes. The ratio between the effective stress and lithostatic load for different vertical profiles in the Bovec area indicated a state of overpressure, with pore-pressure close to the value of the lithostatic load. Thus, our results help define a scenario in which the porosity wave could have originated. Indeed, the formation of the domains of interconnected fractures, such as during the formation of a porosity wave, increases the permeability values, thereby relieving overpressure and restoring a state of equilibrium.
Ultralow frequency (ULF) to low frequency (LF) electromagnetic radiation represents one of the most promising effects of brittle rock strain and microcracking that might be potentially helpful for ...short term earthquake forecasting. In this study the results of a six month monitoring campaign are presented from Obir Cave in the eastern Alps. Direct experimental observations of electromagnetic radiation have been made using a customised broadband data logger installed next to the Obir Fault - this seismogenic fault near the Periadriatic Lineament is known to be related to at least three large prehistoric earthquakes. On the basis of these measurements it has been possible to characterise a number of distinct signals: artificial constant narrowband signals at discrete frequencies; short serial broadband impulses; high energy broadband impulses; and low energy broadband impulses. The narrowband artificial signals were removed from the electromagnetic radiation time series analysis so that the natural signals were enhanced and could be compared more easily to meteorological parameters and rock strain indicators. Critically, the high energy broadband impulses show a strong correlation with lightning activity across much of central Europe and the eastern Mediterranean while the low energy broadband impulses appear to be associated with local rock strain in and around Obir Cave. Unfortunately, it seems certain that some of the essential strain related impulses are likely to have been overprinted by the larger lightning related impulses.
•ULF to LF electromagnetic radiation was monitored at a seismogenic fault•A broadband data logger was installed in a cave to reduce the environmental noise•A number of distinct signals were identified in the spectrum•The low energy broadband impulses show a clear association with local rock strain•Unfortunately, they were overprinted by the larger lightning-related impulses
Due to the development of denser permanent Global Navigation Satellite System (GNSS) networks over the last decade, the observation of transient deformations has significantly increased, mainly in ...high strain-rate zones. We analyzed the data from a group of permanent GNSS sites on the N-Adria microplate, where anomalous southward tilting and low-frequency tremors preceded the 1976 Mw=6.4 earthquake. We present records from different stations of a transient signal with an approximately 2-year period that propagated through the northern edge of Adria, in a region 150km wide. This represents the first time a transient deformation event has been observed in a continental collision area. We exclude surface and groundwater hydrological load effects because we corrected the data for such effects at seasonal, annual, and multiyear scales. The movement is initially upward, except in one location, with slight tilting parallel to the direction of the main tectonic structures. Later, the opposite behavior is observed.
The novel methods used include earthquake location techniques and tomographic inversion of the arrival times. These methods demonstrated that the transient source was located spatially and temporally close to the 2004 Mw=5.2 event in Bovec (Slovenia), attributed to the activity of the Ravne fault. We interpret the transient rises as the expression of a porosity wave, possibly produced by fault valve behavior of the NW tip of the Ravne fault. The propagation velocity is consistent with this hypothesis. As a further test, we invert the arrival times of the transient through hydraulic tomography to obtain hydraulic diffusivity: the values are compatible with the lithotypes present in the region and the literature. By substituting the tomographic velocity and diffusivity in the solitary/porosity wave equation, we infer an initial effective stress of approximately 0.23bar, sufficient to alter the equilibrium of some fault segments and influence the subsequent seismicity.
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•We detected a transient GPS signal in N-Adria, after removing hydrological effects•The transient is compatible with a porosity wave propagating through the region•Through tomography, we locate its source on the NW-continuation of Ravne fault•It originated about 3.5months before the Bovec (Slovenia) 2004 Mw=5.2 event•The causal ~0.23bar effective stress is due to a valve behavior of Ravne fault
Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, ...fixed‐bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two‐dimensional structure observed in the laboratory, is highly three‐dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear‐layer dynamics. In each run, detailed three‐dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing‐layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re‐examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.
We test a recently developed ShakeMap model for the Friuli Venezia Giulia (NE Italy) region (Moratto et al., 2009) by comparing the macroseismic observations, related to three past earthquakes ...(Cansiglio 1936, Friuli 1976 and Bovec-Krn 1998) which occurred in this region, with the instrumental intensities obtained from the ShakeMaps, and based on synthetic seismograms computed from available source models of these events. Because a comparison requires that the simulation resembles as close as possible the real situation, ShakeMaps are computed from synthetics calculated at receivers placed at the sites where the recording instruments are already operative and at the sites where the instruments are going to be installed in the near future. Since the available instrumental recordings are very few, the resulting instrumental intensity maps are compared with the macroseismic observations wherever the latter are available. We make use of various relationships between the ground motion parameters and the macroseismic intensity during the comparison. The best results are obtained when the macroseismic intensities (I
≤
VII) are estimated using the relationship proposed by Faenza and Michelini (2010).
► In this study we test the ShakeMap model for the South-Eastern Alps area. ► We compute synthetic seismograms for three earthquakes occurred in the area (Cansiglio 1936; Friuli 1976; Bovec 1998). ► PGM values are extracted and ShakeMaps are generated for PGA, PGV and instrumental intensity. ► The comparisons between instrumental intensity ShakeMaps and macroseismic observations show good agreement.
► The reliability of magnetic ratio changes as earthquake precursors are investigated. ► Nine cases are considered which include 17 earthquakes. ► In the analysis running averages of the Σ
K
p ...geomagnetic index are taken into account. ► These increases are actually caused by the normal geomagnetic activity variation.
Following the paper by
Fraser-Smith et al. (1990), many scientists have focused their research on the ULF geomagnetic field pulsations in the hope of finding possible anomalous signals caused by the seismic activity. Thereafter, many papers have reported ULF geomagnetic field polarization ratio increases which have been claimed to be related to the occurrence of moderate and strong earthquakes. Even if there is no firm evidence of correlation between the polarization ratio increase and seismic events, these publications maintain that these “anomalous” increases are without doubt precursors of pending earthquakes. Furthermore, several researchers suggest that these seismogenic signals may be considered a promising approach towards the possibility of developing short-term earthquake prediction capabilities based on electromagnetic precursory signatures. On the contrary, a part of the scientific community emphasizes the lack of validation of claimed seismogenic anomalies and doubt their association with the seismic activity. Since earthquake prediction is a very important topic of social importance, the authenticity of earthquake precursors needs to be carefully checked. The aim of this paper is to investigate the reliability of the ULF magnetic polarization ratio changes as an earthquakes’ precursor. Several polarization ratio increases of the geomagnetic field, which previous researchers have claimed to have a seismogenic origin, are put into question by a qualitative investigation. The analysis takes into account both the temporal evolution of the geomagnetic field polarization ratio reported in previous papers, and the global geomagnetic activity behaviour. Running averages of the geomagnetic index
K
p are plotted onto the original figures from previous publications. Moreover, further quantitative analyses are also reported. Here, nine cases are investigated which include 17 earthquakes. In seven cases it is shown that the suggested association between the geomagnetic field polarization ratio increases and the earthquake preparation process seems to be rather doubtful. More precisely, the claimed seismogenic polarization ratio increases are actually closely related to decreases in the geomagnetic activity level. Furthermore, the last two investigated cases seem to be doubtful as well, although a close correspondence between polarization ratio and geomagnetic activity cannot be unambiguously demonstrated.
In the frame of national research and innovation project MASPREM exposure maps of inhabitants, buildings and infrastructures to landslide occurrence were developed for five selected Slovenian ...municipalities. Maps represent an upgrade of the landslide susceptibility maps that were elaborated based on synthesis of analysis of event-based landslide inventory and field investigations. Exposure maps were developed for five municipalities: Bovec, La{ko, Slovenj Gradec, Trbovlje and Železniki. Exposure of inhabitants, construction and infrastructures to landslide occurrence was analysed using simple cross-analysis of landslide susceptibility maps at scale of 1:25,000 with locations of exposed elements. All analyses were conducted in the GIS with software tool ArcMap. Exposure maps, based on landslide susceptibility, were classified into six classes, with values ranging from one to six where class one represents areas with negligible exposure and class six areas with very high exposure to landslide occurrence. Exposure maps of selected municipalities provide the basis for further assessment of risk and consequentially better risk management.
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