Ivica Sović

Subtraction digital holography

Overlapping of the desired (first-order) and undesired (zero-order) terms originating from the recorded primary-fringe patterns in digital holography is a problem without a real-time solution. We propose a procedure for suppressing the zero-order disturbance that is realizable in real time. The procedure is based on the stochastic change of the speckles in the primary-fringe patterns and on the subtraction of two such subsequent patterns. The theoretical description of the procedure is given and experimental results presented.

Historical Seismicity of the Rijeka Region (Northwest External Dinarides, Croatia)—Part I: Earthquakes of 1750, 1838, and 1904 in the Bakar Epicentral Area

We present macroseismic analyses of three historical earthquakes that occurred in 1750, 1838, and 1904 in the Bakar epicentral area (Rijeka region, Croatia). Using various historical sources, we were able to compile intensity maps and assess macroseismic parameters for each of these events. The estimated epicenters lie close to known active fault systems, and the epicentral intensities are lower than those listed in catalogs. We have also identified a number of strong foreshocks and aftershocks which are currently missing from all catalogs. The three events analyzed, together with the Klana earthquake of 1870, are the largest known events to have occurred in these tectonically active parts of the northwest Dinarides. As such, they are important in characterizing seismic hazard in the vicinity of the main Croatian port, the city of Rijeka.

Sensitivity increase in digital holographic interferometry

When using the interferometric techniques, the optical path changes induce the wavefront deformations that in turn cause the appearance of fringes. One general characteristic of such an approach is the measurement sensitivity. The actual sensitivity of a holographic interferometer is a function of, on one hand, the parameters of the measuring system (such as the wavelength of the light used) and, on the other hand, the environmental conditions in which the measurements are being made. The later depend predominately on statistical fluctuations inherent to the particular laboratory conditions. In many applications the sensitivity is near the limits of the deformation detectability. In such cases, it is of vital interest to increase the number of interferometric fringes thus improving the quality of the acquired data. In this paper, we give an overview of the sensitivity increase through various experimental and numerical approaches. We also present a new numerical iterative method in which every cycle doubles the number of interferometric fringes. The method has shown to be especially useful in applications with sub-wavelength wavefront deformations.

Historical Seismicity of the Rijeka Region (Northwest External Dinarides, Croatia)—Part II: The Klana Earthquakes of 1870

The Klana earthquake sequence with the mainshock on 1 March 1870 (Imax = VIII MSK) is one of the most important events that occurred in the Rijeka (Croatia) epicentral region. It is remarkable not just because of its impact to the local community and its significance for the seismic hazard in the Rijeka area, but also because it provided some of important observations on earthquake phenomena during the infancy of seismology. In particular, D. Stur (1871) seems to have been the first to attempt to explain the observed variation in damage by interaction of different local geological units during shaking ; R. Hoernes (1878) heavily relied on Stur’s data from the Klana epicentral region when he proposed that the vast majority of global earthquakes is related to the mountain-building process, and thus named them tectonic earthquakes. In spite of this, the Klana earthquake so far has escaped any detailed consideration by seismologists. In this study we present the historical data sources consulted to extract useful reports on damage and other observed effects, compile macroseismic maps for the five strongest events, and invert the estimated intensity data to compute macroseismic hypocenters and epicentral intensities for the mainshock and the two strongest aftershocks. We also present an earthquake chronicle with data for 92 identified earthquakes in the area for the period January–August 1870. The mainshock’s macroseismic epicenter is located near the active Rasa fault–Klana-Novi Vinodolski fault system, which we thus propose to be the primary source of the sequence. Macroseismic epicenters of the aftershocks lie close to the surface trace of the nearby Skadanscina-Rijeka reverse fault.

The 1667 Dubrovnik earthquake–some new insights

The year 2017 marks the 350th anniversary of the great 6 April 1667 Dubrovnik earthquake that caused extensive damage in a wide area around this old Dalmatian town (today in Croatia). This article presents the effects of the 1667 earthquake and examines the first few weeks following the catastrophe. Macroseismic data are reanalysed, for the first time available data are collected of the damage on the territory of Bosnia and Herzegovina (the territory which was in the 17th century under the Ottoman reign) and a new map of macroseismic intensities is presented. This map is in good agreement with the macroseismic field modelled using the SAF (Strong Attenuation at Fault Zones) model. We highlight some problems in the collection of macroseismic information, which are mainly a consequence of the complex political situation in the areas affected by the earthquake. The 1667 earthquake heavily impacted Dubrovnik and the Dalmatian coast. This event is thought to be the biggest one in the history of Dalmatia and practically defines seismic hazard in the coastal area of Croatia. For this reason, the main goals in this article are the improvement of the epicenter location and the determination of the moment magnitude.

Undersampled digital holographic interferometry

In digital holography, primary holographic fringes are recorded using a matricial CCD sensor. Because of the low spatial resolution of currently available CCD arrays, the angle between the reference and object beams must be limited to a few degrees. Namely, due to the digitization involved, the Shannons criterion imposes that the Nyquist sampling frequency be at least twice the highest signal frequency. This means that, in the case of the recording of an interference fringe pattern by a CCD sensor, the inter-fringe distance must be larger than twice the pixel period. This in turn limits the angle between the object and the reference beams. If this angle, in a practical holographic interferometry measuring setup, cannot be limited to the required value, aliasing will occur in the reconstructed image. In this work, we demonstrate that the low spatial frequency metrology data could nevertheless be efficiently extracted by careful choice of twofold, and even threefold, undersampling of the object field. By combining the time-averaged recording with subtraction digital holography method, we present results for a loudspeaker membrane interferometric study obtained under strong aliasing conditions. High-contrast fringes, as a consequence of the vibration modes of the membrane, are obtained.