Davorka Herak

Body-wave velocities in the circum-Adriatic region

Abstract The paper presents body-wave velocities in the crust and upper mantle of the circum-Adriatic region obtained by analyzing travel times of regional phases of 419 selected earthquakes with epicenters in the central part of the External Dinarides. The studied region was divided into five areas and two kinds of velocity models were determined for each of them. The first kind ( inter-area models ) gives average velocities that characterize the zone between and including the epicentral region and the area in question, while the second kind ( intra-area models ) presents velocities within the respective area. The locations of hypocenters considerably improved when the new models were used instead of the standard one for the Balkan region. The large-scale variation of average velocities obtained by inter-area modeling was found to be relatively small both in the crust and in the upper mantle. On the other hand, the magnitude and distribution of station corrections and the resulting intra-area velocities point to relatively pronounced lateral velocity variation within some of the studied regions. The obtained velocities in most cases do not differ significantly when compared to the values reported in other studies. The most important exceptions occur in the epicentral area (central part of the External Dinarides) where the P-wave velocity in the upper crust is much higher then elsewhere, while the velocity in the upper mantle is found to be lower than that determined by DSS experiments. There is also an indication of pronounced variation of the Poisson ratio in this area.

Relationship of MLSM and magnitudes determined by regional seismological stations in South-Eastern and Central Europe

Abstract We show how by comparing M L SM (computed from strong motion accelerograms within 150 km from the epicenter) and M STA (reported by different seismological stations in Yugoslavia, Italy, Albania, Greece, Austria and Czechoslovakia) it is possible to calibrate amplitude-distance attentuation equations used in M STA definitions. This, more accurate, magnitude determination for regional networks is essential for: (1) better and more accurate integration of the engineering estimates of siesmic risk; and (2) for more accurate and quick estimation of strong motion amplitudes in epicentral region of destructive earthquakes.

Attenuation of coda waves in the contact zone between the Dinarides and the Adriatic Microplate

The attenuation of coda waves is analysed for nine seismic stations in the area of convergent motion of the Adriatic microplate and the Dinarides. The frequency dependent coda quality factor of the form Qc = Q0 fn is estimated for up to seven central frequencies (1.5, 3, 6, 9, 12, 18 and 24 Hz) and for 21 successive 30 s long time windows. Q0 was found to increase from 68–353 for short lapse times of 20–50 s, to 158–373 for lapse times of 90–100 s. Parameter n is observed to vary between 0.46 and 0.89, with a pronounced tendency to decrease with increasing Q0, and vice versa. Both Q0 and n seem to stabilize for lapse times larger than 50–80 s, indicating that a change in rock properties controlling coda attenuation occurs at depths of about 100–160 km. The spatial distribution of observed Q0 is well correlated with observed seismicity and inferred tectonic activity. In particular, we observe significant negative correlation of Q0 with the peak ground acceleration (PGA) estimate for the return period of 475 years. The degree of frequency dependence n, is the smallest for stations on the islands, where the crust is the thinnest. The largest n is observed over the thickest crust in the region, where the Moho lies at depths of over 55 km.

Numerical Modeling of the Ston-Slano (Croatia) Aftershock Sequence

The rate of aftershock occurrence after the M6 Ston-Slano (Croatia) earthquake is modeled as the Epidemic Type Aftershock Sequence (ETAS). Increase of the modeled cumulative number of aftershocks with time was fitted to observations by the least-squares criterion using the combined grid-search and Monte-Carlo approach. This enabled not only the estimation of the most probable ETAS parameters, but also the determination of their confidence limits, as well as the estimation of the bias between them. It has been found that the bias is significant for some of the parameter pairs, regardless of the threshold magnitude assumed. Residual analyses revealed that all strong aftershocks (ML≥ 4.5) occurred during the periods of normal to high aftershock activity. There were two periods of quiescence in the sequence, both of which were followed by a strong aftershock.

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.

Recent Efforts Toward the Establishment of the Lonjsko Polje Geomagnetic Observatory

More than ten years ago, the Geophysical Institute initiated the installation of a geomagnetic observatory in Croatia. Over the past decade, extensive surveys and studies have been conducted in order to determine the proper location for the observatory. Finally, in 2012, the observatory was established in Lonjsko Polje. This paper presents the first data recorded in the period 2012.5-2015.0. Also presented are the technical aspects and data processing techniques of this remotely operated observatory. Analysis of data quality and comparison with data from the surrounding INTERMAGNET observatories is discussed in detail. Although remote observatories cannot provide the ideal environment for magnetometers, the obtained results accentuate the potential of the new observatory to provide high-quality data. The establishment of this observatory paves the way for scientific and professional development of geomagnetism in Croatia.

Recent Measurements of Ambient Vibrations in Free-Field and in Buildings in Croatia

Recent measurements of ambient vibrations in Croatia include free-field measurements in Zagreb and Ston, as well as those done within buildings. Free-field data at both localities are consistent with the properties of shallow geological structures there. In Zagreb, horizontal-to-vertical spectral ratios (HVSR) of microtremors point to existence of thick alluvial cover (over 100 m) that gradually gets thinner as it reaches the southern slopes of the Mt. Medvednica. In Ston HVSR profiles reveal several tens of meters thick sedimentary cover over the bedrock which gets exposed at the Stovis hill. Over 100 measurements of ambient vibrations in buildings that have recently been performed across Croatia, were analysed by a suite of Matlab routines yielding frequencies and damping for the fundamental and higher modes of oscillation. Applications to real buildings proved that analyses of shaking induced by ambient vibrations in most cases lead to well constrained, reliable, and time independent estimates of frequencies and damping of the building vibrational modes. The measurements done so far form the initial nucleus of the Croatian building inventory, a collection of observed fundamental periods, damping, and spectral shapes, which can prove important in documenting building structural integrity and in assessing the degree of possible damage in future earthquakes.

Consistency of magnitudes of the largest earthquakes of the first half of the 20th century determined on the basis of Gottingen and Zagreb Wiechert seismograms

The magnitudes (MS, mbP, mbS) of the largest historical earthquakes which occurred in the first half of the 20thcentury, calculated on the basis of records of Wiechert horizontal seismographs in Göttingen (Germany) and Zagreb (Croatia), are compared with one another, as well as with the magnitudes reported in worldwide catalogues. Systematic trends are observed in the data regarding the temporal stability of magnitude estimations in Göttingen, as well as the apparent non-linearity of the instrument responsle in the case of the Wiechert seismograph in Zagreb. We were unable to clearly identify their causes – possible explanations include effects caused by the interaction of the seismometers frame and mass, as well as local soil conditions, but nonhomogeneity of the reference catalogues cannot be ruled out. The results indicate that a careful re-examination and cross-checking of the reported magnitude figures for the earthquakes from the first half of the 20th century is required.

Anomalous seismicity of the Knin area (Yugoslavia) prior to the M= 5.5 earthquake of 1986

Abstract A case of seismicity changes prior to the M= 5.5 Knin (Yugoslavia) earthquake has been analysed. In addition to the well-expressed seismic quiescence, significant b-value, strain release and seismicity rate temporal changes have been observed. The estimated precursor time of T = 6.8 yrs is longer than expected on the basis of published log(T)— M relationships. A much better agreement is obtained if log(T) is compared to the longest dimension of the aftershock

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.