Empirijski kriterij za određivanje točnosti lokacije epicentra potresa na području Hrvatske

This paper presents the empirically based ground truth criteria, or shorter GT criteria, for the estimation of the epicentral location accuracy of the seismic events recorded at network stations within 400 km around the city of Zagreb. The criteria are based only on the network coverage metrics and the GT5 level represents an absolute location error lower than 5 km. They have been developed using a bootstrap resampling method: same earthquakes have been relocated many times but with different, randomly selected seismic stations. We used 330 reference events taken from the pages of ISC (ISC Reference Event Bulletin, 2008) and showed that the location accuracy is most affected by the distance to the farthest station in the seismic network, while not at all influenced by the distance to the nearest. The developed GT criteria for GT5_95% level of accuracy require 10 or more network stations, all within 125 km from the epicentre, and the secondary azimuthal gap (the largest gap when any given station is removed from the network) less than 200°, or the network quality metric (the deviation between the optimal uniformly distributed network and the actual network) less than 0.41. The obtained results revealed that the global criteria are too restrictive and unsuitable for the studied area since they require more regular networks. With our criteria, it is possible to achieve higher accuracy for the networks with a bigger secondary azimuthal gap or greater network quality metric. In addition, our criteria limitations are shown for the areas with simpler geological structure.U ovom radu određen je empirijski GT kriterij (engl. ground truth) za područje definirano polumjerom od 400 km oko grada Zagreba. Kriterij koristi mrežnu pokrivenost kao mjeru točnosti lokacije epicentra, a GT5 razina predstavlja apsolutnu pogrešku lokacije manju od 5 km. Primijenjena je metoda ponovnog uzorkovanja, gdje su Monte-Carlo postupkom velik broj puta relocirani isti potresi, ali s različitim slučajno odabranim seizmološkim postajama. Korišteno je 330 referentnih događaja preuzetih sa stranica ISC-a (ISC Reference Event Bulletin, 2008). Pokazali smo da na točnost lokacije najviše utječe udaljenost do najudaljenije postaje mreže, dok uopće ne utječe udaljenost do najbliže. Određeni GT kriterij proučavanog područja za postizanje GT5_95% razine točnosti zahtjeva da se mreža seizmoloških postaja mora nalaziti unutar 125 km od epicentra i da sekundarna azimutalna razlika (tj. najveća razlika između azimuta dviju postaja koja se dobiva uklanjanjem bilo koje postaje iz mreže) mora biti manja od 200° ili mjera kvalitete mreže (tj. srednje apsolutno odstupanje između optimalne uniformno distribuirane mreže postaja i stvarne mreže) manja od 0,41. Dobiveni rezultati pokazuju da su globalni kriteriji previše restriktivni za naše područje budući da zahtijevaju pravilnije mreže. Našim kriterijem je moguće postići višu točnost za mreže s većom sekundarnom azimutalnom razlikom i većom mjerom kvalitete mreže. Također, pokazali smo ograničenost našeg kriterija za područja jednostavnije građe

Velocities of refracted longitudinal Pn waves in the Dinarides area

The refracted longitudinal wave velocities in the Dinarides area have been determined on the basis of the modified time-term method for the dipping Moho discontinuity. The homogeneity of the Earth crust is assumed. The results (ranging between 7.85 and 8.45 km s-1) divide the research area into three parts and confirm the complexity of the Dinarides region. The arithmetic mean of the Pn wave velocities (calculated for the entire Dinarides area) is 8.10 km s-1. This value is comparable to the average velocity of 8.14 km s-1 obtained for the Dinarides region by deep seismic sounding

Seizmičnost Hrvatske u 1989.godini i potres od 6. prosinca u blizini planine Kamešnice

The seismicity of Croatia and its surrounding areas in 1989 was analysed on the basis of the earthquake catalogue consisting of 361 earthquakes. Its completeness threshold was estimated to be MLC ≥ 3.0. Seismically the most active was the coastal part of Croatia, where the strongest earthquake in 1989 occurred on December 6 with the focus beneath the hill-sides of the Kamešnica Mt. The fault plane solution for this event indicates the presence of a tectonic stress-field directed approximately SW-NE, which is compatible with the assumed anticlockwise rotation on the Adriatic microplate around the pole in Northern Italy, and the associated subduction of the Adriatic plate under the Dinarides. The aftershocks of the Kamešnica Mt. event were numerous, with hypocenters at depth up to 20 km. Macroseismic investigations confirm the frequently observed fact that seismic energy is much more efficiently absorbed perpendicularly to the direction of the Dinaric belt than along it

Fault plane solutions for earthquakes (1956-1995) in Croatia and neighboring regions

The study presents (re)evaluation of fault-plane solutions for 40 earthquakes that occurred in the period 1956-1995 in Croatia and neighboring regions. All events were analyzed in the same manner, using the best available velocity models and the most recently updated earthquake catalogue for that region. For the most important earthquakes our solution is briefly discussed and compared to the results of previous studies when they were available. The results are presented with all related data thus enabling estimation of each solution’s reliability

Neka obilježja seizmičnosti u Hrvatskoj i susjednim područjima u 1988. godini

Based on the catalogue of all located earthquakes in Croatia and the surroundings areas in 1988, the map of epicenters (Figure 1), and on the macroseismic data analysis for individual earthquakes, certain seismic features have been analyzed. During 1988, 588 earthquakes with 6 or more onset time data were located and the catalogue was found to be complete for the magnitudes ≥ 3.2. Seismically most active were the coastal part of Croatia (the Jablanac-Velebit area and the valley of the river Neretva with Metković) and the Adriatic submarine area near the island Palagruža. The Appendix includes the main parameters for the 42 earthquakes with a magnitude ML ≥ 3.5. All hypocenters have been located by means of the HYPOSEARCH location program (Herak, 1989a)

Seizmičnost Hrvatske u razdoblju 1997–2001

During the 1997 - 2001 period seismic activity of Croatia was confined to the previously identified seismically active areas. All together 1925 earthquakes were located. Seismically the most active was the coastal part of Croatia, especially its southernmost part where the Ston-Slano epicentral area exhibited the continuation of the great earthquake sequence after the September 5, 1996 main shock. The strongest aftershock was recorded on April 26, 1997 at 07:30 (ML = 4.5, Imax = VI °MSK). The earthquake with the same magnitude ML = 4.5, recorded in the Zrmanja river valley, near Obrovac, on November 9, 2000 at 03:01 (Imax = VI °MSK). These two events were the strongest ones recorded in Croatia during the studied period.Tijekom razmatranog petogodišnjeg razdoblja seizmičnost je u Hrvatskoj bila ograničena na poznata epicentralna područja. Ukupno je locirano 1925 potresa. Najaktivniji je bio obalni dio Hrvatske, posebno njegov najjužniji dio, gdje je nastavljena pojačana aktivnost u epicentralnom području Ston-Slano nakon velikog potresa iz 1996. godine. Njegov najjači naknadni potres zabilježen je 26. travnja 1997. u 7:30 (ML = 4.5, Imax = VI °MSK). Potres jednake magnitude dogodio se i u dolini rijeke Zrmanje blizu Obrovca 9. studenog 2000. godine u 03:01 (Imax = VI °MSK). Ova su dva potresa ujedno i najjača u Hrvatskoj u razmatranom razdoblju

Seizmičnost Hrvatske u razdoblju 2002–2005

During the 2002–2005 period a total of 3459 earthquakes were located in Croatia and its neighbouring areas with 15 main events registering magnitudes from 4.0 to 5.5. Seismically the most prominent were the two strongest earthquake sequences recorded in the central part of the Adriatic Sea, near Jabuka Island (the first one with the mainshock on March 29, 2003, 17:42, ML = 5.5, and the second, weaker, with the mainshock on November 25, 2004, 6:21, ML = 5.2). In the epicentral area W and NW of the Jabuka Island 781 earthquakes were confidently located (28 events with magnitudes equal to or larger than 4.0). Seismically active coastal part of Croatia, especially its southern part exhibited the seismicity within well-known epicentral areas. The earthquake with the magnitude ML = 5.5, recorded in the Imotski–Grude area, on May 23, 2004 at 15:19 (Imax = VI–VII °MSK) was the second strongest event during the studied period. Continental part of Croatia experienced moderate seismicity during the observed period, with earthquakes of magnitudes ML ≤ 3.9.U Hrvatskoj i susjednim područjima locirano je 3459 potresa u razdoblju od 2002 do 2005 godine. Zabilježeno je 15 glavnih potresa s magnitudama od 4.0 do 5.5. Seizmički najaktivnije bilo je područje u blizini otoka Jabuka u sredi{njem djelu Jadranskog mora gdje su zabilježene dvije velike serije potresa (prva s glavnim potresom koji se dogodio 29. ožujka 2003. u 17:42, ML = 5.5, i druga slabija s glavnim potresom koji se dogodio 25. studenog 2004., u 6:21, ML = 5.2). U epicentralnom području zapadno i sjeverozapadno od otoka Jabuka lociran je 781 potres (28 potresa s magnitudama većim od 4.0). Seizmička aktivnost obalnog područja Hrvatske bila je ograničena na do sada poznata epicentralna područja. Potres koji se dogodio 23. svibnja 2004. u 15:19, u seizmičkom području Imotski–Grude, magnitude ML = 5.5 (Imax = VI–VII °MSK) bio je drugi najja~i potres zabilježen u promatranom razdoblju. U kontinentalnom djelu Hrvatske zabilježena je umjerena seizmičnost u promatranom razdoblju, s potresima M ≤ 3.9

Calibration of shaking intensity models of the “Extremum” system to simulate loss due to the 2020 Croatia earthquakes

This paper is devoted to applications of the “Extremum” loss simulation system to two damaging earthquakes which occurred in Croatia in 2020. We provide a calibration procedure of mathematical models used for shaking intensity simulation. The regional macroseismic field parameters, such as the coefficients in the macroseismic field equation; the ratio between the longer (b) and the shorter (a) axes of the higher elliptical isoseismals (the flattening ratio k); the angle that specifies the orientation of the macroseismic field, in particular, the azimuth of the longer axis in the isoseismal ellipse, were all based on extensive macroseismic data acquired for the Balkan region and on the data for an analogous area with similar seismotectonic parameters in the Caucasus. We obtained a fairly good consistency between the results of simulation applied to the impact of the 2020 Croatia earthquakes and observations, confirming that the calibration of the macroseismic model by the Extremum system was both reasonable and effective for enhancing reliability for real time loss estimation