Macroseismic intensity attenuation modelling

Izoseiste potresa koji su se dogodili u središnjem dijelu Vanjskih Dinarida izdužene su u smjeru pružanja ovog planinskog lanca. Odstupanje od kružnog oblika izoseista je uobičajeno i u drugim područjima, no u slučaju dinaridskih potresa omjer duljina i širina izoseista iznosi i do 4:1. Tako velika razlika duljine i širine izoseista ukazuje ili na veliki prirast intenziteta u smjeru pružanja ili na snažno prigušenje normalno na pravac pružanja orogenskog pojasa. S obzirom da su Dinaridi krško područje gdje se tradicionalno gradilo na stijenovitoj podlozi, anizotropija makroseizmičkog polja vjerojatno nije posljedica prirasta intenziteta zbog uvjeta tla jer skoro sve točke intenziteta imaju u pravilu vrlo sličnu dolomitnu ili vapnenačku podlogu. Osim toga, u radu je pokazano da potresi s različitim mehanizmima pomaka u žarištu, i oni koji su se dogodili na rasjedima uzdužnim na dinaridskipravac pružanja kao i oni koji su nastali na poprečnim rasjedima, imaju sličan oblik izoseista što isključuje emisiju izvora kao važan faktor u oblikovanju makroseizmičkog polja. U ovom je radu, stoga, pretpostavljeno da rasjedne zone koje se pružaju uzdužno Dinaridima, jače prigušuju valove koji kroz njih rasprostiru te tako umanjuju intenzitet što ima za posljedicu karakteristično spljoštene izoseiste. Anizotropija makroseizmičkog intenziteta je do sada uglavnom razmatrana kao intrinzično svojstvo valnog sredstva šireg područja uzrokovano usmjeravanjem kristala i pukotina pod djelovanjem tektonskih sila (Bottari i sur., 1984; Ko i Jung, 2015). U ovom se radu pristupilo anizotropiji makroseizmičkog intenziteta kao posljedici prigušenja i preraspodjele energije valova na uskim, širine usporedive s valnom duljinom valova koje prigušuju, rasjednim zonama. Rasjedi su sekundarne geološke strukture (Davis i sur., 2011), pukotine u stijenama Zemljine kore po kojima je došlo do pomaka jedne strane u odnosu na drugu stranu pukotine, odnosno, jednog krila rasjeda u odnosu na drugo. Definicija rasjedne zone se razlikuje kod raznih autora, no sve one opisuju zonu koja ima ista fizikalna svojstva za rasprostiranje valova. Davis i suradnici definiraju rasjednu zonu kao mnoštvo blizih rasjednih ploha odvojenih razlomljenim stijenama, odnosno, kao skup subparalelnih rasjeda (Davis i sur, 2011). Billi i suradnici pak definiraju rasjednu zonu kao područje rasjedne jezgre koja obuhvaća glavni rasjed te prijelazne i razlomljene zone na oba krila rasjeda, pri čemu se jezgra razlikuje od razlomljene zone po pojavi kataklazita karakterističnog za jezgru (Billi i sur., 2003). Pukotine u stijenama se nikad ne pojavljuju same već u skupinama (Pollard i Aydin, 1988), a njihova međusobna udaljenost ovisi o fizikalnim svojstvima stijene. U sedimentima, od kojih su građeni i Dinaridi, udaljenost između pukotina je proporcionalna debljini slojeva te drugom korijenu Youngovog modula elastičnosti i modula smicanja (Lachenbacher, 1961; Hobbs, 1967; Ladeira i Price, 1981). Ponekad nekoliko paralelnih rasjednih zona mogu doseći širinu i od nekoliko kilometara. U krškim predjelima su rasjedne zone vrlo često saturirane vodom što efikasnije prigušuje valove potresa zbog čega takve strukture efikasnije disipiraju energiju valova. Osnovna pretpostavka modela prikazanog u ovom radu je da pri nailasku elastičkog vala izazvanog potresom na rasjednu zonu od razlomljenog, nekonsolidiranog materijala koji može biti saturiran podzemnom vodom, makroseizmički intenzitetbitno jače atenuira nego pri prolasku elastičkog vala kroz kompaktnu stijenu. U okviru ovog rada je napravljen je numerički model makroseizmičkog polja za središnji dio Vanjskih Dinarida. Modelirana je vrijednost makroseizmičkih intenziteta u čvorovima merže gustoće 0.1x0.1o. Model radi tako da umanjuje intenzitete izotropnog polja izračunatog Koevesligethyevom atenuacijska jednadžbom ako zraka vala koji putuje iz izvora do čvora mreže prolazi kroz rasjednu zonu. Intenzitet se umanjuje tako da se u trećem članu Koevesligethyeve jednadžbe hipocentralnoj udaljenosti doda umnožak broja prolazaka zrake vala kroz rasjedne zone, srednje širine rasjedne zone i omjera koeficijenata apsorpcije unutar i izvan rasjedne zone. Model je validiran tako da su za testni set od 10 potresa izračunata makroseizmička polja modelom opisanim u ovom radu i izotropnim modelom. Usporedbu modela nije bilo moguće provest analizom varijance zbog svostva makroseizmičke ljestvice da se ponaša kao grubi trobitni analogno-digitalni pretvornik (Sović i Šariri, 2016). Zbog toga su uspoređivane modelirane i empirijske izoseiste novom metodo usporedbe pomoću afinih invarijanti slike. Korišteno je šest afinih invarijanti momenata slike a mjerilo sličnosti je bila euklidska udaljenost u 6D prostoru invarijanti.In this work a simple model of anisotropic macroseismic field (SAF model) is presented. It is known that macroseismic field could be modulated by the source mechanism, regional and local geological structures. The main assumption of the SAF model is that basically isotropic field is modulated only by regional geology becouse influence of source mechanism and local geology can be neglected under some conditions. Influence of the rupture size and orientation could be neglected for small and moderate earthquakes because rupture is long only several km, comparable with the earthquake depth. The wave front become spherical in distance larger than 1.5 length of rupture according to Gusev (1983). Local geological structures, like water saturated stratified media, may increase intensity level by multiple reflections, constructive interference and resonant effects, but inelastic attenuation, significantly stronger in water saturated soils, as well as destructive interference, may decrease intensities. Large geological structures like fault zones decrease intensities due to energy redistribution and inelastic attenuation. This model has been developed for the Karst region of the Outer Dinarides where site effects may be neglected because most of a buildings are founded on a rock. Neglecting of site effects simplifies the model, so the only one modulator of macroseismic field remaind fault systems. According to this asumprion, the shape of macroseismic field depends on spatial distribution of faults and parameters of the earthquake (coordinates of the epicenter, depth and intensity at the epicenter). Contrary to the methods which treat anisotropy as an azimuthal property of elastic medium, it have been supposed that attenuation is uniform and isotropic except at a limited number of narrow fault zones. Intensity is diminished gradually each time a ray crosses the fault zone in the first five km of depth. After several crosses, the value of the attenuation function is numerically equal to the value of attenuation function for much larger epicentral distance when related to ray paths that do not cross fault zones (Figure I: Geological cross section, ray path and graphs of attenuation function modulated by fault zones versus unmodulated function). Enhanced attenuation of fault zones is introduced in Koevesligethy's attenuation function as an "additional distance" mΔr in the third term (αi is intrinsic α and m is the number of fault zones crossed by the S-waves): [inserted formula]. Number of crosses m has been calculated using map of the faults, so the additional distances have introduced spatial distribution of the faults in attenuation function. In order to demonstrate how the model works macroseismic field has been calculated for set of earthquakes which occurred in the central part of Outer Dinarides. Modelled macroseismic field and empirical isoseismals for 25th of November 1986 M=5.5 earthquake are shown in Figure II [Model (shaded areas and Arabic numerals) and empirical (dashed lines and Roman numerals) isoseismals 25th of November 1986 M=5.5 earthquake with epicenter near Knin. The results for all grid points are shown, although the macroseismic field for the sea area does not exist. Coast line and islands are not displayed.] as an example of the results. Grid of the model has been 0.1x0.1 deg. Computed values have been presented in figures as grey scale areas. On the first sight computed macroseismic field is quite similar to empirical but visual method is not good for fields comparation. On the other hand, standard statistical tests like T and F test are not sensitive enaugh for such rough data as intensities. In order to demonstrate how good the model is, the new method of comparation isoseismal maps was introduced. This method compares two isoseismal maps by using affine transformations of the isoseismals. Ten isoseismal maps obtained by the SAF model and by isotropic circular model were compared with empirical ones. It was find out that the SAF isoseismals are 31.4% more similar to the empirical ones than the isoseismals obtained by isotropic circular model. The model does not take into account site effects and source mechanisms, so it is limited to the small and moderate earthquakes and Karst or similar areas with uniform local geology. In spite of those deficiencies, the model is simple, easy to use and gives satisfactory results. It could by useful first step in developing more complex model which will include site effects and source parameters

Interferometer as seismometer displacement transduce

Displacements of the pendulum of S-13 Teledyne Geotech seismometer were measured using a small Michelson interferometer. The oscillations of the pendulum were simultaneously recorded by the PDAS-100 digital seismograph. These records were integrated and compared with those made by the interferometer. The pendulum was simulated by sine (0.5, 1 and 2 Hz) and random signals. Michelson interferometer was simplified by using the cube beam splitter with mirror film deposited onto one of its faces. The source of radiation was thermally stabilised collimated laser diode (LD) driven with constant current. Wavelength of LD was 670 nm. Fringes were counted with 16-bit presentable up/down CMOS counter. Counter state was sampled by Atari ST 1040 computer with sampling frequency equal to 33.4 Hz. Resolution of the transducer was 335 nm. The transducer would be able to record oscillations of the pendulum caused by an earthquake with magnitudes between 1.1 and 5.2 at epicentral distance of 0.1°, and could therefore find the application in a displacement strong-motion seismograph

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

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

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 1993-1996. i potres u području Stona i Slanog iz 1996. godine

Seismic activity in Croatia and surrounding areas in the 1993-1996 period was mostly confined to the previously identified seismically active areas. Nine events (excluding aftershocks) with the magnitude equal to or exceeding 4.5 occurred during that time. The most important earthquake sequence is the one that started on September 5, 1996 (Ml=6.0, Imax=VIII MSK) in the Ston-Slano area (greater Dubrovnik region). Microseismic locations of hypocentres of thousands of aftershocks, as well as the best double-couple CMT solution for the main-shock indicate that the earthquakes occurred on the NW-SE striking reverse fault system dipping towards NE

Seizmičnost Hrvatske i susjednih područja u razdoblju 1990-1992

Based on the catalogue of all located earthquakes in Croatia and the surrounding areas for the period 1990 – 1992 and on the macroseismic data analyses for individual earthquakes, certain seismicity features have been analysed. All together, 1188 earthquakes were located and the catalogue was found to be complete for the magnitudes ML ≥ 3.0. Seismically the most active was the coastal part of Croatia, where the strongest earthquake in the analysed period occurred on November 27, 1990 with the epicentre beneath the Dinara Mt. The composite fault-plane solution for the main shocks indicate the presence of a tectonic stress-filed directly approximately SSW-NNE which is compatible with the assumed anticlockwise rotation of the Adriatic plate and its subduction under the Dinarides