Geodynamics of Zagreb and Pokupsko fault systems

Kako bi se što preciznije mogao odrediti seizmički hazard, potrebno je što bolje poznavati geodinamiku promatranog područja, odnosno poznavati potresnu povijest, seizmotektoniku i geologiju tog područja te bazu podataka o zabilježenim akceleracijama. Potres magnitude 5.3 koji se dogodio u nedjelju 22. ožujka 2020. u 6:24 SEV s epicentrom u blizini Markuševca i Čučerja, 7 km od Zagreba, najjači je instrumentalno zabilježeni potres iz Zagrebačkog epicentralnog područja. Ovaj potres, intenziteta VII-VIII °MSK, je odnio jedan ljudski život i ostavio veliku materijalnu štetu posebice u samom centru Zagreba, gdje su najviše stradale građevine izgrađene do 1920. godine. Približno je procijenjeno da je gibanje tla od 0.16 - 0.19 g barem dvostruko amplicirano. Petrinjska serija potresa započela je vrlo jakim potresom magnitude 5.0 28. prosinca 2020. u 6:28 SEV, a nakon malo više od jednog dana uslijedio je glavni potres magnitude 6.2 u 12:19 SEV. Epicentri glavnog i najjačeg prethodnog potresa ove serije bili su na udaljenosti manje od 7 km jugozapadno od Petrinje, dok su im hipocentri bili na dubini od približno 9 km. Glavni potres intenziteta VIII-IX °EMS odnio je sedam ljudskih života te uzrokovao velike materijalne štete posebice u centru Petrinje. Procijenjeno je da je vršno ubrzanje tla bilo izmeu 0.4 i 0.6 g, ovisno o tipu tla i udaljenosti od epicentra. Veliku ulogu u nastaloj šteti, nakon oba glavna potresa ove dvije serije, imala je nepropisna i strukturno neadekvatna gradnja. Pomoću prostorne razdiobe žarišta potresa iz obje navedene serije potresa, u softveru MOVE su se pomoću IDW metode modelirale rasjedne ravnine koje najbolje opisuju aktivirane rasjede. Tako su za Zagrebačku seriju modelirana tri duboko ukopana rasjeda, tzv. slijepi rasjedi, koji nemaju manifestacija na površini. Rasjed 1 ili primarni rasjed akomodira glavni potres i najjače naknadne potrese. Ovaj rasjed dobro se slaže s parametrima dobivenim u računu žarišnog mehanizma za glavni potres, koji je i pokazao da se potres dogodio na reversnom rasjedu nagiba prema jug-jugoistoku te on najbliže opisuje Sjeverni rubni medvednički rasjed. Rasjed 2 je modeliran na temelju hipocentara potresa srednje jačine. Ovaj rasjed približno prati vrhove Medvednice i vrlo vjerojatno je reversan. Posljednji modeliran Rasjed 3 je približno paralelan Kašinskom rasjedu. Aktivnost ovog rasjeda može ukazati na to da je i Kašinski rasjed još uvijek seizmogen. Iz vremenskih prikaza može se vidjeti da se u kasnijim fazama ove serije potresa nisu aktivirali dodatni rasjedi ni manje strukture zbog čega je za očekivati da se neki dodatni rasjedi neće ni aktivirati, čime će se seizmičnost ovog prostora lagano smanjivati. Slično kao u Zagrebačkoj seriji, i u Petrinjskoj seriji su se aktivirala barem dva, vrlo vjerojatno slijepa, rasjeda. Modeliran je primarni Rasjed 1 koji akomodira glavninu hipocentara potresa iz kataloga, uključujući i potrese najvećih magnituda. Rasjed 1 opisuje Pokupski rasjed te se gotovo savršeno slaže s izračunatim žarišnim mehanizmom za glavni potres koji je dao subvertikalni rasjed s desnim pomakom po pružanju. U ovoj seriji se aktivirala i grupa potresa s epicentrima sjeveroistočno od Pokupskog rasjeda te su kroz tu grupu modelirana dva rasjeda: Rasjed 2 (Petrinjski rasjed) približno okomit na Pokupski te Rasjed 3 približno paralelan Pokupskom. Prostorna razdioba prethodnih potresa ukazuje na aktivnost sekundarnog rasjeda (vjerojatno Petrinjskog) koji je vrlo vjerojatno potaknuo aktivnost Pokupskog rasjeda na kojem se glavni potres i dogodio. Zbog toga se ne može zanemariti mogućnost da su se u ovoj seriji potresa ipak aktivirala sva tri modelirana rasjeda. Katalozi potresa sadrže još uvijek preliminarno određene potrese koji su još uvijek podložni promjenama pa su tako i rezultati ovog rada preliminarni. Svrha ovog rada bila je dati bolji uvid u aktivirani sustav rasjeda te tako osigurati temelje za daljnja istraživanja i preciznije odreivanje seizmičkog hazarda.To contribute to a better and more precise assessment of the seismic hazard for a certain area, it is important to understand the geodynamics of that area. That means we should know earthquake history, seismotectonics and geology of that area as well as having a database of recorded accelerations. On early Sunday morning on 22nd of March 2020, at 6:24 CET, an earthquake M5.3 occurred with epicenter near Markuševac and Čučerje, 7km far from the Zagreb city center. That earthquake is the strongest, instrumentally recorded earthquake in the Zagreb epicentral area. With a maximum intensity of VII - VII °MSK, that earthquake took one life and left severe material damage, especially in the center of Zagreb where the most severely damaged buildings were built until 1920. The first-order assessment of seismic amplication (due to site conditions) in the Zagreb area shows that ground motion of approximately 0.16 - 0.19 g were amplied at least twice. Petrinja earthquake series started with a strong M5 foreshock on early Monday morning on 28th of December 2020, at 6:28 CET. The day after, mainshock M6.2 occurred at 12:19 CET. Epicenters of those two earthquakes are alost at the same place, almost 7 km SW from Petrinja and at the depth of approximately 9 km. The mainshock, with maximum intensity of VIII-IX °EMS, took seven lives and caused severe material damage, especially in the center of Petrinja. It was estimated that peak ground acceleration of mainshock was in between 0.4 and 0.6 g, depending on local soil type and epicentral distance. The main reasons for a big material damage, caused by the mainshocks from both earthquake series, was improper construction of buildings. In this research, spatial distribution of hypocenters from both earthquake series were analysed in structural geological modelling software MOVE using IDW metode, to describe activated faults. For the Zagreb earthquake series three faults that accommodate the vast majority of hypocenters (as well activated faults) were modeled as deep-seated. Fault 1, interpreted as a primary thrust fault, accommodates mainshock and the strongest aftershocks. This fault describes well parameters given in the calculated focal mechanism which as well indicates that mainshock and the strongest aftershock happened on a south-southeast-dipping thrust fault. Fault 1 describes the Northern Medvednica Boundary Fault the best. Fault 2, modelled on hypocenters of moderate earthquakes, is of unknown character (probably thrust fault as well) and it dissects the Medvednica Mt. generally acros the highest peaks. Lastly modeled is Fault 3 which is almost parallel to the Kašina fault and can indicate its activity. From the time dependent 3D model it is visible that in later phases of this earthquake series no additional faults and structures were activated. Because of that, it is expected that the seismic activity of the Zagreb epicentral area is slowly calming down. Similar to the Zagreb earthquake, in the Petrinja earthquake series activated at least two, probably blind, faults. Primar Fault 1 is modeled to accommodate the vast majority of earthquakes from catalogue, including the strongest ones. Fault 1 describes Pokupsko fault and it describes almost perfectly the parameters of the dextral strike-slip calculated in the focal mechanism of mainshock. Smaller group, NE from Pokupsko fault, activated as well, therefore two faults were modeled to accommodate those earthquakes: Fault 2 (Petrinja fault) perpendicular to Pokupsko fault, and Fault 2 almost parallel to the primar Pokupsko fault. Spatial distribution of foreshocks implies the activity of secondary fault (possibly Petrinja fault) which most likely initiated activity of the Pokupsko fault on which the mainshock happened. Because of that, the possibility that all three modeled faults were activated in this earthquake series cannot be ignored. Earthquake catalogues used in this research still contain preliminary analysed earthquakes that can still be changed and therefore results given in this research are preliminar. The main purpose of this research is to give a better insight into the activated fault system to insure foundations for further research and more precise seismic hazard assessment