Title Analiza seizmičkog nemira na devet seizmoloških postaja Hrvatske mreže seizmografa Title (english) Ambient seismic noise analysis for nine seismic stations of the Croatian seismic network Author Nina Matsuno Mentor Iva Dasović (mentor)Committee member Iva Dasović (predsjednik povjerenstva)Committee member Marijan Herak (član povjerenstva)Committee member Ivana Herceg Bulić (član povjerenstva)Committee member Josip Stipčević (član povjerenstva)Committee member Zvjezdana Bencetić Klaić (član povjerenstva)Granter University of Zagreb Defense date and country 2021-05-03, Croatia Scientific / art field, NATURAL SCIENCES Abstract Da bi se odredila razina seizmičkog nemira i njezina vremenska promjenjivost, analizirani su kontinuirani zapisi s devet seizmoloških postaja Hrvatske mreže seizmografa u razdoblju između 1. siječnja 2018. i 30. lipnja 2020. godine. Primijenjena
je metoda računanja gustoće spektra snage (engl. power spectral density, PSD) i pripadajuća funkcija gustoće vjerojatnosti (engl. probability density function, PDF) prema radu McNamare i Bulanda (2004). Za svaku postaju izračunate su vrijednosti PSD za svaki jednosatni niz s preklapanjem od 50% u cijelom promatranom razdoblju na sve tri komponente. Iz dobivenih PSD krivulja, izračunate su PDF krivulje za svaku komponentu i svaku postaju. Dobiveni rezultati pokazali su da se PDF krivulja u pravilu nalazi između krivulja niske i visoke razine nemira (NLNM i NHNM) definiranima u radu Petersona (1993), iako na T > 10 s na horizontalnim komponentama nekih postaja postoji nezanemariva vjerojatnost premašivanja razine nemira NHNM. To ukazuje na zadovoljavajuću kvalitetu zapisa razmatranih seizmoloških stanica. U području mikroseizmičkog nemira (T = 1 – 10 s) vidljiva su u pravilu dva šiljka, čiji se maksimumi približavaju za postaje udaljenije od morske obale, za koje se pretpostavlja da su posljedice djelovanja oceanskih i lokalnih morskih valova na obalu, odnosno dno. Na svim postajama vidljive su dnevne promjene na T < 1 s, a na nekim postajama postoje i na T ≈ 20 – 40 s. Na T < 1 s uočene su tjedne promjene u obliku smanjenja razine nemira za vrijeme vikenda, pogotovo nedjelje. Sezonske promjene najviše se ističu na srednjim periodima (1 – 10 s), pri čemu je njihov utjecaj najjači zimi. Analiza razine seizmičkog nemira za vrijeme snažnih epizoda juga pokazala je izraženi porast razine nemira na T < 1 s na svim vertikalnim komponentama, kao i visokog vodostaja rijeke Kupe u blizini postaje OZLJ. Za vrijeme ljetne turističke sezone, primijećen je porast razine nemira na T ≈ 0.04 – 0.1 s na postajama NVLJ, VIRC i PLIT, dok je na postaji ZAG primijećeno smanjenje razine nemira zbog odlaska stanovnika na ljetovanje izvan grada. Također, na postaji ZAG jasno se vidi smanjenje razine nemira nakon proglašavanja zabrane kretanja zbog pandemije COVD-19, dok se slična pojava nazire još i na postajama PLIT i RUJC.
Abstract (english) To determine the noise levels and their time dependency, continuous datasets of nine seismic stations of the Croatian seismic network were analyzed in the time span of 1st January 2018 to 30th July 2020. The applied methods were based on the work of McNamara and Buland (2004), in which the power spectral densities and their probability density functions were calculated. PSDs were calculated for every onehour-long segments with 50% of overlapping for all three components of each station. Computed PDFs fall between the new low-noise model (NLNM) and the new high-noise model (NHNM), as defined by Peterson (1993), but there is a considerable probability that the PDF exceeds the NHNM at T > 10 s on the horizontal components. Analysis of PDFs show the acceptable quality of seismic station. There are two significant peaks in the microseism range (T ≈ 1–10 s): the one at the shorter periods is assumed to be a result of the interaction between local sea waves and seashore and seabed and it become closer as the distance from the sea increases to the one at longer periods which is assumed to be a result of the ocean waves. All stations shows diurnal variation of noise level at T < 1 s, while some stations even shows diurnal changes at T ≈ 20–40 s. Weekly variations were observed at T < 1 s as the noise level reduce on weekends, especially on Sundays. The seasonal variation was remarkable in the T ≈ 1–10 s and showed the highest noise level during winter. Seismograms recorded during days with strong southern wind (sirocco) shows a rise of noise level at T < 1 s on vertical components, as well as high levels of the Kupa river near OZLJ station. During summer touristic season on periods 0.04–0.1 s, noise level rises on NVLJ, VIRC and PLIT station while it drops at ZAG station because many residents leave the city for vacation. On the ZAG station, a significant drop of noise level was observed after the lockdown caused by the pandemic of COVID-19. A similar but smaller drop of noise level was noticed at PLIT and RUJC station.
Keywords
razina seizmičkog nemira
dnevne promjene
sezonske promjene
gustoća spektra snage
Keywords (english)
seismic noise level
diurnal change
seasonal change
power spectral density
Language croatian URN:NBN urn:nbn:hr:217:123584 Study programme Title: Graduate university study pf Physics - Geophysics; specializations in: Seismology and Physics of the Earth's Interior, Meteorology and Physical Oceanography Type of resource Text File origin Born digital Access conditions Open access Terms of use Created on 2021-05-07 09:05:03
