Elastička svojstva kore u sjeverozapadnim Dinaridima određena na temelju analize seizmičkog nemira

master's thesis

• Title (english): Monitoring the elastic properties of the crust in the northwestern Dinarides using ambient seismic noise
• Author: Helena Latečki
• Mentor: Josip Stipčević (mentor)
• Committee member: Marijan Herak (predsjednik povjerenstva)
• Committee member: Josip Stipčević (član povjerenstva)
• Committee member: Željko Večenaj (član povjerenstva)
• Committee member: Ivana Herceg Bulić (član povjerenstva)
• Committee member: Ivica Sović (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Geophysics); Zagreb
• Defense date and country: 2019-09-20, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Geophysics; Seismology and Physics of the Earth's Interior

Abstract

Da bi se pratile varijacije brzina na području sjeverozapadnih Dinarida, u ovome su radu analizirani desetogodišnji (2009.–2018.) kontinuirani zapisi seizmičkog nemira vertikalnih komponenti seizmograma zabilježenih na ukupno četrnaest postaja. Primijenjena je metoda analize križnih spektara kliznih prozora (engl. skraćeno MWCS) od Clarke i sur. (2011) koja se provodi u frekventnoj domeni, a bazira na usporedbi tzv. referentne i višednevne CCF (kratica od engl. cross-correlation function; funkcija križne korelacije). Metoda je provedena za četiri frekventna raspona između 0.1 i 2 Hz pri čemu je referentna CCF definirana za čitavi desetogodišnji niz, a višednevne CCF za 90-dnevna razdoblja. Vremenski pomaci δti dviju CCF određeni su pomoću križnih-spektara za dio signala koji se odnosi na kodu. Uz pretpostavku da se relativne promjene brzina površinskih Rayleighjevih valova (δv/v) manifestiraju kao razvlačenje višednevne u odnosu na referentnu CCF, tj. da vrijedi δv/v=−δt/t, dobivene su krivulje promjena brzina koje pokazuju jasne sezonske izmjene minimuma tijekom ljeta i maksimuma tijekom zime. Uočeno je kako se smanjivanjem širine frekventnog raspona gubi sezonski oblik što se objašnjava nedostatkom koherentnih faza koje se koriste u metodi, ali i nekih drugih faktora. Iz tog su razloga provedene dvije dodatne analize kojima se htjelo utvrditi jesu li udaljenosti između postaja i lokacija izvora seizmičkog nemira dominantni uzročnici nestabilnosti sezonskog oblika kod užih frekventnih raspona. Sezonske varijacije brzina imaju uzročnike različitog porijekla i da bi se u potpunosti moglo utvrditi koji je najznačajniji potrebna je detaljnija analiza. Bez obzira na to, da bi se pratile male promjene svojstava sredstva čisto tektonskog porijekla, provedena je jednostavna korekcija sezonskog oblika krivulja δv/v. Modificirane δv/v krivulje uspoređene su s histogramima broja potresa za šest serija koje su se dogodile na području od interesa. U svim je slučajeva primijećena promjena u brzini tijekom razdoblja oko povećane seizmičke aktivnosti. Međutim, da bi se to sa sigurnošću i potvrdilo, potrebno je definirati općenitiji model koji uključuje direktne uzročnike sezonskih promjena brzina kako bi modificirane δv/v krivulje u potpunosti odražavale samo promjene mehaničkih svojstava sredstva.

Abstract (english)

In order to monitor seismic velocity variations in the northwestern part of Dinarides, ten years (2009–2018) of continuous vertical component recorded on fourteen stations were analyzed and the Moving Window Cross-Spectral Analysis (MWCS) by Clarke et. al. (2011) is employed. MWCS method operates in the frequency domain and is based on the comparison of two waveforms, the reference and the current CCF (cross-correlation function). Basic assumptions are homogeneity of relative velocity changes δv/v and it’s manifestation as a stretching of the current CCF relative to the reference function (δv/v = −δt/t). Therefore, to study velocity changes of surface Rayleigh waves, relative travel time perturbations (δt/t) between all station pairs have been measured in four different frequency bands between 0.1 and 2 Hz. The reference function was constructed by stacking the one-day CFs over the whole ten year period and current CCFs were obtained by stacking one-day CFs over 90-day period. Only the coda part of the CCFs was used to estimate the velocity changes. On average, between all station pairs in the studied area, a seasonal signal in the relative velocity variation is observed with peaks during winter and troughs during summer time. The stability of the signal decreases with decreasing frequency bandwidth, but generally stays stable with increasing inter-station distance and does not depend on the azimuthal coverage. Other factors such as lack of coherent phases might be the cause of such a behavior of the signal. The seasonality of the signal is a result of several effects and to fully understand which of them is dominant further analysis is needed. Nonetheless, a simple model to correct the seasonality of the signal is made, allowing to track small changes in the mechanical properties of the crust linked to tectonic effects. The corrected δv/v time series are compared with the number of earthquakes occurring during six different series of earthquakes. For all of them velocity changes around the time of higher seismic activity are observed. However, to completely validate such a claim, more detailed analysis is required.

• Keywords: Greenova funkcija; metoda analize križnih-spektara; seizmički nemir; sjeverozapadni Dinaridi
• Keywords (english): Green’s function; MWCS method; ambient seismic noise; northwestern Dinarides
• Language: croatian
• URN:NBN: urn:nbn:hr:217:884994
• Study programme: Title: Graduate university study pf Physics - Geophysics; specializations in: Seismology and Physics of the Earth's Interior, Meteorology and Physical Oceanography; Course: Seismology and Physics of the Earth's Interior; Study programme type: university; Study level: graduate; Academic / professional title: magistar/magistra fizike – geofizike (magistar/magistra fizike – geofizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2019-10-02 12:59:46