Prostorna analiza atenuacije visokofrekventnih seizmičkih valova u središnjem dijelu Vanjskih Dinarida

master's thesis

• Title (english): Spatial analysis of high-frequency seismic waves attenuation in the central part of the External Dinarides
• Author: Lucija Golub
• Mentor: Iva Dasović (mentor)
• Committee member: Iva Dasović (predsjednik povjerenstva)
• Committee member: Marijan Herak (član povjerenstva)
• Committee member: Antun Marki (član povjerenstva)
• Committee member: Josip Stipčević (član povjerenstva)
• Committee member: Željko Večenaj (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Geophysics); Zagreb
• Defense date and country: 2022-11-28, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Geophysics; Seismology and Physics of the Earth's Interior

Abstract

Istraživana je atenuacija izravnih P- i S-valova (Q_P^−1 i Q_S^−1) lokalnih potresa na području središnjih Vanjskih Dinarida. Korišteno je 844 seizmograma za izračun atenuacije P-valova, te 762 seizmograma za S-valove zabilježenih na 10 postaja u tom području. Provedena je metoda normiranja kodom i inverzija kako bi se dobile varijacije atenuacije izravnih valova te faktor geometrijskog širenja γ. Postupak se provodio za četiri središnje frekvencije f_c = 3, 6, 12, 18 Hz. Korišten je programski paket MuRAT (engl. Multi-Resolution seismic Attenuation Tomography), pisan u Matlabu, za provođenje inverzije u prostoru te prostorni prikaz varijacija atenuacije seizmičkih valova. Cilj je vidjeti primjenjivost MuRAT-a na području središnjih Vanjskih Dinarida te kako bi se metoda mogla unaprijediti za kvalitetnije rezultate.

Ova analiza pokazala je da se izračunati faktori geometrijskog širenja γ smanjuju se s frekvencijama tako da je za niž frekvencije vrijednost bliska γ = 1 što odgovara prostornim valovima, a za veće frekvencije faktor γ bliži je vrijednosti 0.5 koja se obično pretpostavlja za površinske valove što ukazuje da na višim frekvencijama prevladavaju površinski valovi. Dobiveni srednji Q-faktori pokazuju da se radi o području visoke atenuacije, a oni se povećavaju s frekvencijama tako da je raspon srednje vrijednosti faktora dobrote Q za P-valove ⟨Q_P ⟩ od 137 (za 3 Hz) do 622 (za 18 Hz), a raspon srednje vrijednosti ⟨Q_S⟩ od 196 (za 3 Hz) do 525 (za 18 Hz). Iako se malo razlikuju, ove vrijednosti u skladu su s iznosima dobivenima u prethodnim istraživanjima za ovo područje, posebno kad se uzme u obzir da su analizirani različiti skupovi podataka te da se razlikuje faktor geometrijskog širenja.

Prostorne varijacije atenuacije izravnih seizmičkih valova ukazuju na to da je područje vrlo heterogeno. Varijacije su vidljive do otprilike 25 km dubine. Provedeni su testovi rezolucije – tzv. checkerboard i spike testovi u kojima se dobro vide područja s manjkom rezolucije te s više zamućenosti i razmazanosti. Rezultati checkerboard testa ukazuju na to da su rješenja najmanje stabilna za 3 Hz zbog najveće mutnoće i razmazanosti, a spike testovi reproduciraju anomaliju dobro na svim frekvencija osim na 3 Hz.

Abstract (english)

Attenuation of direct P- and S-waves is analyzed in the area of the central External Dinarides. In the analysis 844 seismograms of local earthquakes were used to determine direct P-wave attenuation and 762 seismograms for S-wave attenuation recorded on 10 stations. The coda normalization method and inversion procedure were applied to obtain variations of the attenuation factor Q_P,S and geometrical spreading factor γ. The method was applied to four central frequencies f_c = 3, 6, 12, 18 Hz. MuRAT (Multi-Resolution seismic Attenuation Tomography), a Matlab package for attenuation of seismic waves, was used for inversion in space. The goal was to investigate if MuRAT is applicable to the central External Dinarides and what can be done to achieve better results.

Obtained geometrical spreading factors γ decrease with frequency in a way that for lower frequencies factor γ is closer to 1, which is an assumed value for direct wave energies, and for higher frequencies the value of γ is closer to 0.5, which is an assumed value for surface waves and it indicates that for higher frequencies surface waves prevail. Average quality factors Q indicate that this area exhibits high attenuation and they increase with frequency. The range of average P-wave quality factors ⟨Q_P ⟩ values is 137 (for 3 Hz) and 622 (for 18 Hz), while the range of ⟨Q_S⟩ values is 196 (for 3 Hz) and 525 (for 18 Hz). Obtained values were compared to those calculated from frequency dependand quality factors obtained in earlier research of this area – values do not differ much, even though completely different data set and geometrical spreading factors were used.

Spatial variations of direct P- and S-wave attenuations are obtained and they indicate that the area is highly heterogeneous. Attenuation variations are resolved to the depth of approximately 25 km due to the high ray density. Checkerboard and spike tests are used to determine areas with more smearing and less resolution. The results of the checkerboard test indicate least stability for 3 Hz, with more smearing and blurring than for other frequencies, while spike tests reconstruct the anomaly from input model well for all frequencies except for 3 Hz.

• Keywords: atenuacija; metoda normiranja kodom; MuRAT; Dinaridi; tomografija; testovi rezolucije
• Keywords (english): attenuation; coda normalization method; MuRAT; Dinarides; tomography; resolution tests
• Language: croatian
• URN:NBN: urn:nbn:hr:217:349799
• 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: 2022-11-30 14:43:10