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Moment Magnitude Estimates for Central Anatolian Earthquakes Using Coda Waves

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Moment Magnitude Estimates for Central Anatolian Earthquakes Using Coda Waves Solid Earth, 10, 713–723, 2019 https://doi.org/10.5194/se-10-713-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Moment magnitude estimates for central Anatolian earthquakes using coda waves Tuna Eken Department of Geophysical Engineering, the Faculty of Mines, Istanbul Technical University, 34469 Maslak, Sarıyer, Istanbul, Turkey Correspondence: Tuna Eken ([email protected]) Received: 14 January 2019 – Discussion started: 21 January 2019 Revised: 28 April 2019 – Accepted: 30 April 2019 – Published: 23 May 2019 Abstract. A proper estimate of moment magnitude, which is of reliable seismicity catalogs. Moreover, accurate informa- a physical measure of the energy released at an earthquake tion on source parameters could be important when develop- source, is essential for better seismic hazard assessments in ing regional attenuation properties. tectonically active regions. Here a coda wave modeling ap- Conventional types of magnitude scales (ML, mb, MS), proach that enables the source displacement spectrum mod- such as the empirically derived results of using direct wave eling of the examined event was used to estimate moment analyses, can be biased due to various effects such as source magnitudes of central Anatolia earthquakes. To achieve this radiation pattern, directivity, and heterogeneities along the aim, three-component waveforms of local earthquakes with path since they may cause drastic changes in direct wave am- magnitudes 2:0 ≤ ML ≤ 5:2 recorded at 69 seismic stations, plitude measurements (e.g., Favreau and Archuleta, 2003). which were operated between 2013 and 2015 within the Several early studies depending on the analysis of local framework of the Continental Dynamics–Central Anatolian and/or regional coda envelopes have indicated that coda wave Tectonics (CD–CAT) passive seismic experiment, were uti- amplitudes are significantly less variable by a factor of 3 to 5 lized. An inversion on the coda wave traces of each selected compared to direct wave amplitudes (e.g., Mayeda and Wal- single event in the database was performed in five different ter, 1996; Mayeda et al., 2003; Eken et al., 2004; Malagnini frequency bands between 0.75 and 12 Hz. The resultant mo- et al., 2004; Gök et al., 2016). In fact, local or regional coda ment magnitudes (Mw coda) exhibit a good agreement with waves are usually considered to be generally composed of routinely reported local magnitude (ML) estimates for the scattered waves. These wave trains can be simply explained study area. Apparent move-out that is particularly significant by the single-scattering model of Aki (1969), which has been around the scattered variation of ML–Mw coda data points proven to be virtually insensitive to any source radiation pat- for small earthquakes (ML < 3.5) can be explained by pos- tern effect in contrast to direct waves due to the volume- sible biases of wrong assumptions to account for anelastic averaging property of the coda waves sampling the entire attenuation and seismic recordings with a finite sampling in- focal sphere (e.g., Aki and Chouet, 1975; Rautian and Khal- terval. Finally, I present an empirical relation between Mw turin, 1978). In Sato and Fehler (1998) and Sato et al. (2012) coda and ML for central Anatolian earthquakes. an extensive review of the theoretical background of coda generation and advances in empirical observations and mod- eling efforts can be found. There have been several approaches used for extracting in- 1 Introduction formation on earthquake source size via coda wave analyses. These approaches can be mainly divided into two groups. Robust and stable knowledge of source properties (e.g., mo- The first group of studies can be considered the paramet- ment magnitude estimates) is crucial in seismically active ric approach and essentially employs a coda normalization countries such as Turkey for a better evaluation of seismic strategy in which measurements require a correction for hazard potential as this highly depends on the establishment Published by Copernicus Publications on behalf of the European Geosciences Union. 714 T. Eken: Moment magnitude estimates for central Anatolian earthquakes using coda waves empirically derived quality factors representing seismic at- In the current work I present source spectra as the output tenuation parameters (e.g., intrinsic and scattering). In this of a joint inversion of S-wave and coda wave parts extracted case, the adjustment of final source properties is achieved from 487 local earthquakes with magnitudes 2:0 < ML < 4:5 with the help of some reference events whose seismic mo- detected in central Anatolia. The approach used here em- ments are previously estimated based on waveform inver- ploys an isotropic acoustic RTT approach for the forward sion methods. For the forward generation of synthetic coda calculation of synthetic coda envelopes. Gaebler et al. (2015) envelopes, either single-backscattering or more advanced have observed that modeling results from isotropic scattering multiple-backscattering approximations are used. An exam- were almost comparable with those inferred from relatively ple for this group is an empirical method originally devel- more complex elastic RTT simulations with anisotropic scat- oped by Mayeda et al. (2003) to investigate seismic source tering conditions. The use of a joint inversion technique is ad- parameters such as energy, moment, and apparent stress drop vantageous since it is insensitive to any potential bias, which in the western United States and in the Middle East. They could be introduced by external information, i.e., source corrected observed coda envelopes for various influences, for properties of a reference that is obtained separately from instance path effect, S-to-coda transfer function, site effect, other methods for calibration. This is mainly because of the and any distance-dependent changes in coda envelope shape. fact that I utilize an analytical expression of a physical model Empirical coda envelope methods have been successfully ap- involving source- and path-related parameters to describe the plied to different regions with complicated tectonics such as scattering process. Moreover, the type of optimization during northern Italy (e.g., Morasca et al., 2008), Turkey and the joint inversion enables estimates for source parameters of rel- Middle East (e.g., Eken et al., 2004; Gök et al., 2016), and atively small-sized events compared to the one used in coda the Korean Peninsula (e.g., Yoo et al., 2011). normalization methods. The second type of approach depends on estimating source and structural properties through a joint inversion tech- nique. This technique employs a simultaneous optimization 2 Regional setting of source-, path-, and site-specific terms via a fitting proce- dure between a physically derived synthetic coda envelope The present tectonic setting of Anatolia and the surround- and an observed coda envelope within a selected time win- ing regions is mainly the outcome of northward-converging dow that includes both the observed coda and direct S-wave movements among the African, Arabian, and Eurasian plates. parts. Although the conventional coda normalization method To the west, the subducting African Plate with slab rollback essentially relies on a correction for undesired effects of the dynamics beneath Anatolia along the Hellenic Trench has source and site amplifications, it may fail for small events led to back-arc extension in the Aegean and western Ana- with a shorter coda. This mainly stems from random seis- tolia, while compressional deformation to the east around mic noise that dominates the coda, which does not satisfy the Bitlis–Zagros suture was explained by collisional tec- the requirement of the homogeneous distribution of energy tonics (e.g., Taymaz et al., 1990; Bozkurt, 2001) (Fig. 1). in space. In the present study, I avoid this shortcoming by in- Central Anatolia is located between an extensional regime volving source excitation and site amplification terms in the to the west due to subduction and a compressional regime to inversion process. To achieve this, radiative transfer theory the east due to collisional tectonics. There are several fault (RTT) is employed for analytic expression of synthetic coda systems responsible for ongoing seismic activity in the re- wave envelopes. The method was originally developed by gion. The major fault zone, the Central Anatolian Fault Zone Sens-Schönfelder and Wegler (2006) and successfully tested (CAFZ) (Fig. 2), which primarily represents a transtensional on local and regional earthquakes (4 ≤ ML ≤ 6) detected by fault structure with a small amount of left-lateral offset dur- the German Regional Seismic Network. Further, it has been ing the Miocene (e.g., Koçyigit˘ and Beyhan, 1998), can be applied to investigate source- and frequency-dependent at- considered a boundary between the carbonate nappes of the tenuation properties in different geological settings such as Anatolide–Tauride block and the highly deformed and meta- the upper Rhine Graben and Molasse basin regions in Ger- morphosed rocks in the Kır¸sehirblock. To the northwest of many, western Bohemia–Vogtland in Czechia (Eulenfeld and the CAFZ, the Tuz Gölü Fault Zone (TGFZ) (Fig. 2), which Wegler, 2016), the entire United States (Eulenfeld and We- is characterized by a right-lateral strike-slip motion with a gler, 2017), and the central and western North Anatolian significant oblique-slip normal component, appears to be col- Fault Zone (Gaebler et
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