Seismic clustering in the Sea of Marmara: Implications for monitoring earthquake processes
Introduction
Laboratory rock deformation experiments typically show foreshocks and other signals associated with preparation of large events (e.g. Goebel et al., 2013; Selvadurai et al., 2017; Renard et al., 2018). Foreshock activity has been observed before some large earthquakes such as the August 1999 MW 7.4 Izmit earthquake along the North Anatolian fault (e.g. Bouchon et al., 2011; Ellsworth and Bulut, 2018). However, other large events including the November 1999 Mw 7.1 Düzce earthquake to the east of the Izmit event were not preceded by clear foreshocks (e.g. Wu et al., 2014). Analysis of pre-shock activity along the North Anatolian fault and other major faults has not been done systematically, in part because of the lack of high quality seismic catalogs. Refined hypocenter catalogs offering improved spatial resolution and lower magnitude of completeness allow for detailed studies of foreshocks. This is of particular importance for fault segments near densely populated regions, such as the Marmara section of the North Anatolian Fault Zone in Turkey, that are late in their seismic cycle. Below the eastern Sea of Marmara close to the Istanbul metropolitan region, several foreshocks have recently been observed preceding a MW 4.4 event (Malin et al., 2018).
Earthquake cluster identification is essential for understanding the dynamics of seismicity. Systematic analysis of earthquake clusters in a region can provide a context for local variations of foreshocks and other informative patterns of seismicity. The number and structure of earthquake clusters can vary in space and time on a range of scales (e.g., Ben-Zion, 2008; Zaliapin and Ben-Zion, 2016a). Analytical and numerical results in a viscoelastic damage rheology models suggest that basic properties of earthquake clustering are controlled by the effective viscosity of the deforming medium (Ben-Zion and Lyakhovsky, 2006). This implies that heat flow and the presence of fluids should play an important role in determining key properties of earthquake clustering (Zaliapin and Ben-Zion, 2013b).
Repeating earthquakes representing overlapping rupture areas and similar earthquake magnitudes are also important for quantifying regional seismic hazard, and are seen as indicators for fault creep. Observations of repeating earthquakes along the Western High and Central Basin of the Sea of Marmara suggested that aseismic slip may occur at these locations (Schmittbuhl et al., 2016a; Bohnhoff et al., 2017). Earthquake repeaters are commonly identified by employing waveform cross-correlation to find highly similar seismic waveforms (e.g., Poupinet et al., 1984; Nadeau and McEvilly, 2004; Peng and Ben-Zion, 2005). Recently, using analysis of earthquake clusters, it was found that fluid induced seismicity tends to display an unusually high concentration of events characterized by a relatively short distance and long time to the events initiating the clusters (Schoenball et al., 2015; Zaliapin and Ben-Zion, 2016b). Such events share some key features with the classical earthquake repeaters; however, the precise relation between these two types of events requires further exploration.
In this study we utilize a recently derived high-resolution seismicity catalog (Wollin et al., 2018) and nearest-neighbor cluster identification and classification techniques (Zaliapin and Ben-Zion, 2013a, Zaliapin and Ben-Zion, 2013b) to analyze clusters of seismicity in the Sea of Marmara region of the North Anatolian Fault, Turkey. Our main goals are to (1) estimate the spatial distribution of mainshock and aftershock rates and use it to infer the proximity to failure on different fault segments, (2) test the potential of the nearest-neighbor cluster approach to identify areas with enhanced occurrence of earthquake repeaters, and (3) characterize areas with enhanced foreshock activity. In the next section we describe the state-of-the-art knowledge on the seismotectonics and crustal properties of the analyzed fault segments in the Sea of Marmara. The examined seismicity catalogs, the nearest-neighbor methodology and the statistical approach employed are described in Section 3. The main results of the analysis that concerns the spatial distribution of clusters and the relative proportions of foreshocks, mainshocks, and aftershocks are described in Section 4. The implications of the results are discussed in the final Section 5.
Section snippets
Fault segmentation in the Sea of Marmara region
The North Anatolian Fault Zone (NAFZ) is a major dextral strike-slip plate-boundary that spans >1200 km across the northern boundary of the Anatolian Plate from east to west (Barka, 1992; Sengör, 2005; Bohnhoff et al., 2016). The eastern and central portions of the NAFZ are composed of a single well-developed fault. In the west the NAFZ splits into at least two or three main branches forming a horse-tail structure.
The Marmara section is the only portion of the NAFZ that was not activated in a
Earthquake catalog
We analyze two seismicity catalogs of different quality containing different number of events. The main seismicity catalog is a ten-year (January 2006–March 2016) catalog containing the seismicity from the region around the Sea of Marmara (Wollin et al., 2018). The catalog covers the region within 26.5°–30.5°E and 40°–41°N and includes 4744 relocated events. After removing areas of suspected quarry activities, 3974 events are identified as earthquakes (Fig. 1, see Wollin et al., 2018 for
Results
In the following, we present results obtained for the higher quality catalog from Wollin et al. (2018). A comparison with results for the KOERI catalog (documented in supplementary materials) is provided in the discussion.
Consistency of results between the catalogs
We analyzed two earthquake catalogs of varying quality in the Sea of Marmara region to investigate the consistency of the clustering features between the catalogs. In Section 4 we focused on the Wollin et al. (2018) catalog of larger quality. Here we first compare these results with those obtained using the KOERI catalog.
The obtained proportion of background seismicity and aftershocks as well as their spatial distributions are very similar in the KOERI catalog (Fig. S3, Fig. S4). Setting an
Conclusions
We analyzed clusters of seismicity in the Sea of Marmara region, NW Turkey, utilizing a high-quality relocated hypocenter catalog and the nearest neighbor earthquake distance approach. The main conclusions of our analysis are as follows:
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About 70% and 24% of the hypocenter catalog are identified as mainshocks and aftershocks, respectively. Largest background rates are observed around the Tekirdag Basin and the Cinarcik Fault. The largest density of aftershocks is observed around the Western
Acknowledgements
We thank Robert Shcherbakov, an anonymous reviewer and Editor Kelin Wang for thoughtful and constructive comments. PMG acknowledges funding from the Helmholtz Association in the frame of the Helmholtz Young Investigators Group SAIDAN (VH-NG-1323). YBZ and IZ acknowledge support from the Earthquake Hazards Program of the USGS (grants G17AP00086 and G17AP00087) and the National Science Foundation (grants EAR-1723033 and EAR-1722561).
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