An Alternative View of the Microseismicity along the Western Main Marmara Fault

Abstract

A detailed study, based on ocean-bottom seismometers (OBSs) recordings from two recording periods (3.5 months in 2011 and 2 months in 2014) and on a high-resolution, 3D velocity model, is presented here, which provides an alternative view of the microseismicity along the submerged section of the North Anatolian fault (NAF) within the western Sea of Marmara (SoM). The nonlinear probabilistic software packages of NonLinLoc and NLDiffLoc were used for locating earthquakes. Only earthquakes that comply with the following location criteria (e.g., representing 20% of the total amount of events) were considered for analysis: (1) number of stations≥5; (2) number of phases≥6, including both P and S; (3) root mean square (rms) location error≤0.5 s; and (4) azimuthal gap≤180°. P and S travel times suggest that there are strong velocity anomalies along the Western High, with low Vp, low Vs, and ultrahigh Vp/Vs in areas where mud volcanoes and gas-prone sediment layers are known to be present. The location results indicate that not all earthquakes occurred as strike-slip events at crustal depths (>8 km) along the axis of the Main Marmara fault (MMF). In contrast, the following features were observed: (1) a significant number of earthquakes occurred off-axis (e.g., 24%), with predominantly normal focal mechanisms, at depths between 2 and 6 km, along tectonically active, structural trends oriented eastwest or southwest-northeast, and (2) a great number of earthquakes was also found to occur within the upper sediment layers (at depths<2 km), particularly in the areas where free gas is suspected to exist, based on high-resolution 3D seismics (e.g., 28%). Part of this ultra-shallow seismicity appears to occur Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. in response to deep earthquakes of intermediate (ML∼4-5) magnitude. Resolving the depth of the shallow seismicity requires adequate experimental design ensuring source-receiver distances of the same order as hypocentral depths. To reach this objective, deep-seafloor observatories with a sufficient number of geophone sensors near the fault trace are needed.

Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul

“…The seismicity patterns beneath both geodetic networks significantly differ but are consistent with the respective geodetic observations, where the fault segment with no seismicity seems locked, whereas the fault segment with a high level of seismicity is interpreted as being partially creeping. Although this observation is common for onshore faults, submarine faults may have a different behaviour due to the possible high amount of gas migrating upwards 42,44,45 and the large water content of the shallow sediments. The fault segment east of our geodetic network related to the Çınarcık Basin (CB) segment is characterized by seismicity which significantly increased 6 after the M w 7.4 1999 Izmit earthquake and was suggested to be related to stress increase imposed from the 1999 Izmit rupture zone to the eastern Sea of Marmara 6 .…”

Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers

“…Most of the events could be clearly identified on the seismograms allowing some earthquakes to be relocated (e.g. Batsi et al 2018). No particular site effect was noted, when comparing the amplitudes of a given earthquake on the network (Fig.…”

Improved detection and Coulomb stress computations for gas-related, shallow seismicity, in the Western Sea of Marmara

“…They were deployed in the Central and Western parts of the SoM, mainly in and around the Tekirdag and Central basins as well as on the Western High (Figure 1). This zone is targeted due to its significant seismic activity [Tary et al, 2011;Schmittbuhl et al, 2016b;Batsi et al, 2018] and to its intense fluid manifestations including: i) the expulsion of gas bubbles in the water column [Géli et al, 2008;Dupré et al, 2015]; ii) the presence of gas and gas hydrates within the superficial sediments [e.g., Thomas et al, 2012]) along with the presence of mud accumulation and expulsion identified on the high-resolution bathymetry and seismic profiles [Grall et al, 2013] (Figure 2).…”

“…The cross-correlation is carried out solely for stations with time picks for the parent event. This could be complemented by additional picks using ray-tracing and an appropriate velocity model but we found that the calculated travel-times using the available 1-D velocity models [Tary et al, 2011;Géli et al, 2018] come with significant errors that might then prevent the phase identification for the newly detected events.…”

“…In the SoM, the seismic activity along the NAF is characterized by zones of intense activity organized as clusters [Schmittbuhl et al, 2016a;Tary et al, 2011] or delineating asperities [Bohnhoff et al, 2013], and zones with relatively low seismicity such as the Istanbul-Siliviri segment extending across the Kumburgaz basin, from the Central High to the Central Basin [Schmittbuhl et al, 2016b]. In 2011, a magnitude 5 earthquake with a dextral strike-slip focal mechanism occurred below the Western High at a depth of about 11.5 km [Batsi et al, 2018] (Figure 1). Apart from several aftershocks that occurred at roughly the same depth of the mainshock, this relatively unusually strong event for this zone also apparently triggered numerous events at depths shallower than 5 km (Figure 1).…”