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

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 ultra-high 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 east–west 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 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.