Spatio-temporal distribution of seismicity in the northern Armutlu Peninsula (northwest Turkey)&#160;
Authors: Bocchini Gian Maria , Martínez-Garzón Patricia , Verdecchia Alessandro , Harrington Rebecca M. , Bohnhoff Marco , Turkmen Taylan , Nurlu Murat 
Published: March 2022

&lt;p&gt;The Armutlu Peninsula, bounded between two major sub-branches of the North Anatolian Fault (NAF) at the eastern Sea of Marmara, hosts the only onshore NAF segment along the Marmara seismic gap. It also hosts intense seismic and hydrothermal activity and documented episodes of aseismic slip. Here, we investigate the spatio-temporal distribution of seismicity in the northern Armutlu Peninsula to identify primary deformational mechanisms (i.e. seismic vs aseismic) and investigate the processes driving the seismicity. We employ multi-station matched-filter techniques to generate an enhanced seismicity catalog using up to 30 seismic stations, including regional permanent stations augmented by temporary stations from the SMARTnet network. We detect &lt;span&gt;7,677 events&lt;/span&gt; between 2019.01.25 and 2020.02.10, &lt;span&gt;and successfully relocate &lt;/span&gt;&lt;span&gt;4,182 of them using double-difference methods. The enhanced seismicity catalog reveals four week-long sequences with up to ~&gt; 200 events per day alternating in month-long periods with only &lt; 10-20 events per day. Earthquakes primarily concentrate within a narrow region of ~80 km&lt;/span&gt;&lt;sup&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sup&gt;&lt;span&gt; between 40.540&amp;#176;-40.600&amp;#176; N and 28.920&amp;#176;-29.025&amp;#176; E, forming linear structures striking from NW-SE to N-S at 5-12 km depth. Nearest-neighbor cluster analysis shows a gradual decrease of the ratio between swarm-like and burst-like activity, accompanied by a decrease of the background activity rates from the first to the fourth seismic sequence. Periods with predominantly swarm-like behavior and increased background activity exhibit a higher &lt;/span&gt;&lt;span&gt;&lt;em&gt;b&lt;/em&gt;&lt;/span&gt;&lt;span&gt;-value. We invert focal mechanism solutions of background seismicity and obtain an extensional stress regime for the broader Armutlu Peninsula and a transtensional stress regime for the narrow, most seismically active region. Within the narrow seismically most active region the minimum compressive stress (&amp;#963;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;3&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) is approximately horizontal and well defined, while the maximum (&amp;#963;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;1&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) and intermediate (&amp;#963;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) compressive stresses are close in magnitude and less well constrained. Moreover, in the most seismically active region, we observe that the principal stress orientations obtained from aftershocks is similar to that estimated from background seismicity. In contrast, the respective orientations of &amp;#963;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;1 &lt;/span&gt;&lt;/sub&gt;&lt;span&gt;and &amp;#963;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; inferred from foreshocks switch from vertical and horizontal to horizontal and vertical. Clusters of both normal faulting and strike-slip events identified through waveform based clustering analysis are optimally oriented with respect to the regional stress field, where normal faulting kinematics are predominant. We observe negligible seismic activity associated with the onshore segment of the NAF in the Marmara seismic gap. In contrast, we observe seismicity at 5-12 km depth that highlights the geometry of a major normal fault structure, the Waterfall fault, in the northern Armutlu Peninsula. The seismicity distribution and stress-field orientation suggest that the Waterfall fault exerts a primary control in the deformation of the northern Armutlu Peninsula.&lt;/span&gt;&lt;/p&gt;