Abstract
Ayvacik district of Canakkale province was hit by four moderate size earthquakes on 6 February 2017 03:51 M w 5.2, 6 February 2017 10:58 M w 5.1, 7 February 2017 02:24 M w 5.2, 12 February 2017 13:48 M w 5.3. Centroid moment tensor solutions for 60 events with moment magnitudes (M w ) between 3.4 and 5.3 are computed by applying a waveform inversion method on data from the Turkish and Greek broadband seismic networks. The time span of data covers the period between 1 January and 28 February 2017. The mainshock is a shallow focus normal event with strike-slip component at a depth of 10 km. Focal depths of aftershocks range from 3 to 24 km. The seismic moment (M o ) of the mainshock is estimated as 7.62 × 10 16 Nm. The calculated rupture duration of the Ayvacik mainshock is 3 s. The focal mechanisms of the aftershocks are mainly normal faulting with a minor strike-slip component. The geometry of focal mechanisms reveals a normal faulting regime with NE-SW trending direction of T -axis in the entire activated region. We perform stress tensor inversion to acquire more accurate picture for the regional stress field. The stress tensor inversion results indicate a predominant normal stress regime with a WNW-ESE oriented maximum horizontal compressive stress (S H ). With respect the newly determined focal mechanisms, the effect of the propagation of the North Anatolian Fault into Aegean Sea is very clearly pronounced. According to the double-difference relocation algorithm for the 2017 Ayvacik seismic sequence, ten clusters are revealed. In order to find these clusters, we use HypoDD software. According to the relative relocations from HypoDD, we define 10 clusters along the five profiles. The aftershock activity in the observation period between 1 January and 28 February 2017 extends from W to E direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the ruptured segments. The western cluster is associated with a fault plane trending mainly N- S and dipping vertical, while the eastern is related to a fault plane trending NE-SW and dipping towards NW. The best constrained focal depths operate in the approximate depth range from 3 to 24 km. Coulomb stress analysis is performed to calculate the cumulative effect of all the earthquakes of the swarm and correlate it with the activated region. Positive lobes with stress more than 3 bars are obtained, these values are large enough to trigger failure towards NW-SE of the swarm activity.
Highlights
The Ayvacık earthquake (EQ) occurred at 03:51:40.0 Generic Mapping Tools (GMT) on 6 February 2017 in Çanakkale province
The Ayvacık EQ is directly located in the westernmost part of Biga Peninsula (BP) along the western North Anatolian Fault (NAF) strike-slip fault system (Figure 1a)
The main seismotectonic structures of the BP region are the Edremit Fault Zone (EFZ), Çan-Biga Fault (ÇBF), Evciler Fault (EF), Gülpınar Fault (GF) and Kestanbol Fault (KF) that compose transform plate boundaries where the Eurasian plate (EU) to the north is sliding relative to the Anatolian (AT) and Aegean Sea plates (AS) to the south
Summary
The Ayvacık earthquake (EQ) occurred at 03:51:40.0 GMT on 6 February 2017 in Çanakkale province. The focal mechanisms of some moderate recent events are related to the NAFZ strike-slip and oblique system combination We interpret their orientation and spatial distribution in terms of the major strike-slip fault with normal motions. Mesimeri et al [2018] indicated that spatial distribution of Ayvacık EQ sequences revealed a south-dipping causative fault along with secondary and smaller antithetic segments In this respect, the seismicity started at the western part and migrated to the eastern part to the activated area. A stress tensor inversion of earthquake focal mechanism data is performed to obtain a more accurate picture of the regional stress field For this purpose, seismic waveforms at local and regional distances are used to calculate source parameters of the 60 events (5.3 ≥ Mw ≥ 3.4) of the 2017 Ayvacık EQ sequence using the waveform inversion method [Nakano et al, 2008]. Determination of accurate stress field orientations and event locations using data from local and regional seismic networks are crucial for investigations of the seismotectonics in and around southwestern Marmara Sea region
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