ALBERT

Description: The Marmara region in Turkey is an important geological setting, both from a tectonic and a seismic hazard/risk perspective. We present a new map of crustal thickness variation across this complex region to better understand the interplay of past and present tectonic processes that have formed present‐day structure. Maps of crustal thickness are created using Ps converted phases and receiver function (RF) analysis of earthquakes recorded at all publicly available seismic stations and stations in the national monitoring network (run by AFAD Disaster and Emergency Management Authority Turkey). RFs are converted from time to depth using a local 3‐D full‐waveform tomographic model and are combined in multiphase common conversion point stacks. Direct P to S converted arrivals and associated multiples are mapped to produce continuous maps of the Moho discontinuity. Results show Moho depths ranging from 26–41 km with a regional trend of westward thinning reflecting the effects of the extensional regime in western Anatolia and the neighboring Aegean Sea. The thinnest crust is observed beneath the western end of the Sea of Marmara, attributed to transtensional basin opening. A distinct region of increased crustal thickness bounded by the West Black Sea Fault in the west, and the northern strand of the North Anatolian Fault in the south, defines the ancient crustal terrane of the Istanbul Zone. Isostatic arguments indicate that the thickened crust and lower elevation in the Istanbul Zone require it to be underlain by thicker lithosphere, a conclusion that is consistent with its hypothesized origin near the Odessa shelf.

Description: Given its intense seismic activity and damaging earthquake generation potential, the western part of the North Anatolian Fault constitutes a serious natural hazard. As a result, the fault is monitored with a broad range of seismological and geodetic instrumentation making it a natural laboratory environment for scientific studies. One of the long-term projects in this region is GONAF (Geophysical Borehole Observatory at the North Anatolian Fault) which is the first borehole seismometer network project in Turkey. GONAF is a joint research project that started in 2011 as joint initiative of the Turkish Ministry of Interior, Disaster and Emergency Management Presidency AFAD and GFZ and the German Research Center for Geoscience Helmholtz Center Potsdam. The aim of GONAF is to detect, examine, and monitor the microseismic activity in the region and to observe the physical processes before, during and after a large Marmara earthquake (M 〉 7.0) that is expected to rupture the western part of the North Anatolian Fault, below the Marmara Sea along the Princes Islands segment or even further to the west. For this purpose, the permanent GONAF observatory was established consisting of 7 borehole seismometer arrays installed down to a depth of 300 m. In this paper, we report on regional stress changes in the western part of the North Anatolian Fault Zone (NAFZ) using instrumental data and the Coulomb stress method. We also present preliminary results of the observation and evaluation of microseismic activity obtained from the GONAF observatory. For the automatic evaluation of real-time data, Seiscomp3, RTQUAKE, and Earthworm Softwares were used. Within the scope of automatic earthquake detection studies, between March, 2016 and November, 2017, a total of 2568 earthquakes were detected using the RTQUAKE software. Of these, 1459 could be analyzed. While the magnitude of the analyzed earthquakes varies between 0.8 and 4.2, the depth of these events ranges from 2 to 30 km.

Description: Various geophysical observations show that seismic and aseismic slip on a fault may occur concurrently. We analyze microseismicity recordings from a temporary near‐fault seismic network and borehole strainmeter data from the eastern Marmara region in northwest Turkey to track seismic and aseismic deformation around the hypocentral region of an Mw 4.5 earthquake in 2018. A slow transient is observed that lasted about 30 days starting at the time of the Mw 4.5 event. We study about 1200 microseismic events that occurred during 417 days after the Mw 4.5 event around the mainshock fault rupture. The seismicity reveals a strong temporal clustering, including four episodic seismic sequences, each containing more than 30 events per day. Seismicity from the first two sequences displayed typical characteristics driven by aseismic slip and/or fluids, such as the activation of a broader region around the mainshock and swarm‐like topology. The third and fourth sequences correspond to typical mainshock–aftershock sequences. These observations suggest that slow slip and potentially fluid diffusion along the fault plane could have controlled the seismicity during the initial 150 days following the Mw 4.5 event. In contrast, stress redistribution and breaking of remaining asperities may have caused the activity after the initial 150 days. Our observation from a newly installed combined dense seismic and borehole strainmeter network follows an earlier observation of a slow transient occurring in conjunction with enhanced local seismic moment release in the same region. This suggests a frequent interaction of seismic and aseismic slip in the Istanbul–Marmara seismic gap.

Description: The Main Marmara Fault (MMF) in NW Turkey south of Istanbul is a segment of the North Anatolian Fault Zone (NAFZ) that constitutes a right-lateral continental transform fault. Several well-documented strong (M7+) earthquakes indicate that the MMF poses a great risk to the Istanbul metropolitan region. A 150 km long stretch of the MMF has not ruptured since 1766 and the recurrence time of 250 yrs for M7+ events derived from historical records indicate that the fault is overdue. We introduce a new project addressing how the rheological configuration of the lithosphere in concert with active fluid dynamics within the crust and mantle influence the present-day deformation along the MMF in the Marmara Sea region. We test the following hypotheses: (1) the seismic gap is related to the mechanical segmentation along the MMF which originates from the rheological configuration of the crust and lithosphere; (2) variations in deformation mechanisms with depth in response to variations in temperature and (fluid) pressure exert a first-order control on the mode of seismic activity along the MMF, and, (3) stress and strain concentrations due to strength and structural variability along the MMF can be used as an indicator for potential nucleation areas of expected earthquakes. To assess what mechanisms control the deformation along the MMF, we use data from the ICDP GONAF observatory (International Continental Drilling Programme – Geophysical Observatory at the North Anatolian Fault) and a combined work flow of data integration and process modelling to derive a quantitative description of the physical state of the MMF and its surrounding crust and upper mantle. Seismic and strain observations from the ICDP-GONAF site are integrated with regional observations on active seismicity, on the present-day deformation field at the surface, on the deep structure (crust and upper mantle) and on the present-day stress and thermal fields. This will be complemented by numerical forward simulations of coupled thermo-hydraulic-mechanical processes based on the observation-derived 3D models to evaluate the key controlling factors for the present-day mechanical configuration of the MMF and to contribute to a physics-based seismic hazard assessment.

Description: We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an Mw 4.7 foreshock and Mw 5.8 mainshock) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian fault in the central Sea of Marmara, northwest Turkey. To investigate in detail the seismicity evolution, we apply a matched‐filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two largest events are clearly seen, exhibiting two different behaviors: a long‐term activation of the seismicity along the entire fault segment and a short‐term concentration around the epicenters of the large events. We suggest a two‐scale preparation phase, with aseismic slip preparing the mainshock final rupture a few days before, and a cascade mechanism leading to the nucleation of the mainshock. Thus, our study shows a combination of seismic and aseismic slip during the foreshock sequence changing the strength of the fault, bringing it closer to failure.