Work package number: WP7
Work package title: Re-evaluation of the seismo-tectonics of the Marmara Region
WP Leader: ITU
Objectives of the WP7 are three-fold:
- To generate a GIS database and relevant bibliography on the evolution of the North Anatolian Fault from an earlier shear zone in and around the Sea of Marmara;
- To synthesise all data to prepare an active fault map of the Sea of Marmara with appropriate explanation of nature, age, distribution of deformation and history of activity of the fault population;
- To evaluate the earthquake history of the Sea of Marmara and surrounding region that is important in both predicting future events and seismic risk assessment in Istanbul and the Marmara region
Description of work
The main task of the seismotectonics group is to establish the geography of the active fault system in the Sea of Marmara, its history of activity, both in terms of geologic time (since its origination in the medial Miocene) and historical time (past earthquake record). The past earthquake record requires collaboration with historians and we hope to have such supports from expert historians. The mapping of the fault population under the Sea of Marmara is almost complete. The only outstanding bit is the connection with the Thrace Basin and a part of it has been mapped using the Turkish Petroleum Company’s multichannel seismic data. Once that mapping is complete we shall have complete coverage of the active fault population of the Sea of Marmara. For the criterion of activity we simply take activity in the Quaternary as a sufficient condition, following the recommendation of Allen (1975). We acknowledge that faults may turn on and off within the time span represented by the Quaternary, but no fault belonging to a family as large and as active as the North Anatolian Fault becomes definitively dormant until the entire zone ceases its activity.
Parallel with the mapping of the faults, we shall evaluate the earthquake history of the Sea of Marmara and surrounding regions. The historical material available is huge and it is not possible for one person to deal with it. We shall have the cooperation of a historian team at the Marmara University, but also hope to consult the restoration department of the Faculty of Architecture of our own university. One serious problem is assessing historical seismicity is ascription of earthquakes to individual fault segments. Here we hope to be able to trench the suspected faults wherever possible on land. However, the closeness and on-going activity of major fault strands make definitive ascriptions difficult and we hope, at least, to be able to specify error margins in our ascriptions and produce a reliable epicentre map of historical earthquakes around the Sea of Marmara.
To understand the nature and distribution of activity along the North Anatolian Shear Zone during the geological past is of vital importance for this project. The previous estimates of the age and distribution of deformation were based on assumptions that did not stand the test of time. A serious problem is the distribution of the Neogene sediments within the Sea of Marmara. Previous estimates were based on assuming a basin-wide Neogene depocentre. More recent seismic profiling and studies of sedimentation rates on cores obtained during the numerous sea-borne missions showed that none of the Marmara basins can have any sediment older than top Pliocene at best. Almost all probably formed in the Pleistocene. This necessitates re-evaluation of the on-land sedimentary and geomorphological record and definition of the Neogene depocentres and areas of denudation in and around the Sea of Marmara. We believe that only after such a work one can make a true geological synthesis of the entire area from the late Cretaceous to the present.
Task 1. Re-evaluation of the seismo-tectonics and geohazards
Plate motion between Eurasia and Anatolia is known from geodesy, and models of slip partitioning between active faults in the transition from the Anatolian to the Aegean domains have been proposed (Meade, 2002; Le Pichon et al., 2003; Flerit et al., 2004; Reilinger et al., 2006; Hergert et al., 2010). However, the distribution of slip between offshore faults in the Sea of Marmara remains problematic. Although most of the plate motion may occur on the Main Marmara Fault of Le Pichon et al. (2001), geomechanical models suggest that other fault branches could accommodate part of it, and thus present earthquake and tsunami hazards (Hergert and Heidbach, 2010). This is in agreement with estimate of geological (10,000 year time-scale) slip-rate estimate carried out along the submerged North-Anatolian Fault system (Polonia et al., 2004; Gasperini et al., 2011a).To date, deformation rates inferred from stratigraphy and geomorphology (Sorlein et al., in press; Grall et al., 2012; Beck et al., 2007; Armijo et al., 2005; Seeber et al., 2006) are the main observations available to constrain models. Model outputs have been compared with results of local studies (Hergert et al., 2011; Muller and Aydin, 2005) but a more complete confrontation with data is needed, also taking into account variations with time and the effect of sediment compaction. The aim of the proposed work is to fully integrate available constraints from geology into kinematic and mechanical modelling efforts.
Another important task to model the behaviour of seismogenic faults is to carry out reliable co-seismic estimate of deformation associated with major historical earthquakes. This has been recently attempted in the Sea of Marmara by different working groups, which applied the methods of Earthquake Geology to the submarine environment (Polonia et al., 2002; Armijo et al., 2005; Pondard et al., 2007; Gasperini et al., 2011b).
The very large marine geophysical data set acquired in the Sea of Marmara combines observations over a range of scales: multibeam bathymetry and imagery, micro bathymetry from ROV and AUV surveys, THR sounder profiles, High resolution 3D seismic and 2D profiles, deep penetration multichannel seismic, wide angle seismic surveys and tomography. Most are now accessible and structural interpretations have been published. Available age models from sediment core analysis, and stratigraphic interpretations in term of eustatic cycles, constrain sedimentation rates over the last 10 to 500 ka. These data will be integrated with heat flow data in basin subsidence models. At a few locations, geomorphologic and 3D stratigraphic interpretations yield constraints on horizontal displacement, which also need to be taken into account. The re-evaluation of fault kinematic models will thus proceed in three steps: (a) synthesis of data offshore (structure, geomorphology, stratigraphy, heat flow) and onshore (b) modelling of basin subsidence, sediment compaction and heat flow, (c) critical assessment of kinematic and geomechanical models.
Task 2. Integration of faulting parameters from paleoseismic and historical data for hazard assessment
The North Anatolian Fault (NAF) splays into several branches in the Marmara Region (Barka and Kadinsky-Cade, 1988; Sengor et al. 2005). The most active northernmost branch prolongs between Düzce and Izmit as the on land section and enters into the Sea of Marmara in the Izmit Gulf. GPS data and elastic block models clearly show that the southern branch has relatively lower strain accumulation. However, this section of the NAF produces large and destructive earthquakes as well. These branches are located very close to large cities such as Izmit, Bursa, Istanbul, which have very dense population and are centres of the industry in Turkey. It is well known that large earthquakes affected these settlements in the past and created many casualties, heavy destruction and economical loss.
Even many previous paleoseismological studies have been done along the North Anatolian Fault (NAF) in the Marmara Region, there are still lots of uncertainties for the past earthquakes (e.g.: Rockwell et al, 2001; Hitchcock et al., 2003; Klinger et al., 2003; Ferry et al., 2004; Pavlides et al., 2006; Pantosti et al., 2008; Dikbas and Akyüz, 2011). These studies and additional data from the Marmara Sea bottom (e.g.: Sari and Cagatay, 2002; McHugh et al., 2006; Beck et al., 2006; Cagatay et al., 2012) will be combined and integrated into a GIS-based database in this project. At least 2000-years earthquake history of the western NAF is aimed to be included within this database. The lack of precise paleoearthquake data for the southern branch of the NAF will be completed based on the fault segmentation models. Any lack of data for separate segments or gaps in historical records will be examined with new paleoseismological trench studies both on northern and southern branches of the NAF in the Marmara Region. It is believed that this study with its deliverables will provide important data for predicting future events and give strong background for seismic risk assessment in Istanbul and the Marmara region.