Determining the geometry of the North Anatolian Fault East of the Marmara Sea through integrated stress modeling and remote sensing techniques
Introduction
Due to the effect of fault geometry on modeled stress and strain fields, accurately representing the active fault geometry is a critical component of the modeling process (Bilham and King, 1989, Lesne et al., 1998). Longer, linked faults play host to the majority of earthquakes – particularly high magnitude earthquakes – in a fault system (Barka and Kadinsky-Cade, 1988, Slemmons and Depolo, 1986), and are the most influential in the development of the regional stress and strain field (Bilham and King, 1989, Lesne et al., 1998, Schwartz and Coppersmith, 1986). In strike-slip systems, short extensional faults linking echelon fault segments allow for long-term transfer of fault slip, thus affecting the stress field distribution, and may also act as kinetic barriers impeding or arresting rupture propagation (King, 1986, King and Nábëlek, 1985, Sibson, 1986). Fault geometry is sometimes hard to observe as a trace on the surface when there is little to no topographic expression, or if the fault has been covered by young sediments. In this study, we examine the relationship between fault geometry, and numerically determined stress and strain fields along a strike-slip fault in Turkey by utilizing a methodology adapted from McElfresh et al. (2002).
The North Anatolian Fault (NAF) system has seen several devastating and high magnitude earthquakes, particularly in the Marmara Sea region (Bohnhoff et al., 2013, Parsons, 2004). The 1999 earthquake of August 17th was of particular note for being the largest recorded earthquake in Turkey by modern digital networks (Özalaybey et al., 2002) with a devastating death toll of over 17,000 people (Scawthorn and Johnson, 2000). In the region impacted by the 1999 earthquake, the through-going NAF splits into two strands just west of the town of Bolu (Barka et al., 2002, Șengör et al., 2005, Hergert and Heidbach, 2010): the Düzce and Karadere faults (NE portion of the northern strand), which we refer to as a single fault called the Düzce–Karadere and the Mudurnu fault (SE portion of the southern strand) (Fig. 1). Two potential geometries of the active through-going strands of the NAF are proposed. One active fault geometry suggests that two strands almost converge (west of Bolu) and then diverge as distinct fault traces (Fig. 1B) (Akyüz et al., 2002, Barka et al., 2002). The fault strands continue on to border the Marmara Sea in the north and south respectively (Fig. 1B): the northern strand as the Izmit fault, and the southern strand as the Iznik fault (Armijo et al., 2002, Șengör et al., 2005). The second geometry is similar, except that the southern and northern faults described above are linked by a fault through the Mudurnu valley (Fig. 1C). A study of fracture zones attributed to major earthquakes of the 20th century by Koulakov et al. (2010) shows this link, as does previous work to understand stress transfer on the NAF by Stein et al. (1997). Studies of dextral displacement of Eocene volcanic rocks in the region between the northern and southern also imply an extensional linking fault (Armijo et al., 1999, Hisarli et al., 2011, Yılmaz et al., 1997). Seismic data from the Mudurnu valley show a normal sense of motion, indicating that the link may be an extensional fault between two major echelon fault segments (Mudurnu and Izmit faults) (Heidbach et al., 2008, International Seismological Centre, 2011, Neugebauer et al., 1997). A 100 year record of earthquakes (KOERI-UDIM, 2012) clusters around the northern strand, but also illustrates that the Mudurnu valley is seismically active, suggesting a connecting fault and supporting the second geometry (Fig. 2A). Additionally, it is proposed that the rupture of the 1967 M7.1 Mudurnu earthquake, the most recent of a series of historical earthquakes at its location (Palyvos et al., 2007), reached Lake Sapanca (along the Izmit fault) as distributed deformation (Ambraseys and Zatopek, 1969, Muller et al., 2003). Elevation data from a 10 meter Digital Elevation Model (DEM) – courtesy of the (GLCF) (USGS, 2008) – highlight pronounced topographic features that are a result of the southern and northern fault strands. The southern strand is located along narrow river valleys, while the northern strand controls the sharp transition from high to low topography (Fig. 2B). This fault-controlled topography has led to the inference of no connecting fault between the two segments. A potential linking fault from the Mudurnu fault to the Izmit fault can be traced through a narrow river valley, but loses any topographic expression as it nears the northern strand. Previous studies evaluating earthquake ruptures on this portion of the NAF system identify the Mudurnu fault in the southern Mudurnu valley, but terminate the fault trace before it links to the northern strand (Akyüz et al., 2002, Barka et al., 2002). This raises a question regarding the significance of a fault linking the two strands as part of the through-going NAF geometry within the region, and which geometry will best duplicate the regional stress and strain in geophysical models.
Determining the geometry of the active fault system would allow for more detailed and accurate models of the stress accumulation along the NAF, one of the world's largest active strike-slip faults. Stress accumulation can be used to define a region with a higher probability for seismic risk (Bowman and King, 2001, Stein et al., 1997). This study utilizes simplified fault geometries and friction values, surficial geology, rock physics parameters, and GPS velocities as model inputs. We created simplified two-dimensional models for each of the fault geometries and processed them with a finite element method to evaluate primary stress orientations for the region. The stress orientations from the two models are compared to lineament analyses and a record of focal mechanisms from the region to determine which geometry of active faults best reproduces the inferred stress field within an 85 km wide by 111 km long region centered around the Mudurnu valley. We evaluate the accuracy of the generated stress field from the finite element model (FEM) by comparing it to regional focal mechanisms, and inferred paleostress orientations determined from relating lineaments to potential structures.
We utilize PyLith – a finite element code tectonic deformation software – (Aagaard et al., 2012) to calculate the stress field in the region of interest for both models and compare the σ1 orientation frequency and magnitude of the stress field to the principal stress orientations suggested from lineament analyses and focal mechanisms. The overall goal of this study is to identify the active fault geometry of the NAF within the region of interest, which best duplicates the regional stress field as determined from focal mechanism data and lineament analyses.
Section snippets
Geologic background
The NAF forms the most prominent part of a strike-slip dominated belt of deformation between the Eurasian and Anatolian plates (Șengör et al., 2005). It extends from the junction of the NAF at the East Anatolian Fault (EAF) near the town of Karliova Turkey (Fig. 1) to the north Aegean region (Șengör et al., 2005) nearly 1200 km, paralleling the southern margin of the Black Sea. East–west widening of a dextral shear zone associated with the NAF system continues across the northern Aegean sea,
Fault parameters
A wide range of proposed friction coefficients (μ) have been suggested for the NAF (Hergert and Heidbach, 2011, Jiménez-Munt et al., 2006, Kasapoglu and Toksöz, 1983, Provost et al., 2003, Stein et al., 1997). Provost et al. (2003) created a 3D mechanical model of the NAF and determined the friction coefficients (μ = 0.05 to μ = 0.1) necessary to match the calculated velocity field to the GPS velocity field. Stein et al. (1997) determine that the friction is μ = 0.75 based on laboratory rock
Model processing
We use PyLith (Aagaard et al., 2012), to process the models using finite elements. A triangular mesh over the region of interest was created with a grid spacing of 5 km along the outer boundaries and a finer grid of 2 km near the faults. The grid spacing is used as input to create a finer triangular mesh near the faults, which grades into a coarser mesh towards the outer boundaries. The models are processed using the rock physics and fault parameters described in Section 3, and the block's
Inferring maximum paleostress orientations
Tectonic features such as faults and fractures may exert a strong control on topographic patterns due to these features creating pathways for weathering and erosion. Using digital imagery, we can highlight topographic lineaments and evaluate the potential control of tectonic induced deformation on topography. Tectonic features that may strongly influence topographic patterns include fault, large fracture systems and joints. These linear features can be mapped using digital imagery. Lineament
Interpretation
To evaluate which of the two models best represents the active fault geometry, we compare the maximum principal stress axis orientations from the finite element method results with the inferred maximum principal stress axis from the lineament analysis. The maximum principal stress orientations of model 1 (115°–120°) and model 2 (120°–125°) have peaks that nearly coincide with the spread of inferred directions of σ1 from the lineament analyses (115°–130°); however, model 2 results in a σ1
Conclusion
The methodology presented in this paper can be used to remotely identify approximate active fault geometries, which may not have well-developed surface expressions. Through an integrative approach of stress modeling with remote sensing techniques, the active fault geometry with the linking fault in model 2 best explains the primary stress orientations as observed from deformation patterns and earthquake focal mechanisms within the region of interest. The absence of a linking fault (model 1) led
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