Preliminary Results on Seismicity and Fault Zone Structure Along the 1944 Rupture of the North Anatolian Fault East of Ismetpasa

The North Anatolian Fault (NAF), a continental plate boundary similar in some respects to the San Andreas Fault (SAF), is of great importance to Turkey in terms of seismic hazard. The geological history, lithology & topography suggest that the NAF is generally a bimaterial interface separating different rock bodies. Theoretical and observational studies suggest that there may be fundamental differences between properties of earthquakes and seismic radiation generated by ruptures along interfaces that separate similar and dissimilar solids (e.g., Weertman 1980; Ben-Zion 2001; Dor et al. 2008; Ampuero & Ben-Zion 2008; Zaliapin & Ben-Zion 2010). High-resolution imaging of the internal fault structure in various locations can be used to test hypotheses associated with bimaterial ruptures, and estimate expected shaking hazard based on the results. It is also important to clarify the geometry and seismic potential of various sections through high-resolution studies of seismicity. To begin such studies, we conducted a pilot seismic experiment east of Ismetpasa on the 1944 rupture of the NAF with a line of 6-11 seismometers that cross the fault. The location was chosen because it is within the area where Dor et al. (2008) found strong asymmetry of rock damage that may reflect repeating ruptures on a bimaterial fault interface. The location also coincides with a section of the NAF that is partially creeping at least at shallow depth. The creep rate decayed from a maximum of 4-5 cm/yr following the 1994 earthquake to a present value of to 0.7 cm/yr (Cakir et al. 2005). The small local network has been operating for ~2.5 yr. Earthquake detection was done by a manual inspection of automatic identification of candidate events. To date we were able to detect only ~235 events in the magnitude range -1 to 2.5 within a radius of 45 km from the center of the network. Using template earthquakes for detecting more events was not successful so far, as the signals produced by the employed events are too similar to the background noise. Only 15-20 events are located within 3 km of the rupture and they all have Ml<1.0. The small number of events is in mark contrast to the numerous microearthquakes along the creeping section of the SAF, suggesting that the NAF east of Ismetpasa does not creep at depth. We are planning to use other potential detection methods to increase the number of events, and to analyze the waveforms for high-resolution local velocity structure.