Imaging the Mudurnu Segment of the North Anatolian Fault Zone From Waveforms of Small Earthquakes. Issue 1 (22nd January 2018) - Najdahmadi, Bita
Abstract: We analyze waveforms of local earthquakes occurring before, between, and after the two consecutive 1999 M w > 7 İzmit and Düzce earthquakes in NW Turkey. The waveforms were recorded at three seismic stations DOK, EKI, and GOK located around the Mudurnu segment of the North Anatolian Fault Zone. We focus on the interpretation of a distinct secondary phase contained in the P wave coda that is well separated from the direct P wave. The phase is visible in many waveforms of most seismicity clusters and has a specific constant time delay after the direct P wave arrivals at each station, irrespective of epicentral distance, hypocentral depth, or back azimuth. Based on a polarization analysis of records at station GOK, this secondary phase is interpreted as a PS wave converted at an interface near the stations. Its particle motion is consistent with the direct S wave and displays S wave splitting produced by the anisotropic upper crust. Synthetic modeling indicates that this PS phase can be converted either at a horizontal interface or at a steeply inclined interface. The steep Mudurnu fault zone with the near‐surface setting indicating a juvenile pull‐apart structure fits well into these interpretations, which are in agreement with the eastward progressing transtensional tectonics known for the region. Key Points: Detailed study of waveforms from seismic activity at the İzmit‐Düzce rupture zone in NW Turkey is presented New advanced waveform processing results in the detection of prominent secondary phases in P wave coda well separated from direct P wave Polarization analysis reveals PS phase converted at the upper crustal structure in agreement with near‐fault setting of the Mudurnu segment