Imaging the Mudurnu Segment of the North Anatolian Fault Zone From Waveforms of Small Earthquakes

Abstract

We analyze waveforms of local earthquakes occurring before, between, and after the two consecutive 1999 Mw > 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.

Stress Inversion of Regional Seismicity in the Sea of Marmara Region, Turkey

“…This tectonically young development still evolves and leads to the progression of the transtensional system towards the east. Younger and thus less well developed pull-apart structures are currently progressing further east below the eastern tip of the Sea of Marmara (Cinarcik Basin) (Le Pichon et al 2001;Karabulut et al 2002;Acarel et al 2014) and also extend further on land along the 1999 Izmit rupture (Akyazi Plain) (Bohnhoff et al 2006;Najdahmadi et al 2016Najdahmadi et al , 2018.…”

A Numerical Study of Elastic Wave Arrival Behavior in a Naturally Fractured Rock Based on a Combined Displacement Discontinuity-Discrete Fracture Network Model

“…Elastic waves in natural fracture systems involving systematic fracture sets and variable fracture lengths may exhibit much more complicated wavefield phenomena compared to those in idealized fracture networks. A typical example is the velocity anisotropy in the crust (Liu and Crampin 1990 ; Crampin and Lovell 1991 ; Liu et al 2004 ; Fontaine et al 2009 ; Al-Harrasi et al 2011 ; Frietsch et al 2015 ; Robinson et al 2020 ) intrinsically caused by the complicated spatial distribution of fractures (Kohler et al 2003 ; Matonti et al 2017 ; Zhang et al 2020 ), which often results in a highly variable arrival behaviors of seismic waves (Majer et al 1988 ; Spetzler and Snieder 2001 ; Santos et al 2015 ; Shao and Pyrak-Nolte 2016 ; Xu et al 2018 ; Najdahmadi et al 2018 ). Studying the linkage between the wave arrival behavior and the fracture network distribution can be useful for characterizing crustal heterogeneities in many rock engineering applications.…”