A shallow fault-zone structure illuminated by trapped waves in the Karadere-Duzce branch of the North Anatolian Fault, western Turkey

Abstract

S U M M A R YWe discuss the subsurface structure of the Karadere-Duzce branch of the North Anatolian Fault based on analysis of a large seismic data set recorded by a local PASSCAL network in the 6 months following the M w = 7.4 1999 Izmit earthquake. Seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long-period oscillations in both P-and S-wave trains that do not exist in nearby off-fault stations. Examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone. The anomalous features in fault-zone seismograms produced by events not necessarily in the fault may be referred to generally as fault-zone-related site effects. The oscillatory shear wave trains after the direct S arrival in these seismograms are analysed as trapped waves propagating in a low-velocity fault-zone layer. The time difference between the S arrival and trapped waves group does not grow systematically with increasing source-receiver separation along the fault. These observations imply that the trapping of seismic energy in the Karadere-Duzce rupture zone is generated by a shallow fault-zone layer. Traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3-4 km. The synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m, a velocity decrease relative to the surrounding rock of approximately 50 per cent and an S-wave quality factor of 10-15. The results are supported by large 2-D and 3-D parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones. The inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower-type structure. The motion amplification associated with fault-zone-related site effects increases the seismic shaking hazard near fault-zone structures. The effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large.

Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit

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Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

“…As a final step, the seismograms are convolved with 1/t 1/2 to convert a point source response to that of an equivalent line dislocation source (e.g. Igel et al 2002;Ben-Zion et al 2003). The inverted model parameters are: (1-3) S velocities of the two quarter spaces (assumed different based on Section 3.3.2) and the fault zone layer, (4-5) width and Q value of the fault zone layer, (6) location of contact between the fault and left quarter space, and (7) propagation distance within the fault zone layer.…”

“…Li & Leary 1990;Ben-Zion et al 2003;Lewis & Ben-Zion 2010). In this study the focus is on identifying and analysing Lovetype FZTW that follow the direct S wave (Ben-Zion 1998).…”

From mechanical modeling to seismic imaging of faults: A synthetic workflow to study the impact of faults on seismic

“…Other works focus on the response of trapped waves within large fault zones in order to relate the anomalous behavior of the seismic wavefield to a possible fault zone (e.g. Ben-Zion et al, 2003;Shtivelman et al, 2005). These techniques are however not applicable to standard industry seismic data and faults at large depths.…”