NEW_Accomplishments.indd - IRIS

2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES
Systematic High-Resolution Imaging of the Karadere-Düzce Branch of the North
Anatolian Fault
Zhigang Peng • University of Southern California, now at University of California, Los Angeles
Yehuda Ben-Zion • University of Southern California
The spatial extent and material properties of the damaged fault zone rock have important implications for many aspects of
earthquake behavior. Fault zone structures with material discontinuity interfaces and low-velocity layers of damaged fault zone
rock can produce several indicative wave propagation signals, including scattering, anisotropy, non-linearity, and guided head
and trapped waves. We perform a systematic analysis of such signals from seismic data recorded by a PASSCAL seismic network
deployed along and around the Karadere-Düzce branch of the north Anatolian fault during the 1999 Mw 7.4 İzmit and Mw
7.1 Düzce earthquake sequences. Our results can be summarized as follows: The observed fault zone trapped waves are generated
by relatively shallow structures that extend generally only over the top ~3-4 km of the crust. The shallow trapping structure
is ~100 m wide and is surrounded by broader (~ 1 km) anisotropic and scattering zones that are also confined primarily to the top
3 km. The average delay times for ray paths that propagate along the rupture zone are larger than for the other paths. The apparent
crack density in the damaged shallow fault zone rock is about 7%. Systematic analyses of anisotropy and scattering measured
from waveforms generated from repeating earthquakes do not show precursory temporal evolution of properties before the
Düzce mainshock. The anisotropy results show small co-seismic changes. However, the scattering results show clear co-seismic
changes and post-seismic logarithmic recovery after the Düzce mainshock. A strong correlation between the co-seismic delays
and intensities of the strong ground motion generated by the Düzce mainshock implies that the radiated seismic waves produce
the velocity reductions in the top portion of the shallow crust.
Figure 1. (a) Hypocentral distribution of ~18000
earthquakes recorded by the PASSCAL seismic
experiment along the Karadere-Düzce branch
of the north Anatolian fault. (b) An example
of fault zone waveforms recorded by stations
on (VO) and off (FP) the fault. (c) An example
of rotated horizontal seismograms showing
splitted shear waves. (d) A summary plot of
average splitting parameters (bars) in our study
area. The bars are oriented parallel to the average
fast direction and scaled by the average
delay time. (e) Median delay times for the early
S-coda waves plotted against the earthquake
occurrence times for the vertical-component
seismograms generated by 36 repeating clusters
and recorded at station VO. (f ) A schematic summary
of the proposed fault zone model around
the study area. See the text and references for
more information.
Ben-Zion, Y., Z. Peng, D. Okaya, L. Seeber, J.G. Armbruster, N. Ozer, A.J. Michael, S. Baris and M. Aktar. A shallow fault zone structure illuminated by
trapped waves in the Karadere-Düzce branch of the north Anatolian fault, western Turkey, Geophys. J. Int., 152, 699-717, 2003.
Peng, Z. and Y. Ben-Zion. Systematic analysis of crustal anisotropy along the Karadere-Düzce branch of the north Anatolian fault, Geophys. J. Int., 159,
253-274, 2004.
Peng, Z. and Y. Ben-Z
Turkey, earthquake sequences, Geophys. J. Int., 160, 1027-1043, 2005.
Peng, Z., and Y. Ben-Zion. Temporal changes of shallow seismic velocity around the Karadere-Düzce branch of the north Anatolian fault and strong ground
motion, submitted to Pure Appl. Geophys, 2005.
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