Monitoring fault zone environments with correlations of earthquake waveforms

S U M M A R Y We develop a new technique for monitoring temporal changes in fault zone environments based on cross-correlation of earthquake waveforms recorded by pairs of stations. The method is applied to waveforms of∼10 000 earthquakes observed during 100 d around the 1999M 7.1 Duzce mainshock by a station located in the core damage zone of the North Anatolian Fault and a nearby station. To overcome clock problems, the correlation functions are realigned on a dominant peak. Consequently, the analysis focuses on measurements of coherency rather than traveltimes, and is associated with correlation coefficient of groups of events with a reference wavelet. Examination of coherency in different frequency bands reveals clear changes at a narrow band centred around 0.8 Hz. The results show a rapid drop of ∼1–2 per cent of the coherency at the time of the Duzce event followed by gradual recovery with several prominent oscillations over 4 d. The observed changes likely reflect evolution of permeability and fluid motion in the core damage zone of the North Anatolian Fault. Compared to noise correlation processing, our analysis of earthquake waveform correlation (i) benefits from high level of coherencewith short duration recorded signals, (ii) has considerably finer temporal sampling of fault dynamics after mainshocks than is possible with noise correlation, (iii) uses the coherence level to track property variations, which may be more robust than traveltime fluctuations in the coda of noise correlations. Studies utilizing both earthquake and noise waveforms at multiple pairs of stations across fault damage zones can improve significantly the understanding of fault zone processes.