The studies focus on systematic efforts to detect and quantify evolutionary changes of earthquake and rock properties that precede and follow large seismic events. We use multiple techniques to examine evidence for accelerated faulting process before the Mw7.1 Düzce earthquake on the North Anatolian fault, and to provide detailed results for postseismic effects following both the Düzce and the preceding Mw7.4 Izmit earthquakes. The study employs extensive waveform data set recorded by a tight fault zone array, with several stations located within the rupture zones of the Ãzmit and Düzce events, which operated from a few days after the Ãzmit mainshock until ~3 month after the Düzce event. The data set contains many events recorded in close proximity to the hypocenter of the Düzce mainshock, over a time interval including aftershocks of the Ãzmit earthquake, foreshocks and aftershocks of the Düzce mainshock, and the Düzce event itself. Previous studies based on ~26000 events detected in triggered-mode seismograms with standard techniques provided high-resolution information on the fault zone structure and co/postseismic changes of seismic velocities. Uncovering additional small events buried in the noise of the recorded waveforms can increase the available data significantly and offer unprecedented opportunities for tracking spatio-temporal changes of earthquake and fault properties.
To examine in detail evolutionary fault zone processes, the investigators perform research focusing on the following tasks: (1) Use the recently developed waveform matched filter technique to detect all possible additional earthquakes during the operation period of the network. (2) Use the updated catalog with many previously-undetected small events to search for patterns indicative of accelerated pre-earthquake activity around the hypocenter of the Düzce mainshock, as well as in the entire region covered by the data. (3) Identify clusters of repeating events in the complete updated data set, and use waveforms of the repeating event clusters to study temporal evolution of seismic velocities and earthquake source properties at various locations across the time of the Düzce mainshock. The newly-detected additional small events will increase significantly the resolution of results compared to those of previous studies. The project is expected to provide the most detailed results to date on temporal changes of fault processes and rock properties near and around the hypocenter of a large continental strike-slip earthquake. The study can provide fundamental in-situ results on the earthquake initiation process, postseismic effects and other aspects of earthquake and fault physics.
This grant provided support to clarify the extent of accelerating or other precursory seismic activity before the 1999 Mw7.1 Duzce earthquake, along with details of postseismic processes of the event. These topics were addressed using several observational studies, some of which involving the development of new techniques. The studies focused on the following research directions. (i) Use the waveform matched-filter technique to detect small events that were buried in the noise in the space-time domain near the initiation of the Mw7.1 Duzce earthquake. (ii) Develop improved techniques for systematic detection and location of small seismic events. (iii) Use correlations of earthquake waveforms to quantity postseismic changes in the rupture zone of the Duzce event. (iv) Use recent techniques on detection and classification of earthquake clusters to track the evolution of seismicity related to the initiation of the Mw7.1 Duzce earthquake. The results do not indicate a localizing foreshock process that accelerates in time and/or involves progressive concentration of activity around the Mw7.1 Duzce hypocenter during the preceding 65-hour period. Instead, we find that the Duzce source region becomes less active during the ~20 hours immediately before the main shock. Our results, together with other recent studies, suggest that progressive acceleration and localization of foreshocks around the main shock epicenter is not a general phenomenon. The results on post-seismic changes show a rapid drop of ~1-2% of the coherency of waveforms at the time of the Duzce event followed by gradual recovery with several prominent oscillations over four days. The observed changes likely reflect evolution of permeability and fluid motion in the core damage zone of the North Anatolian fault. The research provided one PhD student and one postdoctoral fellow with state-of-the-art techniques for detecting very small seismic events. Four peer-reviewed papers were published in prime seismological journals.
