Abstract. The Marmara section of the North Anatolian Fault Zone (NAFZ) runs under water and is located less than 20 km from the 15-million-person population center of Istanbul in its eastern portion. Based on historical seismicity data, recurrence times forecast an impending magnitude M>7 earthquake for this region. The permanent GONAF (Geophysical Observatory at the North Anatolian Fault) has been installed around this section to help capture the seismic and strain activity preceding, during, and after such an anticipated event.GONAF (Geophysical Observatory at the North Anatolian Fault) is currently comprised of seven 300 m deep vertical seismic profiling stations and four collocated 100 m deep borehole strainmeters. Five of the stations are located on the land surrounding the Princes Islands segment below the eastern Sea of Marmara and two are on the near-fault Princes Islands south of Istanbul. The 300 m boreholes have 1, 2, and 15 Hz 3-C seismometers near their bottoms. Above this are vertical, 1 Hz, seismometers at ∼ 210, 140, and 70 m depths. The strainmeter boreholes are located within a few meters of the seismometer boreholes and contain horizontal strain tensor sensors and 2 Hz 3-C seismometers at their bottoms. This selection of instruments and depths was made so as to ensure high-precision and broad-frequency earthquake monitoring and vertical profiling, all under low-noise conditions.GONAF is the first ICDP-driven project with a primary focus on long-term monitoring of fault-zone dynamics. It has already contributed to earthquake hazard studies in the Istanbul area in several ways. Combining GONAF recordings with existing regional seismic stations now allows monitoring of the NAFZ offshore of Istanbul down to magnitudes M<0. GONAF also improves the resolution of earthquake hypocenters and source parameters, better defining local fault branches, their seismicity, and earthquake potential. Using its vertical distribution of sensors, it has directly measured depth-dependent seismic side-effects for ground shaking studies. GONAF is starting to address fundamental questions related to earthquake nucleation, rupture dynamics, temporal changes in material properties and strain.
A primary hurdle in observing small foreshocks is the detection-limit of most seismic networks, which is typically about magnitude M1-1.5. We show that a start-up test of a borehole-based seismic network with a much lower detection limit overcame this problem for an Mw4.2 earthquake. This earthquake occurred offshore of Istanbul, Turkey, on a fault system that is likely to rupture in an M > 7 event in the coming decades. In the three days before and two after, a total of 62 or more earthquakes, including at least 18 foreshocks, came from the mainshock source area. The signal similarity of the foreshocks shows a clear increase during the hours before the Mw4.2 mainshock. Similar foreshock sequences have recently been reported for a few well monitored M > 7 plate-boundary earthquakes. The sequence surrounding the Mw4.2 gives the impression of stochastic failures that ended up interactively unloading stress concentrations. The Mw4.2 mainshock then resulted from the accumulated release of significantly smaller events, as suggested by other field and laboratory studies.
Based on the earthquake event data accumulated by the Turkish National Seismic Network between 2007 and 2013, the local magnitude (Richter, Ml) scale is calibrated for Turkey and the close neighborhood.
Using the first dataset available from the downhole Geophysical Observatory of the North Anatolian Fault, we investigated near-surface seismic-wave propagation on the Tuzla Peninsula, Istanbul, Turkey. We selected a dataset of 26 seismograms recorded at Tuzla at sensor depths of 0, 71, 144, 215, and 288 m. To determine near-surface velocities and attenuation structures, the waveforms from all sensors were pairwise deconvolved and stacked. This produced low-noise empirical Green's functions for each borehole depth interval. From the Green's functions, we identified reflections from the free surface and a low-velocity layer between ∼90 and ∼140 m depth. The presence of a low-velocity zone was also confirmed by a sonic log run in the borehole. This structure, plus high near-surface P-and S-wave velocities of ∼3600-4100 and ∼1800 m=s, lead to complex interference effects between upgoing and downgoing waves. As a result, the determination of quality factors (Q) with standard spectral ratio techniques was not possible. Instead, we forward modeled the Green's functions in the time domain to determine effective Q values and to refine our velocity estimates. The effective Q P values for the depth intervals of 0-71, 0-144, 0-215, and 0-288 m were found to be 19, 35, 39, and 42, respectively. For the S waves, we obtained an effective Q S of 20 in the depth interval of 0-288 m. Considering the assumptions made in our modeling approach, it is evident that these effective quality factors are biased by impedance contrasts between our observation points. Our results show that, even after correcting for a free-surface factor of 2, the motion at the surface was found to be 1.7 times greater than that at 71 m depth. Our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface.Online Material: Plots of resistivity and caliper logs and the spectra of all 26 events.
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