Noise measurements were carried out in the Cologne area (Germany), and the resonance frequency of each site was estimated from the main peak in the spectral ratio between the horizontal and vertical component. For 32 of these sites, the thickness of the sedimentary cover was known from boreholes, and a clear correlation between resonance frequency and sediment thickness was observed. A formula that correlates cover thickness with frequency of the main peak in the horizontal-to-vertical spectral ratio was derived. In addition, a best-fitting shearwave-velocity distribution with depth, v s (z), as well as a relationship between average shear-wave velocity V ¢ s and thickness of the sedimentary cover, was calculated. By using all of the noise measurements and applying the derived relationships, we obtained a subsoil classification for the Cologne area.

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Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique. *

SUMMARY We derive the rupture history of the 1999 August 17 Izmit (Mw=7.4) and 1999 November 12 Düzce (Mw=7.1) earthquakes in Turkey from teleseismic body waves using broad‐band data of the Global Seismograph Network, aftershock locations and mapped surface breaks. The centroid solutions indicate strike‐slip mechanisms for both events. The Izmit earthquake was characterized by rupture propagating predominantly eastwards. It consisted of a main rupture lasting about 25 s followed within 1 min by two more events of Mw=6.9 and Mw=7.0. With the teleseismic data, we could not resolve the westward extent of rupture into the Marmara Sea. However, an upper bound of the seismic moment release west of the epicentre of the Izmit event is estimated to be  1.9×1019 N m. The Düzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture, in excellent agreement with mapped surface breaks and aftershock locations.

Local S-wave velocity-depth profiles are a key factor in seismic hazard assessment, as they allow the amplification potential of the sedimentary cover to be evaluated. Ambient seismic noise is mainly composed of surface waves, and therefore contains vital information about the S-wave velocity structure, allowing polarization or dispersion curves to be obtained from single station or array noise recordings. At two sites in the area of Cologne, Germany, the extended spatial correlation method was applied to such recordings and apparent phase velocity curves in the frequency range of interest for earthquake engineering were obtained. Using this data, a linearized inversion, the simplex downhill method, and a genetic algorithm yielded similar S-wave profiles. However, the latter method is recommended since it is less dependent upon a good starting model. Importantly, the presence of low-velocity layers in the Cologne area made it necessary to consider in the frequency range of interest higher modes in the inversion procedures. Finally, independent information on the total thickness of the sedimentary cover permitted the estimation of a 2D S-wave velocity profile crossing the Cologne area. Here, the H/V ratio inversion using 20 single-station noise recordings was used, with the results in good agreement with a geological profile.

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