Elcin Gok

Microtremor HVSR Study of Site Effects in Bursa City (Northern Marmara Region, Turkey)

Local site effects are one of the most important aspects in the assessment of seismic hazard. Local site response can be investigated by empirical and theoretical methods. Theoretical methods allow a detail analysis of the parameters considered in the evaluation; however, they require information of the geological structure (Dravinski et al., 1996). Empirical methods are based on seismic records on sites with different geological condition from which relative amplitudes and dominant periods may be determined directly. This approach requires of a large number of earthquakes. In regions with low seismicity, it would be necessary to wait for a long time to obtain a complete data set. For this reason, the use of ambient seismic noise is becoming popular as an alternative (Bard, 1998). Recording and analyzing ambient noise is simple. A few minutes of microtremor data are usually sufficient. Microtremors are present continuously in time and space. A single threecomponent station is the only instrument required. Routine spectral techniques can be easily applied to estimate the dominant frequency of vibration of the sedimantary structure. These frequencies of vibration are closely related to the physical features of the site under study, i.e., layer thicknesses, densities and wave velocities. Estimates of these frequencies are useful to constrain the physical properties at a given site. The Nakamura technique (Nakamura, 1989), based on the horizontal to vertical spectral ratio (HVSR), has been commonly used to estimate the site effects. Later it has been extended to both weak motions (Ohmachi et al., 1991; Field & Jacob, 1993, 1995); and strong motions (Lermo & Chavez-Garcia, 1994; Theodulidis & Bard, 1995; Suzuki et al., 1995). Lermo & Chavez-Garcia (1993) applied this technique to estimate the empirical transfer function from the intense S-wave part of a small sample of earthquake records obtained in three cities of Mexico. Their results showed that the HVSR can estimate the dominant frequency at a site based on earthquake data. Suzuki et al. (1995), using both microtremor and strong motion data in Hokkaido, Japan, showed that the dominant frequency obtained from HVSR was in good agreement with the predominant frequency estimated from the thickness of an alluvial layer. Lermo & Chavez-Garcia (1993) compared transfer functions computed using the Haskell method agreement with the HVSR. Lermo & Chavez-Garcia (1994) verified that the underlying

Tectonic Implications of Microseismic Activity and Fault Plane Solutions around Izmir City

Western Anatolia is one of the most seismically active and expeditiously extending regions in the world. The large-scale deformation in the Western Anatolia is dominated by the collision of the African and Arabian plates with Eurasia. This collision causes a westward migration of the Anatolian block from the east to the Aegean Sea in the west. Izmir city is the capital of Aegean region, and also 3rd biggest city of Turkey with its more than 4 millions inhabitants. This study aims to reveal present micro-seismic activity and seismotectonic features obtained from a local-scale accelerometric array.

WinFoc - A Windows-based Individual Focal Mechanisms Program

We present a software dealing with focal mechanisms solutions for educational and research purposes. It gives reasonable results by performing graphical interface of the beachball diagrams. Plots can facilitate to understand the kinematics of fault ruptures. The program is used to examine the nodal planes, calculate the orientation of the Strike, Dip and Rake, and to plot the possible structural model on the block diagram representing the beachball diagram. It is written in Visual Basic 6, and has all the advantages of the Windows OS.

Evaluation of Spectrum Behaviour on Acceleration and Displacement Records

This study aims; - Using the acceleration records as well as the displacement traces to determine the spectral source parameters described above by using P & S waves and noise records, - Showing the possibility of obtained the corner frequency (Fo) without converting acceleration traces into displacement, - Determining evidences of the parameter Fmax, and, - Comparing the spectrum characteristics obtained from noise, P and S waves both of acceleration and displacement data. Spectral seismograms of 10 earthquakes recorded by acceleration stations of IzmirNet, are presented, here, to discuss spectral behaviour of seismic signals included both acceleration and displacement records. Earthquake parameters of events are also given in the text.

Seismic studies from small aperture accelerometric network in Izmir metropolitan city, Turkey

This study deals with the results obtained from IzmirNet local strong-motion network installed in Izmir metropolitan city. Data were interpreted in terms of engineering seismology and earthquake engineering, correlation of noise using accelerometric array, seismological and seismotectonics studies. Site characteristics reveal an amplification factor equal or greater than 6.0 for a dominant frequency interval between 0.6-1.5 Hz on alluvial deposits observed mostly around the Izmir Gulf. Bigger frequencies and less amplifications were also detected for limestones and andesitic volcanic units. Recent study about cross-correlation of noise exposes underground velocity structure by using station-pairs methods. Low group velocities present at the east of the gulf where thick Quaternary-aged deposits exhibit over there. City was under high seismic risk according to the locations of recorded events by IzmirNet. Izmir, Orhanli-Tuzla and Seferihisar faults are candidate to produce a destructive earthquake in the future according to the present seismic activity and focal mechanisms solutions. Stress tensor study illuminates E-W aligned faulting under the N-S extension regime.