Orhan Polat

The seismotectonics of the Marmara region (Turkey): results from a microseismic experiment

The Marmara region is an active tectonic zone characterised by the transition between the dextral strike-slip regime of the North Anatolian Fault (NAF ) and the extension regime of the Aegean Sea. Strong historical earthquakes (M>7) and the presence of known seismic gaps imply a high level of seismic hazard. A synthesis of recent studies of active tectonics in the region is presented, including inland and underwater observations. The branching of the NAF is explained in terms of increasing influence of the extension. Historical information and instrumental seismicity are reinterpreted in order to have a critical appraisal of the existence of large seismic gaps in the central and eastern Marmara Sea. Focal mechanisms of strong earthquakes are used to obtain orientation and shape factor of the deviator of the stress tensor. The resulting tensor is in shear regime (s 2 vertical ) but close to extension (R=0.93) with s 1 oriented N145°0. A microseismic experiment with 48 stations distributed around the Marmara Sea was carried out in October‐December 1995. A total of 137 microearthquakes were located and 23 of those were selected to obtain focal mechanism solutions. The epicentral distribution indicates activity along the system of pull-apart basins north of the Marmara Sea. The segment between Marmara Sea and the Saros Bay, activated in 1912, and the Gulf of Izmit, site of the 1754 earthquake, are now silent. Seismic activity is very linear along the northern branch of the NAF, but it is more diVused on the Bursa and Iznik branches, southeast of the Marmara Sea. The stress tensor obtained from the focal mechanisms of the micro-earthquakes is compared to the one inverted from teleseismic data. The microseismic stress is compatible with a shear (intermediate) regime like the one obtained from strong earthquakes, though not as well constrained, one of the acceptable solutions having the same orientation but diVerent shape (R=0.5).

Analysis and interpretation of the aftershock sequence of the August 17, 1999, Izmit (Turkey) earthquake

A micro-seismic field experiment has been carried out in the Marmara Searegion. The analysis of the events before and after the August 17, 1999Izmit (Turkey) earthquake has been completed. 1446 events have beenwell located out of a total of 3165 recorded within the period from July15 to November 2, 1999. 67% of the aftershocks with magnitudegreater than 4 have occurred within the first 6 days after the main-shock. Earthquakes of the Izmit sequence are distributed in the first 15 km of theearth crust, but major events are located in between 5 km and 15 kmdepth. The seismicity pattern defines a rupture plane extending for about150 km in an E-W direction. The rupture is extremely linear butsegmented, and its complexity increases towards the western endmanifesting bifurcation. A stress analysis has been carried out both at thewestern end and all along the aftershock zone. 96 selected aftershocks,registered between August 21 and October 22, were chosen in order tocompute their focal mechanisms and obtain information about the stressregime after the Izmit earthquake. Strike-slip and normal faultingmechanisms are dominant. The numerous strike-slip mechanisms arecompatible with a dextral motion on an EW oriented vertical fault plane. The best stress tensor solution shows a regime in extension with awell-defined σ3 axis oriented approximately N35°.

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

Determination of Soil Characteristic Using SPAC Method in Karsiyaka-Izmir, Turkey

All surface waves, with the exception of Rayleigh waves which are in an isotropic, half-infinite and homogeneous environment, in spite of showing a certain distribution of velocity (depends on frequency) through the surface, act in a different phase waves.In this study, for the purpose of determining the S wave velocity structure, utilizing from the property of dispersion of surface waves, has been used Spatial Autocorrelation Method (SPAC) and microtremor method which doesn’t require any source and measures the natural noise of surface with seismometers. Spatial auto-correlation method is used and passive sources have been used in this study as the source. S-wave velocities were obtained from a circular seismometer placed in the array. In addition, in order to determine the period of the floor of the dominant cycle values were obtained for each point. Settlements in the northern part of Izmir at the beach, it has a slower Vs velocity and period than the distant point.

A glance at velocity structure of Izmir

In this study; we investigated velocity structure of Izmir and surroundings. We used local earthquake data which was recorded by different type of instruments and obtained high resolution 3D sections. We selected more than 400 earthquakes which were occurred between 2010 and 2013. Examined tomographic sections especially in Izmir along coastal areas (Mavisehir-Inciralti); revealed the low speed zone. Along this low-speed zone; it is consistent with the results obtained from the stratigraphic section and surface geology. While; low velocity zones are associated with faults and water content; high velocity is related to magmatic rocks or compact rocks. Along Karsiyaka, Seferihisar, Orhanli, Izmir fault zones; low P velocity was observed. When examined higher elevations of the topography; which are composed of soured magmatic material is dominated by high P velocity. In all horizontal sections; resolution decreasing with increasing depth. The reason for this; the reduction of earthquakes causes ray tracing p...

Investigation of 1-D crustal velocity structure beneath Izmir Gulf and surroundings by using local earthquakes

In this study; we examined one dimensional crustal velocity structure of Izmir gulf and surroundings. We used nearly one thousand high quality (A and B class) earthquake data which recorded by Disaster and Emergency Management Presidency (AFAD) [1], Bogazici University (BU-KOERI) [2] and National Observatory of Athens (NOA) [3,4]. We tried several synthetic tests to understand power of new velocity structure, and examined phase residuals, RMS values and shifting tests. After evaluating these tests; we decided one dimensional velocity structure and minimum 1-D P wave velocities, hypocentral parameter and earthquake locations from VELEST algorithm. Distribution of earthquakes was visibly improved by using new minimum velocity structure.

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.