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This paper reports the results of a case study of the application of the microtremor exploration method in western Turkey. We have conducted microtremor surveys at strong motion stations in Aydin and Denizli provinces to derive shallow S-wave velocity profiles. Array records of vertical microtremor motion were acquired at 12 and 15 stations in Aydin and Denizli provinces, respectively. A spatial autocorrelation analysis was applied to the records to estimate Rayleigh wave phase velocities in the frequency range of 3–30 Hz. Then, the phase velocity dispersion at each station was inverted to a one-dimensional S-wave profile down to a maximum depth of about 50 m. The soil models at 5 and 11 stations in Aydin and Denizli provinces, respectively, have a bedrock with an S-wave velocity of 400–600 m/s at depths less than 10 m. The spatial distribution of average S-wave velocities in the top 30 m of the revealed profiles shows low values in the Büyük Menderes Graben and the Denizli Basin, and high values in the surrounding hill or mountain areas. We compared our S-wave profiles with those from previous surface-wave studies. S-wave velocities in shallow parts to a depth of 5–10 m are similar in the two studies. However, S-wave velocities in deep parts are different at some of the stations. [ABSTRACT FROM AUTHOR]

This article investigates the stress drop variability of shallow normal-faulting earthquakes in western Anatolia through strong-motion simulations. For this purpose, source characteristics of three moderate to large magnitude events are constrained by the empirical Green's function simulation in a broadband frequency range. Recordings of ten strong-motion stations in 78-km epicentral distance range are utilized for the simulation. Estimated strong-motion generation areas (SMGAs) are 22 km2, 66 km2, and 110 km2 where rise times are 0.6 s, 0.7 s, and 0.6 s, respectively, for the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw 6.3), and 2017 Bodrum-Kos (Mw 6.6) earthquakes. Those values are found to be consistent with global source-scaling relationships. One particular observation is that stress drop ratios between the mainshock and aftershock for all events are relatively large compared with those previously calculated for strike-slip events in Turkey. Stress drops of SMGAs for the Simav and Bodrum earthquakes are in the range of 25 MPa, and this value drops to 19 MPa for the Lesvos earthquake. To further investigate the stress drop variation of earthquakes in Western Anatolia, an earthquake source database (fc-Mo) offered by Yamanaka et al. (IAG-IASPEI 2017, S07-1-03, 2017) is utilized. Brune (Journal of Geophysical Research, 76:5002, 1971) stress drop values of > 360 small to moderate earthquakes (Mw 3.0–6.0) are calculated with the given corner frequency and seismic moment information assuming a constant shear wave velocity. Results indicate that the majority of the earthquakes have a stress drop value < 5 MPa. This value changes to between 5 and 57 MPa for the remaining earthquakes. These high stress drop values support the former findings stating that normal-faulting earthquakes may release higher stress than strike-slip earthquakes. This indicates that the regional stress regime in western Turkey may cause relatively larger stress release during the normal-faulting mainshocks. [ABSTRACT FROM AUTHOR]

In this study, edge effect on the spatial variation of surface ground motion was evaluated by conducting one (1D) and two-dimensional (2D) dynamic analyses on the numerical model of Duzce basin. Duzce is a province in northwestern Turkey and was hit by a devastating earthquake in November 1999. In order to estimate the seismic behavior of Duzce basin, 1D and 2D dynamic response analyses were carried out on its numerical model by using the seismic bedrock motion data derived from the deconvolution analysis of the N-S component of 1999 Duzce earthquake accelerogram. In addition, a series of different accelerograms and slope values were utilized to investigate the possible effects of change in strong ground motion intensity level and edge geometry on the dynamic response of soil layers over inclined bedrock. The finite element model for the edge section of the flat sedimentary basin was set up by using the 2D shear wave velocity profile, geotechnical and geological site conditions in Duzce. The variation of 2D/1D spectral acceleration ratios and aggravation factors with distance from basin edge was obtained for different period values. The results indicate that the aggravation factors reached their maximum values at a specific zone near basin edge and they generally converged to 1 regardless of the bedrock inclination or period values with increasing distance from the edge section. The highest aggravation factor values were calculated as 2.0-2.5 for the period interval of T= 0.2-0.6 s. A relationship was proposed between the aggravation factor and basin edge geometry. [ABSTRACT FROM AUTHOR]

On 24 May 2014, a Mw 6.9 earthquake occurred in the west of Gokceada Island, northern Aegean Sea. The earthquake was close to Canakkale, Enez, Tekirdag cities, and damaged 300 buildings in the Marmara Region, NW Turkey. We simulated its broadband (0.1-10 Hz) ground motions including 1D deep and shallow structures soil amplification effects at the 12 strong ground motion stations in the western Marmara Region. The 1D deep velocity structures from the focal layer to the engineering bedrock with an S-wave velocity of 0.78 km/s in different azimuthal directions were tuned by comparing the observed group-velocity dispersion curves of Rayleigh and Love waves from the mainshock with theoretical ones. We also added the shallow parts from previous surveys into the 1D models. Synthetic seismograms on the engineering bedrock were generated using the discrete wave number method with a source model and the 1D deep velocity structures. Then the surface motion was generated considering shallow soil amplification. The synthetic seismograms are generally in good agreement with the observed low and high-frequency parts at most of the stations indicating an appropriateness of the source model and the 1D structural model. [ABSTRACT FROM AUTHOR]

The promotion of earthquake research in Japan covers comprehensive basic policies for thepromotion of seismic research through the observation, measurement and survey ofearthquakes. HERP (Headquarter for Earthquake Research Promotion) proposed a frameworkof survey and observation plan of earthquakes in Japan. At the first stage of this framework, itconsists of observation of crustal activities and surveying of underground structures. Nextstep is to improve the methods of predicting strong ground motion and increase accuracy,and thus the improvement of seismic hazard maps. In order to fully capture theseismic hazard of a region, we must understand the effects of the regional 2D/3Dunderground structure on wave propagation. Selection and modification of groundmotion time series to represent a specific hazard at a site has a large impact on theresults of nonlinear response history analyses using these input motions, indicatingthe need to establish rational procedures for ground motion selection, scaling andmodification. Validated seismological methods may be used to generate ground motiontime series that incorporate near-fault rupture directivity effects and basin effects,and appropriately represent the duration and long period energy content of theselarge design events. These effects cannot be fully predicted by simply adoptingGMPEs that are developed for other regions. The main objective of this project is todetermine time-histories for distant and near-field earthquakes through ground motionsimulations. This joint project proposes to investigate the validation of the modeling procedure ofdetermination of geological models proposed by HERP for seismic hazard map in Japan, andoutput of this validation will be applied to the Eskisehir basin in Turkey. We canunderstand the performance of the procedure through the application to the Kumamotoarea by the Japanese side (using HERP procedure), and will be compared with theresults from Eskisehir by Turkish side. We also incorporate the methods used in theJapanese procedure into estimation of geological models in the Eskisehir basin. Inthis frame, we attempt to calculate broadband period ground motion time seriesfrom near-field and far-field earthquakes by using source modeling and groundmotion techniques. The effects of ground motions having a broadband period rangecan be studied in two parts; first one is the rupture directivity effects and secondlybasin edge effects. The prediction and simulation of such ground motions needsource modeling, determination of 2D/3D basin geometry and S-wave velocitystructure. We performed both array- and single-station microtremor measurements at a number oflocations so as to characterize the area to determine 1-D S-wave velocity structures down tothe bedrock depth beneath the strong motion sites for the broadband simulation. Finalgeological models obtained will be verified by seismic interferometry method andmicrogravity measurements. As next step, we make ground-motion simulations to get timehistories to analyze the effects of ground motions to buildings. In spite of increasing numberof buildings having different heights and characteristics in urbanized areas in Turkey, a fewstudies exist on the interaction of such buildings with ground motions having broadbandperiods. [ABSTRACT FROM AUTHOR]

Recent studies suggests that variation in stress drop may be dependent to faulting type. In addition, Akkar et al. (2018) performed residual analyses of strong-motion data compiled from the Marmara and Aegean regions in Turkey and stated that between-event residuals of the Aegean region indicate that the ground motion prediction equation tends to overestimate at 1 s and shorter periods. They explain such a discrepancy may be due to the fact that the regional differences in stress drop were not taken into account in the current ground motion prediction equations. These indications motivate us to examine the stress parameters of the well recorded normal faulting earthquakes. The Aegean region is one of the most rapidly moved and seismically active parts of the world. As a result, continuous seismic activity is observed at the region. We selected and analyzed three earthquakes, the 2011 Simav earthquake (Mw 5.8), the 2017 Lesvos earthquake (Mw 6.3) and the 2017 Bodrum-Kos earthquake, (Mw 6.6) in Western Turkey. Source characteristics of these three normal faulting events are investigated by performing empirical Green's function simulation in broadband frequency range. Relationship between strong motion generation area, rise time, and stress drop as a function of seismic moment are examined. Broadband ground motion simulations of the earthquakes revealed that the stress drop ratio between the mainshock as a large event and the aftershock as a small event, C, is relatively large in comparison to those previously calculated for strike slip events in Turkey. This indicates that the regional stress regime in Western Turkey may affect relative larger stress release during the normal-faulting potential mainshocks. [ABSTRACT FROM AUTHOR]