1 SITE RESPONSE ANALYSIS USING MICROTREMORS Boğaziçi University Kandilli observatory and Earthquake Research Institute Department of Geophysics Korhan Umut Şemin

2 OUTLINE Purpose of The Study What is Microtremor? Site response Analysis Techniques Reference site Dependent Techniques Non Reference site Dependent Techniques Examples Results

3 PURPOSE The applicability of microtremor measurements to evaluation of site response. Comparison with other site response estimation methods.

4 What is Microtremor? Ambient seismic noises with very low amplitudes. Amplitude values change between micron. Periods of microtremors change between s. Short period microtremors consist of Rayleigh waves excited locally from traffic disturbances near the recording instruments. Long period microtremors caused by atmospheric perturbations over the oceans, and they propagate as Rg and Lg phases over continental paths.

5 Site Response Analysis Techniques 1.Reference site Dependent Techniques Spectral Ratio of s-waves Spectral Ratio of Coda-waves 2.Non Reference site Dependent Techniques Receiver Function Method (H/V) Nakamura’s Technique

6 Reference Site Dependent Techniques These methods consider the ratio between the spectrum at a site of interest and the spectrum at a reference site. Reference site is usually a nearby rock site. Earthquake aftershock data are used. These methods depend on the availability of reference site. The coda wave method is popular due to the abundance of data provided by microearthquake observation networks.

7 Non Reference Site Dependent Techniques Receiver function technique assumes that the local site conditions are relatively transparent to the vertical motion. This method uses the spectral ratio between the horizontal and vertical components of motion.

8 Nakamura’s Technique This technique relies on the interpretation of microtremors as Rayleigh waves propagating in a single layer over a half space. Neglects any contribution from deep sources. Assuming very local sources, will not affect microtremor motion at the base of the soft soil layer. For all frequencies of interest, H B (w)/v b (w)=1 Site effect : H S (W)/V S (W)

9 EXAMPLES

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15 RESULTS Receiver function type estimates are successful in identifying the frequency dependence of site response. Nakamura’s technique isolates the source (and possibly path) effects from the site effects. It is possible to estimate dominant period and rough amplification level of soft sediments by measuring directly the dominant period of microtremors and its maximum amplitude.