SITE RESPONSE ANALYSIS USING MICROTREMORS

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SITE RESPONSE ANALYSISUSING MICROTREMORS

• Bogaziçi University Kandilli observatory and
• Earthquake Research Institute
• Department of Geophysics

Korhan Umut Semin
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OUTLINE

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

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PURPOSE

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

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What is Microtremor?

• Ambient seismic noises with very low amplitudes.
• Amplitude values change between 0.1-1 micron.
• Periods of microtremors change between 0.05-2 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.

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Site Response Analysis Techniques

• Reference site Dependent Techniques
• Spectral Ratio of s-waves
• Spectral Ratio of Coda-waves
• Non Reference site Dependent Techniques
• Receiver Function Method (H/V)
• Nakamuras Technique

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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.

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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.

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Nakamuras 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, HB(w)/vb(w)1
• Site effect HS(W)/VS(W)

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EXAMPLES
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RESULTS

• Receiver function type estimates are successful
  in identifying the frequency dependence of site
  response.
• Nakamuras 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.